Listeria monocytogenes,Salmonella spp.,and Staphylococcus aureus: Threats to the Food Industry and Public Health

General characteristics

L. monocytogenes is a Gram-positive bacterium, part of the phylum Bacillota, known to cause listeriosis, which is a series of infections that can be fatal in high-risk individuals, especially those immunocompromised, newborns, elderly individuals, and pregnant women (Camargo et al., 2017; Gandhi and Chikindas, 2007). It is a foodborne pathogen that encodes a range of virulence factors in addition to being able to grow at low temperatures, including under commercial refrigeration conditions (Gandhi and Chikindas, 2007). The first reports on L. monocytogenes were one century ago, dating back to 1924, when E.G.D. Murray, a bacteriologist from Cambridge, isolated Gram-positive rods from the blood of laboratory rabbits that died suddenly. He named the new bacterium Bacterium monocytogenes (Hof, 2003). Two years later, Murray et al. published a description of the bacterium and its pathogenicity in rabbits (Schlech and Acheson, 2000). In 1940, Harvey Pirie, a bacteriologist from South Africa changed the genus name of the bacterium to Listeria, in honor of Joseph Lister, the pioneer of antiseptic surgery (Hof, 2003). And in 1949, J. Potel and H.P.R. Seeliger, two German bacteriologists, identified L. monocytogenes as the cause of an epidemic of granulomatosis infantiseptica, a severe disease in newborns or stillborn infants (Hof, 2003). Later, in 1952, A.H. Dack et al. from the U.S. Public Health Service isolated L. monocytogenes from human cases of meningitis and septicemia (Khelef et al., 2006). In 1981, R.J. Mitchell et al. from Canada reported the first outbreak of listeriosis associated with food consumption, involving coleslaw contaminated with L. monocytogenes (Bell and Kyriakides, 1998). Since then, many outbreaks and sporadic cases of listeriosis have been reported worldwide, linked to various food products such as cheese, meat, seafood, vegetables, and fruits (Ribeiro et al., 2023).

Nowadays L. monocytogenes is well accepted and known as a relevant foodborne pathogen, associated with serious health issues and even death of consumers, normally related to its virulence (Batt, 2014). The infections caused by ingestion of L. monocytogenes, called listeriosis, are noninvasive for healthy individuals and invasive for immunocompromised individuals, elderly, newborns, and pregnant women, associated with causing meningitis or miscarriage in pregnant women. However, even healthy individuals can occasionally experience invasive forms of the disease, particularly when consuming large quantities of L. monocytogenes, even though flu-like symptoms and mild diarrhea may be more common in these individuals (Camargo et al., 2017; Matereke and Okoh, 2020). The infective dose for individuals with a generally healthy status is estimated to be around 10⁷–10⁹ CFUs per digested food and around 10⁵–10⁷ CFUs for individuals with immunocompromised status, pregnant women, elderly, or newborns (Quereda et al., 2021). The death rate of listeriosis is in the range of 15–60% for the at-risk groups, representing half of the foodborne infections lethality in Western countries (Disson et al., 2021; Lecuit, 2020; Maury et al., 2016; Wiktorczyk-Kapischke et al., 2021). It is well known that L. monocytogenes infections besides being a serious health issue because of its pathogenicity, L. monocytogenes is also able to survive in a wide range of temperature intervals, pH, and water activity (A_w). The organism can also be associated with biofilm formation properties which give it a better chance of survival in different environmental conditions (soil, water, plants, and animals), including in the gastrointestinal tract of humans and other animals (Anwar et al., 2022; Duze et al., 2021; Lecuit, 2020; Matereke and Okoh, 2020).

Bacteria from the genus Listeria are classified as catalase positive and are able to metabolize glucose to lactic acid (Batt, 2014). The genus Listeria includes, among others, L. monocytogenes, L. ivanovii, L. innocua, L. seeligeri, L. welshimeri, and L. grayi. However, only L. monocytogenes and L. ivanovii are considered pathogens to humans and other animals (Schmid et al., 2005). Moreover, Bouznada et al. (2025), based on the development of biomolecular taxonomic tools, suggested the taxonomic re-evaluation of the genus Listeria, where the division to Listeria senso stricto and Listeria senso lato was created and as a consequence three newly proposed genera: Murraya gen. nov., Mesolisteria gen. nov., and Paenilisteria gen. nov. within Listeriaceae family were proposed.

L. monocytogenes is a non-spore-forming Gram-positive, rod-shaped bacterium (0.5–2.0 µm), a facultative anaerobe, and capable of growth on complex media. This species grows from 0°C to 45°C, with optimum growth temperature ranging from 30°C to 37°C; at 20°C–25°C the bacterium forms the flagella, being motile, produce antigens, and actively express virulence factors (Batt, 2014; Matereke and Okoh, 2020; Wiktorczyk-Kapischke et al., 2021; Zamuz et al., 2021) (Table 1). L. monocytogenes are also characterized by tolerating a wide pH range (4.3–9.6), surviving in A_w ≥0.9, and a NaCl concentration of up to 10% (Wiktorczyk-Kapischke et al., 2021; Zamuz et al., 2021).

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Table 1.

