Salmonella remains one of the most significant pathogens in food microbiology due to its association with frequent and widespread outbreaks of foodborne illness. Responsible for millions of cases of gastroenteritis worldwide each year, this bacterium poses a serious public health threat, particularly in the context of raw and undercooked foods. Its ability to survive in various environmental conditions and its resistance to many common food safety measures make it a major focus of research and prevention efforts in the food industry. Understanding Salmonella’s behavior, persistence, and transmission is crucial for minimizing outbreaks and protecting public health.
1. Cell Structure and Morphology
Salmonella is a gram-negative, facultatively anaerobic bacterium in the family Enterobacteriaceae. These rod-shaped bacteria measure about 2–5 µm in length and 0.7–1.5 µm in width. Salmonella is generally motile due to peritrichous flagella, although non-motile strains exist. The genus comprises two main species: Salmonella enterica and Salmonella bongori, with S. enterica further divided into six subspecies and over 2,600 serovars (Todar, 2020). Among these, Salmonella enterica subsp. enterica is particularly pathogenic, including serovars like S. Typhi (causing typhoid fever), S. Typhimurium, and S. Enteritidis (associated with non-typhoidal salmonellosis). These serovars vary in their host specificity, pathogenic mechanisms, and disease outcomes. The cell wall structure of Salmonella includes lipopolysaccharides (LPS), which contribute to its virulence and ability to evade the host immune system (Jay et al., 2005).
2. Sources and Illness
Salmonella contamination can originate from various food and environmental sources, including raw or undercooked poultry, eggs, meat, seafood, unpasteurized milk, and contaminated fresh produce (CDC, 2022). Livestock, reptiles, amphibians, and wild animals often act as reservoirs. Transmission typically occurs via the fecal-oral route, through consumption of contaminated food or water, or contact with infected animals (Todar, 2020). Salmonella is highly resilient, capable of surviving in a pH range of 4 to 9 and tolerating low water activity (Aw ≥ 0.94). Its optimal growth temperature is 35°C–37°C, though it can survive at temperatures as low as 7°C and as high as 48°C. An infectious dose typically ranges between 10³ and 10⁶ cells, depending on the serovar and the host’s immunity (Jay et al., 2005). Illness caused by Salmonella includes non-typhoidal salmonellosis, which manifests as diarrhea, abdominal pain, fever, and vomiting, and typhoid fever, a more severe systemic illness caused by S. Typhi (CDC, 2022). The incubation period ranges from 6 to 72 hours, with symptoms typically lasting 4 to 7 days. Major outbreaks include the 2018 S. Enteritidis outbreak linked to eggs in the U.S., causing 45 illnesses and 11 hospitalizations across ten states. In 2011, contaminated papayas imported from Mexico caused 100 illnesses in the U.S., and in 2019, a S. Newport outbreak from pre-cut melon sickened 137 people across 10 states (CDC, 2022). Globally, typhoid fever caused by S. Typhi remains a significant public health concern in developing countries, with over 14 million cases annually (WHO, 2023).
3. Identification Methods
Salmonella detection in microbiology laboratories involves a combination of cultural, biochemical, and molecular techniques. Isolation typically begins with pre-enrichment in buffered peptone water to resuscitate stressed cells, followed by selective enrichment in Rappaport-Vassiliadis (RV) broth or tetrathionate broth (ISO 6579-1:2017). Selective agar plates such as Xylose Lysine Deoxycholate (XLD) agar and Hektoen Enteric (HE) agar are used to distinguish colonies, which typically appear red with black centers due to hydrogen sulfide (H₂S) production. Incubation is conducted at 37°C for 24–48 hours. Confirmatory biochemical tests include triple sugar iron (TSI) slants, which indicate glucose fermentation and H₂S production, and serological tests to identify serovars using O and H antigens (Jay et al., 2005). Advanced molecular techniques such as Polymerase Chain Reaction (PCR) and enzyme-linked immunosorbent assays (ELISA) enable rapid and specific detection of Salmonella in food samples, even at low concentrations (ISO 6579-1:2017). One of the most notable Salmonella outbreaks in peanut butter occurred in 2008–2009 and was linked to the Peanut Corporation of America (PCA). This outbreak resulted in one of the largest food recalls in U.S. history. Here’s a brief account: The outbreak was caused by contamination of peanut butter and peanut paste with Salmonella Typhimurium. These products were widely distributed to food manufacturers, leading to secondary contamination of various food items such as cookies, crackers, and ice cream. The Centers for Disease Control and Prevention (CDC) reported that the outbreak caused 714 illnesses across 46 states and was linked to nine deaths. Investigations revealed serious sanitation issues at PCA’s facilities, including water leaks, pest infestations, and the intentional shipment of contaminated products despite positive Salmonella tests. The outbreak led to PCA’s bankruptcy and the criminal conviction of its executives. This incident underscored significant gaps in food safety practices and enforcement, contributing to the enactment of the Food Safety Modernization Act (FSMA) in 2011.
4. Prevention Methods
Prevention Methods Preventing Salmonella contamination requires a multi-faceted approach. Proper cooking is crucial, as internal temperatures of 74°C (165°F) or higher effectively kill the bacteria (Jay et al., 2005). Refrigeration at temperatures below 4°C slows bacterial growth, while pasteurization ensures the safety of dairy products and juices. Natural antimicrobials, such as essential oils from oregano, thyme, and clove, demonstrate strong antibacterial activity against Salmonella in food matrices. Antimicrobial peptides like nisin, as well as organic acids like lactic and acetic acids, are commonly used as preservatives in food products (CDC, 2022). Synthetic antimicrobials, including sodium nitrite and sodium hypochlorite, are also effective in controlling Salmonella during food processing. Additionally, adhering to good manufacturing practices (GMP), regular sanitation of equipment, and avoiding cross-contamination are critical for reducing the risk of Salmonella outbreaks in food production facilities.
References
- Centers for Disease Control and Prevention (CDC). (2022). Multistate Outbreaks of Salmonella Infections. Retrieved from https://www.cdc.gov.
- International Organization for Standardization (ISO). (2017). ISO 6579-1:2017: Microbiology of the Food Chain – Horizontal Method for the Detection, Enumeration, and Serotyping of Salmonella.
- Jay, J. M., Loessner, M. J., & Golden, D. A. (2005). Modern Food Microbiology. Springer. • Todar, K. (2020). Todar’s Online Textbook of Bacteriology: Salmonella. Retrieved from http://textbookofbacteriology.net.
- World Health Organization (WHO). (2023). Salmonella (non-typhoidal) Fact Sheet. Retrieved from https://www.who.int.
- Centers for Disease Control and Prevention (CDC). (2009). Multistate Outbreak of Salmonella Typhimurium Infections Linked to Peanut Butter, United States, 2008–2009. Available at: https://www.cdc.gov/salmonella/typhimurium/
- S. Food and Drug Administration (FDA). (2009). Peanut Corporation of America Recalls and Investigation. Available at: https://www.fda.gov/
- Griffin, P. M., & Hoffmann, S. (2010). Foodborne Illnesses in the United States. Food Safety Magazine, 16(4), 17-22.
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
Ms. Shrestha is currently a Graduate Research Assistant at Louisiana State University, USA and holds a Master’s degree in Food Engineering and Bioprocess Technology from the Asian Institute of Technology (AIT), Thailand. She previously, she worked for the Government of Nepal at the Department of Food Technology and Quality Control (DFTQC), Kathmandu, and also served as a teaching faculty at the College of Applied Food and Dairy Technology (CAFODAT) affiliated with Purbanchal University, Nepal.