Salmonella

Salmonella
source
Caption	Color-enhanced scanning electron micrograph showing Salmonella Typhimurium (red) invading cultured human cells.
Field	Infectious disease, Microbiology

Salmonella. It is a genus of rod-shaped, Gram-negative bacteria belonging to the family Enterobacteriaceae. These facultative anaerobes are primarily known for causing gastrointestinal illnesses, ranging from mild gastroenteritis to severe systemic infections. The genus is named after the American veterinarian Daniel Elmer Salmon, who led the research team that first isolated the bacterium. Two key species, Salmonella enterica and Salmonella bongori, encompass over 2,600 distinct serovars, many of which are significant pathogens for humans and animals worldwide.

Characteristics and classification

The genus is divided into two primary species: Salmonella enterica and Salmonella bongori. Salmonella enterica is further subdivided into six subspecies, with Subspecies I containing most serovars pathogenic to humans, such as ''S. enterica'' subsp. ''enterica''. Classification relies heavily on the Kauffman–White classification scheme, which identifies serovars based on O and H antigens. These bacteria are non-spore-forming and most are motile via peritrichous flagella. They can be cultured on selective media like XLD agar and Hektoen enteric agar, and are identified through biochemical tests and techniques like PCR. The World Health Organization collaborates with reference laboratories like the Institut Pasteur and the Centers for Disease Control and Prevention to monitor global strain diversity.

Pathogenesis and transmission

Pathogenesis involves the bacteria surviving the acidic environment of the stomach and colonizing the small intestine. Key virulence factors include type III secretion systems encoded on Salmonella Pathogenicity Islands, which facilitate invasion of intestinal epithelial cells and M cells. The SPI-1 island is crucial for invasion, while SPI-2 is important for systemic survival within macrophages. Transmission occurs predominantly through the fecal–oral route. Common sources include contaminated food such as undercooked poultry, eggs, raw milk, and produce, as well as contact with infected animals like reptiles and amphibians. Major outbreaks have been traced to products processed in facilities like the Peanut Corporation of America.

Disease in humans

Clinical manifestations depend on the serovar and host factors. Gastroenteritis, often caused by serovars like Enteritidis and Typhimurium, presents with diarrhea, abdominal cramps, fever, and vomiting. Invasive diseases include typhoid fever and paratyphoid fever, caused by serovars Typhi and Paratyphi, which are restricted to humans and can lead to severe bacteremia. Complications may involve reactive arthritis, osteomyelitis, or endocarditis. Diagnosis is confirmed by culturing the organism from stool, blood, or other bodily fluids. Treatment for uncomplicated cases is typically supportive; antibiotics like ciprofloxacin or azithromycin are reserved for invasive infections, though resistance mediated by plasmids is a growing concern noted by agencies like the European Centre for Disease Prevention and Control.

Prevention and control

Prevention focuses on proper food handling and public health surveillance. Key measures include cooking poultry and eggs thoroughly, avoiding cross-contamination in kitchens, and practicing good hand hygiene, especially after contact with animals. Public health agencies like the Food and Drug Administration and the United States Department of Agriculture regulate food safety standards. Vaccines are available for typhoid fever, including the Ty21a live oral vaccine and the Vi capsular polysaccharide vaccine. In healthcare settings, infection control practices are vital to prevent nosocomial spread. Global monitoring networks, such as PulseNet International and the Global Salmonella Surveillance system, track outbreaks and emerging resistant strains to guide interventions.

In animals and the environment

Many serovars have broad host range, causing disease in livestock, poultry, and companion animals. In poultry, infections can lead to fowl typhoid and pullorum disease, impacting industries worldwide. Reptiles and amphibians are often asymptomatic carriers, posing a zoonotic risk. The bacteria can persist for long periods in the environment, including in soil, water, and on surfaces, facilitated by biofilm formation. Agricultural practices, such as the use of contaminated manure or irrigation water, can lead to contamination of crops. Wildlife, including birds and rodents, can act as reservoirs and vectors. Control in animal populations involves biosecurity measures, vaccination programs, and sometimes antibiotic treatment, though this is discouraged due to concerns about promoting antimicrobial resistance.

Category:Gram-negative bacteria Category:Foodborne illnesses Category:Zoonoses