Optimal Growth Conditions for Listeria monocytogenes, Salmonella spp., and Staphylococcus aureus

Category	Listeria monocytogenes	Salmonella spp.	Staphylococcus aureus
Temperature resistance (°C)	1–45	6–46	4–46
Preferential temperature (°C)	30–37	37	37
pH range	4.5–9.6	4.1–9.0	4.8–8.0
Lowest Aw	<0.90	0.93	0.86
NaCl tolerance	25.5%	—	10–20%

L. monocytogenes has several serotypes, each with varying prevalence and implications for human health. The most frequently encountered serotypes in human listeriosis cases are 1/2a, 1/2b, and 4b (Ribeiro et al. 2014). These three serotypes are responsible for approximately 95% of human infections. Moreover, serotype 4b is particularly notable for its association with large outbreaks of listeriosis. While less common, serotypes 1/2c, 3a, 3b, 3c, 4a, 4c, 4d, 4e, and 7 are also recognized. These serotypes can be found in various environmental and food sources. L. monocytogenes is divided into three major phylogenetic lineages. For example, serotypes 1/2a, 1/2c, 3a, and 3c belong to lineage I, while serotypes 1/2b, 3b, and 7 are part of lineage II (Nho et al., 2015). Lineage III strains are rarely isolated and are less frequently associated with human listeriosis. They are more commonly found in ruminants and environmental samples. It was suggested that lineage III has a unique evolutionary status, which may contribute to its specific adaptations and lower prevalence in human infections (Ravindhiran et al., 2023; Zhao et al., 2011).

Virulence factors

L. monocytogenes may invade human cells by three mechanisms: by crossing the intestinal barrier, by crossing the placental barrier, and by crossing the blood–brain barriers (Lecuit, 2020; Maury et al., 2016). Each one of these cases is influenced by specific L. monocytogenes virulence factors and a susceptible individual (Anwar et al., 2022; Maury et al., 2016) (Table 2).

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Table 2.

Principal Virulence Factors Reported for Listeria monocytogenes, Salmonella Spp., and Staphylococcus aureus (Keerthirathne et al., 2016; Lungu et al., 2009; Matle et al., 2020; Otto, 2014)

Variables	Salmonella spp.	Listeria monocytogenes	Staphylococcus aureus
Main virulence factors	Development of salmonellosis in humans after ingestion, mainly by food poisoning	It causes listeriosis, especially in vulnerable groups, mainly by food poisoning	Production of enterotoxins which leads to food poisoning
Toxins	Shiga, Shigella dysenteriae-1 like, trypsin-sensitive cytotoxin, heat-labile Salmonella enterotoxin	Listeriolysin, internalin, phospholipases, protein p60	Staphylococcal enterotoxin (SE), alpha-toxin, bi-component toxins
Infectious dose (CFU/g)	Intake of 105–108	Intake of 106–108	Intake of 105–108
Toxin stability	Highly affected by pH and Aw	Resistant to adverse conditions, including low pH	Extremely resistant to heat and acidic conditions
Transmission	Mainly due to ingestion of contaminated food	Mainly due to the consumption of contaminated food	It occurs due to the consumption of foods with preformed SE
Association with outbreaks	Widely associated with outbreaks of food poisoning	Associated with outbreaks, especially in dairy products	Associated with outbreaks of staphylococcal food poisoning
Susceptibility and severity of the disease	Everyone is susceptible, the severity varies depending on the individual’s health

A_w, water activity.

The virulence factors of L. monocytogenes are grouped in genes that are part of the pathogenicity islands (LIPI-1, LIPI-2, LIPI-3, and LIPI-4). LIPI-1 and LIPI-2 are related to adhesion, binding, invasion, polymerization, and cell-to-cell spread within the host organism; LIPI-3 is related to listeriolysins, toxins (listeriolysin O) with hemolytic activity, and that act as bacteriocins, dysregulating the gut microbiota; LIPI-4 is associated with neural and placental infections (Anwar et al., 2022; Disson et al., 2021; Maury et al., 2016).

During the installation of the infection, the ActA protein (enconded within LIPI-1) helps to shield the pathogen from the phagosomes with host proteins, which propels the bacteria in the cell. ActA plays an essential role in the process of the polymerization of actin, a structural protein in host cells. It mimics host cell proteins to recruit and activate the Arp2/3 complex, which initiates the formation of actin filaments. This process generates an “actin comet tail” that propels the bacterium through the host cell cytoplasm. By inducing actin polymerization, ActA enables L. monocytogenes to move within and between host cells. This motility is critical for the bacterium’s ability to spread from cell to cell, evading the host's immune response. Moreover, ActA facilitates the bacterium’s escape from the phagosome into the cytoplasm and its subsequent spread to neighboring cells. This cell-to-cell spread is a hallmark of Listeria infection and contributes to its virulence. Moreover, ActA can interact with different host cell proteins, including profilin and vasodilator-stimulated phosphoprotein, and as a consequence, it can contribute to actin filament formation and stability (Travier and Lecuit, 2014).

