Salmonella

Salmonella is a genus of gram-negative, rod-shaped bacteria in the Enterobacteriaceae family. On laboratory media it is a non-lactose fermenter that produces hydrogen sulfide (black colonies on Hektoen agar), and unlike its close cousin Shigella it is motile — flagellated — a distinction exams test repeatedly.

Clinically, the genus splits in two. Nontyphoidal Salmonella (such as S. enteritidis and S. typhimurium) causes ordinary gastroenteritis — nausea, vomiting, fever, and diarrhea beginning roughly 12 to 72 hours after eating contaminated poultry, eggs, or dairy, or after handling reptiles like pet turtles. Illness is usually self-limited, and antibiotics are generally reserved for severe or invasive cases since they can prolong carriage. Salmonella typhi, found only in humans, causes typhoid fever: stepwise rising fever, abdominal pain, rose spots on the trunk, and sometimes relative bradycardia; chronic carriers harbor the organism in the gallbladder.

Salmonella invades the intestinal mucosa through M cells in the ileum and can survive inside macrophages, which is how typhoidal disease spreads systemically. Two classic associations round out the picture: patients with sickle cell disease are prone to Salmonella osteomyelitis, and gastric acid normally kills much of an ingested dose, so antacid use and gastrectomy lower the infectious threshold.

USMLE Step 1 tests Salmonella in microbiology (lab identification, Salmonella-versus-Shigella distinctions, typhoid features) and in gastrointestinal pathology, while the CCMA covers salmonellosis among digestive system diseases that medical assistants encounter, including its food-safety prevention points.