Internalins InlA and InlB (LIPI-2) are proteins capable of creating an entrance in the host cell, enabling the bacterium to colonize the cell (Pizarro-Cerda et al., 2024). In addition, these proteins induce the cell to “embrace” the pathogen-synthesizing actin. They are responsible for the internalization of the bacteria into the host cell where it is enclosed in phagosomes. After that, L. monocytogenes produces the listeriolysin O (encoded in LIPI-3), and together with phospholipase A and B (PlcA and PlcB), lysis the phagosome (Vilchis-Rangel et al., 2019). Once in the cytosol, the bacteria multiply and use ActA, as previously mentioned (Pillich et al., 2016). These filaments form a tail that propels L. monocytogenes through the cytoplasm to the periphery of the host cell. L. monocytogenes enters the neighboring cells through protrusions and the cycle of infection initiates again. Other internalins also interact with the bacterium’s capability to cross the blood–brain barrier and the placental barrier (Ireton et al., 2021; Lecuit, 2020; Matereke and Okoh, 2020). Moreover, this variety of virulence facts is a powerful illustrative fact for the potential negative impacts of L. monocytogenes on consumers and evolution adaptation (for review, see Gandhi and Chikindas, 2007; Kathariou, 2002; Radoshevich and Cossart, 2018; Wiktorczyk-Kapischke et al., 2023).

L. monocytogenes has resistance to antibiotics and chemicals applied in the cleaning/sanitizing process. The first reports regarding antibiotic resistance in L. monocytogenes were seen in France in 1988, including the first multidrug-resistant strain. Up through 1999, only sporadic resistance had been observed to antibiotics, including tetracycline, chloramphenicol, erythromycin, and streptomycin (Hanes and Huang, 2022). In clinical terms, antimicrobial resistance is the most important, affecting the listeriosis lethality. This resistance to antibiotics is associated with the following factors: horizontal gene transfer, which allows the bacteria to receive genes from other bacterial species; and the formation of biofilms, with dormant cells in the bottom layer, which contributes to the chances of surviving against antibiotics, disinfectants, temperature, and pressure (Matereke and Okoh, 2020).

Listeria monocytogenes in the food industry

L. monocytogenes is a bacterial species, designated as a serious foodborne pathogen, subject to continuous surveillance and control by the food industry. L. monocytogenes can be found in various natural environments including soil, which can contaminate vegetables and other crops, especially when manure is used as fertilizer; water sources can harbor Listeria and contaminate food during irrigation or processing; L. monocytogenes can be an inhabitant of the intestinal tract of many animals, including livestock. This can lead to contamination of meat during slaughter and processing. Moreover, decaying plant material and silage can be reservoirs of Listeria, and further contaminate crops and animal feed; The feces of healthy animals and humans can contain Listeria and contribute to environmental contamination (Gandhi and Chikindas, 2007). It is a sanitary and economic risk for the food production industry, particularly for the dairy and meat industries, which have products commonly related to outbreaks of listeriosis (Disson et al., 2021; Shamloo et al., 2019; Wiktorczyk-Kapischke et al., 2021). There are several possibilities for cross contaminations, which aligned with the metabolic activity of the organism under refrigeration, makes L. monocytogenes one of the dangerous pathogens in the food industry, particularly for ready-to-eat products (Gandhi and Chikindas, 2007; Ribeiro et al., 2023). Moreover, the capability to form biofilm is an additional major problem in the dairy industry, making the removal of the contamination a challenge (Bahrami et al., 2020; Ribeiro et al., 2023; Townsend et al., 2021). The most affected products are surface-ripened, soft, and semi-soft cheeses (Ribeiro et al., 2023; Wiktorczyk-Kapischke et al., 2021). The L. monocytogenes contamination in food products may come from contaminated raw material or equipment with biofilms, which release the pathogen in all the products of that production line. Depending on the contamination source, different solutions are applied. For example, if the raw material is contaminated, the industry can change the supplier, process, and/or formulation to eliminate contamination. On the other hand, if the contamination comes from the production line, elimination of the organism may require extreme measurements, including the use of alternative methods because the organism may resist traditional treatments.

Biofilm formation can be considered one of the major problems faced by the food industry when the issue is contamination with L. monocytogenes. This capability significantly enhances its survival and persistence in various environments, particularly in food processing settings. The environmental conditions are directly involved in biofilm formation, where several factors, such as temperature, pH, nutrient availability, and the presence of salts may play a role in the persistence of L. monocytogenes formed biofilms (Gandhi and Chikindas, 2007; Lee et al., 2019b). Moreover, the specificity of each strain of L. monocytogenes, in other words, is that its genetic background can predispose capacities for biofilm formation. In general, genes associated with quorum sensing, flagellar synthesis, and extracellular polymer production are upregulated during biofilm formation (Yang et al., 2024). Moreover, resistance to disinfection and antibiotics may contribute as well to biofilm stability or even biofilm may provide a protective environment for L. monocytogenes, making them more tolerant to disinfectants and antibiotics compared with their planktonic counterparts (Møretrø and Langsrud, 2004). The processes of biofilm formation are associated with surface adherence properties of L. monocytogenes, giving advantages to adhering to a variety of surfaces commonly found in food processing environments, such as stainless steel and plastic (da Silva and De Martinis, 2013). However, the persistence of L. monocytogenes in biofilms within food processing facilities can lead to contamination of food products, posing a serious risk to public health. This is particularly concerning for ready-to-eat foods, which do not undergo further cooking to eliminate pathogens (Lee et al., 2019a).

Most common sources of contamination with L. monocytogenes are soil, water, and animal feces, and can contaminate raw vegetables and fruits that are grown or washed with these sources. Moreover, animal feces can also contaminate foods of animal origin, such as meats and dairy products (Camargo et al., 2017; Rodrigues et al., 2016). Raw or unpasteurized milk and dairy products are often pointed out as sources of L. monocytogenes. L. monocytogenes can be present in the udder or milk of cows, goats, sheep, or other animals, and can contaminate raw or unpasteurized milk and dairy products, such as cheese, butter, yogurt, and ice cream (Condoleo et al., 2020; El-Hajjaji et al., 2022; Hamidiyan et al., 2018; Lee et al., 2019a; Rodrigues et al., 2016; Shamloo et al., 2019). In addition, L. monocytogenes can be present in the intestinal tract or skin of animals, such as pigs, cattle, sheep, chickens, or turkeys, and can contaminate raw or processed meat and poultry products, such as sausages, hot dogs, deli meats, pâté, and liver and other processed food commodities of animal origin (Hamidiyan et al., 2018; Liu et al., 2020; Matle et al., 2020; Rodrigues et al., 2016). Raw or smoked fish and seafood products are maybe one of the most associated with L. monocytogenes contaminated food products, since Listeria can be present in the water or on the surface of fish and seafood, such as salmon, trout, tuna, shrimp, crab, or oysters, and can contaminate raw or smoked fish and seafood products, such as sushi, sashimi, smoked salmon, or caviar (Elbashir et al., 2018; Zahedi Bialvaei et al., 2018). Moreover, L. monocytogenes can also be a serious pathogen for pets and transmitted via raw pet food products that contain meat, poultry, fish, or dairy ingredients. Pets can also spread the bacterium in the home environment if they eat food contaminated with L. monocytogenes (Bilung et al., 2018; Davies et al., 2019).

The conventional thermal treatment is capable of reducing the number of L. monocytogenes, but new nonthermal approaches have been studied to solve the L. monocytogenes contamination in food products, such as high pressure, ultrasound, irradiation, cold plasma, ohmic, and ozone (Bahrami et al., 2020). The addition of ingredients with antimicrobial activity has also been explored, such as phenolic compounds (Zamuz et al., 2021), the addition of bacteriocins (nisin is applied worldwide as an antimicrobial agent in the dairy industry), and other bacteria, where interbacterial interactions were explored as part of the biopreservative strategy (Aragon-Alegro et al., 2021; Castilho et al., 2019, 2020; Cavicchioli et al., 2019). In addition, the use of new approaches to biofilms formation also has been explored with the application of bacteriocins, postbiotics, essential oils, and shikonin to disrupt and inhibit biofilm formation (Hossain et al., 2021; Li et al., 2021; Ribeiro et al., 2023; Somrani et al., 2020).

Salmonella spp.

Salmonella is a bacterial genus, part of the phylum Pseudomonadota, family Enterobacteriaceae, associated with various diseases in humans and other animals, including typhoid fever, gastroenteritis, and septicemia (Santos et al., 2003). First reports on Salmonella were from 1880 by Karl Eberth, a German bacteriologist, who observed a bacterium in the spleens of typhoid patients (Monte and Sellera, 2020). Later, in 1884, Georg Theodor Gaffky, a student of Robert Koch, confirmed Eberth’s discovery and isolated the bacteria in pure culture (Ibrahim and Morin, 2018). The genus Salmonella was named only in 1900 by Harvey Pirie, a South African bacteriologist, in honor of Daniel Elmer Salmon, an American veterinarian who worked on hog cholera (Bernstein, 1983). However, the bacterium that Salmon and his colleague Theobald Smith found in pigs was later shown to be different from the one that causes typhoid fever in humans (Schultz, 2008).

The first recorded outbreak of salmonellosis associated with humans and food ingestion was reported in 1919 by Edwin Oakes Jordan, an American microbiologist, who traced the contamination to cheese (Hudson, 1937). In the last (20th) century, numerous outbreaks and sporadic cases of salmonellosis were reported and linked to various food products of animal and plant origin, including meat, poultry, eggs, dairy products, fruits, and vegetables (Eng et al., 2015).

Morphology and serology

Salmonella is a bacterial genus of rod-shaped, facultative anaerobes, non-spore-forming, Gram-negative, predominantly motile bacteria ranging from 0.7 to 1.5 µm in diameter and 2 to 5 µm in length. This genus has more than 2600 serotypes (serovars). It grows in an A_w above 0.93 at temperatures ranging from 2°C to 54°C, with the optimum temperature at 37°C, and the pH range from 6.5 to 7.5, with some strains growing at pH values of 9.5 as well as below 4.05 (Cox and Pavic, 2014) (Table 1). Cellular components such as flagella and fimbriae may not be expressed under extreme pH conditions. Salmonella are environmental bacteria found majorly in the gastrointestinal tract of humans and animals (Ehuwa et al., 2021). Nowadays, the genus Salmonella consists of two species: Salmonella enterica with six subspecies and Salmonella bongori. The six subspecies of S. enterica are: S. enterica subsp. enterica, S. enterica subsp. salamae, S. enterica subsp. arizonae, S. enterica subsp. diarizonae, S. enterica subsp. houtenae, and S. enterica subsp. indica. S. enterica subsp. enterica comprises more than 2600 serovars (Uzzau et al., 2000). Salmonella serovars can be classified as: host-restricted serovars, which are able to cause a typhoid-like disease in a single host species. Examples of this group are Salmonella Typhi, Salmonella Sendai, and Salmonella Paratyphi A, B, and C in humans and Salmonella Gallinarum and Salmonella Pullorum in poultry; host-adapted serovars, which are associated with one host species but are also able to cause disease in other hosts. Salmonella Choleraesuis and Salmonella Typhisuis belong to this group; broad-host-range serovars, which rarely produce systemic infections but are able to colonize the GI tract of a wide range of animals. The vast majority of serovars are in this group, of which the most common strains are Salmonella Typhimurium and Salmonella Enteritidis, are collectively referred to as nontyphoidal serovars (Uzzau et al., 2000). However, most species may not cause a health problem.

Most Salmonella infection cases are not severe, but in immunocompromised individuals, it can develop into a severe disease. The symptoms usually last 4–7 days, and the patient recovers without treatment. For most of the cases, the infectious dose is between 10⁶ and 10⁸ CFUs. These infections are common, and only the United States estimates 1.4 million Salmonella cases annually, with 415 fatal cases. However, in developing countries, these infections affect 21.5 million people annually. But typhoid fever, a disease resultant of S. Typhi contamination, is estimated at 12–33 million cases and 600,000 deaths annually (Jaroni, 2014). Salmonella infections normally are associated with financial costs of around $2.7 billion annually (Jaroni, 2014; Ricke and Gast, 2014). Salmonella, together with L. monocytogenes and Shiga toxin–producing E. coli are responsible for most recalls within the food industry (Qiu et al., 2021).

Virulence factors

Biological diversity between various Salmonella spp. is related to specific serology variety. Moreover, different strains of Salmonella may carry a variety of virulence factors that help them overcome host defenses with a profound effective pathogenicity. Generally, as a result, Salmonella can cause two distinct syndromes which are characterized as gastroenteritis and systemic diseases (Ehuwa et al., 2021) (Table 2).

Several of those virulence factors may have been acquired by horizontal gene transfer and integrated into the bacteria’s chromosome on Salmonella pathogenicity islands (SPIs). Twelve of these SPIs are currently recognized with SPI1 and SPI2 being the most critical for cell invasion. These determinants, along with other virulence traits encoded elsewhere on the chromosome or on plasmids, are common to many strains and work together to allow Salmonella to function as pathogens (Cox and Pavic, 2014; Jennings et al., 2017). The bacteria can also express a specific membrane-bound protein named cytotoxin, which is released intracellularly related to the limited bacterial lysis and can inhibit protein synthesis and consequently lead to host cell lysis. The toxin’s chelation of divalent cations also causes cell membrane disruption (Cox and Pavic, 2014).

Salmonella (LPSs) composition of the O side chains influences virulence, particularly the ability to cause invasive infection, as different serogroup determinants interact differently with components C5–C9 of the complement cascade (Cox and Pavic, 2014). For instance, the length and structure of the O-antigen can affect the bacterium’s ability to evade complement-mediated killing, with longer O-antigen chains providing better protection against the host immune response (Krzyżewska-Dudeket al., 2024) Additionally, variations in the O-antigen structure can influence the rate at which Salmonella is taken up by macrophages, thereby affecting its survival and replication within host cells (Lerouge and Vanderleyden, 2002).

Several of the Salmonella serovars can produce one or more of the different fimbrial structures, some of them being involved in pathogenicity, an important research model for studying the Enteritidis serotype fimbriae. Fimbriae are flagella-like appendages on the surface of the bacteria, involved in the processes of adhesion to host cells, a critical step in initiating the infection process (Dufresne and Daigle, 2017). Type 1 fimbriae are the most studied and are involved in adhesion to epithelial cells; curli fimbriae are associated with biofilm formation and adhesion to surfaces; type IV fimbriae are related to the motility but as well play a role in DNA uptake (Dufresne and Daigle, 2017; Yue et al., 2012). The genes associated with encoding fimbriae are generally organized in clusters known as fimbrial gene clusters. Those specific clusters contain genes for the structural components of the fimbriae, as well as for their assembly and regulation (Yue et al., 2012). Fimbriae do not play a role only for initial adhesion but are involved in biofilm formation processes, and as a consequence contribute to the protection from the host immune system and effects of antibiotics (Dufresne and Daigle, 2017; Thanassi et al., 2007). Salmonella, like many other members of the Enterobacteriaceae, produces two types of chelating molecules, or siderophores, to acquire iron. Strains of various Salmonella serotypes harbor large serotype-specific plasmids, containing an identical piece of DNA known as the Salmonella plasmid virulence (Ibarra and Steele-Mortimer, 2009).

Salmonella biofilm facilitates their capacity for survival and long-term persistence in adverse environmental conditions and increases their ability to transmit to new hosts. Bacterial biofilm members resist various antibiotics, disinfectants, detergents, and UV radiation. Salmonella biofilm formation abilities can lead to increased mortality and morbidity rate, chronic infection, and hospitalization rate, thus the resistance to several antibiotics and evading the host’s immune system (Pradhan et al., 2023).

Salmonella enterica Enteritidis

Salmonella enterica Enteritidis is a subspecies of S. enterica, generally associated with infections transmitted from poultry to humans. Several identified virulence factors contribute to the pathogenic behavior of Salmonella Enteritidis: Salmonella Enteritidis endotoxin, associated with cell wall LPS, increases resistance to attack and digestion by host phagocytes. Heat-labile protein exotoxins are also involved in the virulence of Salmonella Enteritidis. Enterotoxin activity induces secretion by intestinal epithelial cells, while cytotoxin inhibits protein synthesis and causes structural damage to intestinal epithelial cells (Coburn et al., 2007; Ricke and Gast, 2014).

Salmonella Typhi

Salmonella Typhi causes typhoid fever, a febrile systemic illness, atypical of the gastrointestinal syndrome associated with most Salmonella characterized by fever, headache, constipation, malaise, chills, and myalgia. The severe manifestation of the disease is hemorrhagic necrosis of the ileal Peyer’s patches, resulting in tissue perforation, peritonitis, septicemia, and death. However, this disease progression happens in less than 5% of the cases but has a mortality rate of 40%, which can increase to more than 80% if treatment is delayed. It is transmitted by the fecal-oral route, mainly through contaminated food and water. Individuals with typhoid fever can carry bacteria in their intestinal tract. In addition, a small number of individuals, called carriers, recover from the disease but continue to harbor the bacteria in their gallbladder, serving as a reservoir for these pathogens. Both patients and carriers excrete Salmonella Typhi in their faces. Typhoid fever is more common in areas of poor hygiene, and water is likely to be contaminated with sewage. The virulence of Salmonella Typhi depends on its ability to invade cells and produce a protective layer of LPSs and the production and excretion of invasin (inv genes), a protein that helps the bacterium invade nonphagocytic cells, where the bacterium can survive and replicate intracellularly. Salmonella Typhi strains produce two types of siderophores, aerobactin and enterochelin, and toxins, including enterotoxin (Coburn et al., 2007; Jaroni, 2014).

Salmonella in the food industry

Several environmental conditions affect Salmonella growth and survival, which are used to control the contamination level in the products, such as temperature, pH, and A_w. Moderate heat processes, such as pasteurization, quickly eliminate the bacteria. Foods low in moisture and high in solids, particularly protein or fat, are highly protective of the bacterium, increasing its thermal resistance (Liu et al., 2018). Its heat resistance is lower when solutes such as NaCl rather than sugars are used to reduce A_w. Salmonella also survives low temperatures (Cox and Pavic, 2014).

Incidents related to contamination of food products with Salmonella, normally are associated with raw meat and poultry. Beef, chicken, pork, and turkey are common sources, especially if not cooked. Moreover, raw or undercooked eggs can carry Salmonella, even if the shell appears intact. Unpasteurized dairy products, and specially milk and different types of cheeses prepared from unpasteurized milk, raw or undercooked seafood and shellfish, fruits, and vegetables, especially those consumed raw can be carriers for Salmonella. Food processed products such as nut butters, frozen pot pies, chicken nuggets, and stuffed chicken entrees are considered as high risk (Ehuwa et al., 2021).

Salmonella pathogens contamination in the food industry is a significant problem, thus the bacteria contamination source and the capability to grow in conditions that affect specific products. Moreover, the specificity of the food products (A_w, pH, nutrients, storage temperature) can be facilitating the development of Salmonella when present. For example, foods such as mayonnaise made with raw eggs, undercooked food products, and ready-to-eat products can be carried as primary contaminated or cross contaminated during food handling stages. Salmonella is naturally found in poultry and eggs. What makes necessary the use of processes to reduce natural contamination or eliminate it, the most effective method is the application of heat above 70°C and the use of hygienic handling practices (Ehuwa et al., 2021). Environmental spread of Salmonella and industrial mass production of food commodities when hygienic controls are not sufficient can give an opportunity for that microbial species to be found in a variety of food products, including poultry, beef, eggs, dairy products, fruits, and vegetables. Contamination may occur at multiple points along the food production chain, starting from the farm and going all the way up to ready-to-eat products. Salmonella bacteria is the natural inhabitant of the intestines of many farm animals and can be transferred to meat during slaughter and processing. Fruits and vegetables can be in contact with contaminated water or soil. Cross contamination can be an important factor in the spread of Salmonella, and thus proper handling and hygiene practices need to be implemented when manipulating food products (Ehuwa et al., 2021).

Staphylococcus aureus

S. aureus is a Gram-positive bacterium, part of the phylum Firmicutes, that causes various infections in humans and other animals, including skin infections, pneumonia, endocarditis, and food poisoning (Tong et al., 2015). First observations and reports on S. aureus dates back to 1880 by Karl Eberth, a German bacteriologist, who found the bacteria in the spleens of typhoid patients (Contrepois, 1996). In 1884, Georg Theodor Gaffky, another scientist working with S. aureus, confirmed Eberth’s discovery and isolated the bacterium in pure culture (Stillwell, 2013). The genus Staphylococcus was named in 1900 by Harvey Pirie, a South African bacteriologist. In 1928, Alexander Fleming, a Scottish biologist, discovered penicillin, the first antibiotic that could kill S. aureus (Dosani, 2004). However, already in the 1940s, some strains of S. aureus had developed resistance to penicillin and other antibiotics (Bal and Gould, 2005). In 1961, Patricia Jevons, a British microbiologist, reported the first case of methicillin-resistant S. aureus (MRSA), a strain of S. aureus that is resistant to most antibiotics. Methicillin was introduced to the pharmaceutical market as a narrow-spectrum β-lactam antibiotic that was historically used to treat infections caused by certain Gram-positive bacteria, particularly those producing β-lactamase, such as S. aureus. Since then, MRSA has become a major public health problem worldwide, causing serious and sometimes fatal infections in hospitals and communities (Boucher and Corey, 2008; Stefani et al., 2012).

General characteristics

S. aureus is a Gram-positive, with coccoid morphology, catalase, and coagulase-positive bacterium. It forms irregular colonies of convex shape with a yellow color on the surface when grown on solid culture media. Its diameter ranges from 0.5 to 1.5 μm. The optimal growth conditions for S. aureus include temperatures between 7°C and 48°C, with optimal for the production of etoxins around 40°C and 45°C and pH between 4.0 and 9.8, and A_w up to 0.83 (Missiakas and Schneewind, 2013). The microorganism naturally colonizes the skin and mucous membranes of most land and marine animals. In humans, S. aureus can be found in the upper respiratory tract and is part of the skin microbiome. It is estimated that about 30–50% of the population carry the pathogen as a transient or permanent host without any symptoms since the protective epithelial tissue acts as a physical barrier preventing contamination and the entry of the microorganism. Under normal circumstances, colonization of this microorganism is not directly associated with diseases since the bacteria benefit from the nutrients provided by the host’s skin. S. aureus can become a problem when the integrity of this barrier is compromised or there is a failure in the body’s defense mechanisms, a situation that can be seen in individuals with immunodeficiency. This coexistence on the skin confers high resistance to changes in osmolarity since S. aureus is well adapted to a high amount of inhospitable environment, being able to survive outside its host in various environmental conditions (Powers and Wardenburg, 2014; Short and White, 1971) (Table 1). Moreover, some coagulase-negative staphylococci can be even beneficial starter cultures and plays an essential role in meat products’ fermentations (Laranjo et al., 2019).

Serotypes

S. aureus presents a wide variety of serotypes, which are based on the presence of specific surface antigens. There are more than 30 known serotypes, which are classified based on their surface proteins, such as protein A, protein G, fibronectin-binding A and B, and clumping factor protein types A and B. These serotypes are of fundamental importance for epidemiological studies and investigation of infection outbreaks (Powers and Wardenburg, 2014).

Virulence factors

S. aureus produces various types of factors that facilitate its entry and adhesion to the host and enable its survival in a variety of environments. To recognize and eliminate a pathogen, cells must first recognize it through professional antigen-presenting cells (APCs), which present the antigen to T lymphocytes that will produce specific antibodies to neutralize the antigen. Based on this system, S. aureus has many mechanisms that allow it to evade this defense mechanism (Pantosti et al., 2007; Powers and Wardenburg, 2014) (Table 2).

S. aureus virulence is related through the release of superantigen, described as highly pyrogenic factor exotoxin. When activated, this toxin, classified as pyrogenic toxin superantigens (PTSAg), recruits a large amount of immune system cells such as APCs and lymphocytes, and through its interaction with T lymphocyte receptors and major histocompatibility complex (MHC), responsible for preventing a foreign body from entering or spreading throughout the host, results in extensive cytokine release by these cells, resulting in a failure in the functionality of cells such as macrophages and T and B lymphocytes (Tan et al., 2020).

Not limited to the aforementioned, there are several types of PTSAg toxins, each with its specificity and effects on the host, such as the toxin of type 1 toxic shock syndrome (TSST-1), as well as staphylococcal enterotoxins (SEs A-E, G-J, K, L, M, O, and P), exfoliative toxins, hemolysin (α, β, δ, γ, and δ), and leukocidin, among others, which vary according to the strain of the microorganism (Pineda et al., 2023). At a macro level, these toxins cause symptoms such as fever, tissue destruction, toxic shock, and even death (Dinges et al., 2000; Pantosti et al., 2007).

Multiresistance to antibiotics

Using various mechanisms of horizontal gene transfer, bacteria such as S. aureus can acquire and proliferate their abilities responsible for resistance against different classes of antibiotics. Horizontal gene transfer can occur through transduction, transformation, or conjugation, and this passage means that several bacterial generations are not necessary before a mutation responsible for antibiotic resistance occurs. Then, this genetic material is incorporated into the bacterium’s genome, making it available for use. Currently, different types of antibiotics can be applied for the control of S. aureus, each with a specific mechanism of action and recommended for a particular pathogen according to its metabolism and resistance factors. Antibiotics can be divided into classes based on their specific interactions with bacterial groups and cause the microorganism’s death by lysis, while there are also antibiotics that belong to the bacteriostatic group that inhibits pathogen proliferation, making it possible for the immune system to eliminate the invader (Doulgeraki et al., 2017; Pantosti et al., 2007).

Microbial cells express different genes in response to various antibiotics due to specific resistance mechanisms inherent to the targeted microorganisms. In summary, S. aureus uses enzymatic means to deactivate the product, alteration of the antibiotic’s action site, promoting changes in the membrane’s thickness, making it difficult for the antibiotics to penetrate, and even trapping the substance, in addition to efflux pumps. S. aureus strains present many antibiotic resistance mechanisms. Antibiotics resistance is recorded, including penicillin, amoxicillin, cefazolin, cephalothin (β-lactam), ampicillin, cephalexin, (first-generation cephalosporin), ciprofloxacin (aminoglycoside), erythromycin, tetracycline (macrolides), gentamicin (lincosamide), oxacillin (fluoroquinolone), and some strains are resistance to vancomycin (glycopeptide) (Mohammed et al., 2015).

Enzymatic inactivation of antibiotics occurs when bacteria produce enzymes that can break down or modify the antibiotic, resulting in its ineffectiveness. Penicillinase, for example, is an enzyme produced by bacteria that can inactivate penicillin, making it ineffective. At the same time, aminoglycoside-modification enzymes can chemically modify aminoglycoside antibiotics, preventing them from binding to their target site (Dos Santos et al., 2007; Pantosti et al., 2007; Powers and Wardenburg, 2014).

Alteration of the target occurs when bacteria modify the antibiotic’s target site, making it less susceptible to binding by the drug. For example, some bacterial strains, such as MRSA, produce a modified form of penicillin-binding protein 2a, which has decreased affinity for β-lactam antibiotics such as methicillin and penicillin (Dos Santos et al., 2007; Pantosti et al., 2007; Powers and Wardenburg, 2014).

Trapping of the antibiotic occurs when bacteria produce a thick cell wall that can prevent antibiotics from reaching their target. For example, vancomycin-resistant S. aureus can produce a modified form of peptidoglycan precursors called d-Ala-d-Lac that cannot be bound by vancomycin (Dos Santos et al., 2007; Pantosti et al., 2007; Powers and Wardenburg, 2014).

Efflux pumps work by pumping out antibiotics from inside the bacterial cell before they can exert their effect. Fluoroquinolones and tetracycline are some examples of antibiotics that can be pumped out by efflux pumps in S. aureus (Dos Santos et al., 2007; Pantosti et al., 2007; Powers and Wardenburg, 2014).

Growth conditions

S. aureus is a facultative anaerobic bacterium that cannot produce its toxins under restricted or no air conditions. This ability allows it to survive through a wide range of temperature variances, from 7°C to 46°C, and even resist freeze cycles down to −20°C. These factors also depend on the A_w that needs to be at 0.86 or higher and pH levels ranging from 4.5 to 9.3. In anaerobic conditions, the microorganism can double its colony in almost an hour, while in aerobiosis, it can do it in only 25 min (Dos Santos et al., 2007).

S. aureus in different fields: hospital and industry

Due to its resistance to most of the antibiotics available on the market, and its virulence factors, S. aureus is a very problematic pathogen in the hospital setting. Here are some common diseases caused by it, principally in immunodeficient patients. Pneumonia, for example, caused by S. aureus, is a severe lung infection that can occur in healthy people or those with underlying diseases. Symptoms include fever, productive cough, chest pain, and shortness of breath. In severe cases, pneumonia can cause respiratory failure and death. The infection is usually acquired by inhaling respiratory droplets containing the bacteria or through contact with contaminated surfaces. S. aureus produces a series of virulence factors that contribute to the pathogenesis of pneumonia, including adhesive proteins that allow the bacteria to attach to respiratory tract cells and toxins that damage lung tissue. Treatment involves the use of antibiotics and, in some cases, respiratory support. Bacteremia is a bloodstream infection that can be caused by S. aureus. Symptoms include fever, chills, sweating, and in severe cases, septic shock. The infection can occur from a primary source of infection, such as an infected wound, or can be a complication of another infection, such as pneumonia or endocarditis. S. aureus produces a variety of virulence factors that allow the bacteria to evade the host’s immune system and spread throughout the body. Treatment involves the use of intravenous antibiotics and, in severe cases, hemodynamic support. Endocarditis is a heart infection that can be caused by S. aureus. Symptoms include fever, chills, sweating, fatigue, shortness of breath, and chest pain. Endocarditis can occur when the bacteria attach to heart valves, causing damage and blood clot formation. S. aureus produces enzymes that allow the bacteria to adhere to heart valves and virulence factors that promote blood clot formation. Treatment involves the use of intravenous antibiotics and, in some cases, surgery to repair or replace damaged heart valves. Osteomyelitis is a bone infection that can be caused by S. aureus. Symptoms include pain, swelling, redness in the affected area, fever, and general malaise (Dos Santos et al., 2007; Kretschmer et al., 2012).

S. aureus is also a common foodborne pathogen that poses a potential risk to both customers and staff in the food industry. This bacterium can form biofilms, which are complex structures of bacterial cells that provide a protective environment for the pathogen to survive and be able to tolerate many forms of stress. Various factors, such as subtypes, genetic locus, environmental conditions, and adhesion surfaces, can affect the formation and maturation of S. aureus biofilms. Over the past decades, several studies have reported on the formation and development of S. aureus biofilms in the food industry, and the consequences can include contamination of food products, leading to food poisoning outbreaks and substantial economic losses for the sector (Dos Santos et al., 2007).