Gram Negative Rods of the Enteric Tract Gram negative rods can infect many sites in the body including the enteric tract, respiratory tract, or as opportunistic infections of various sites such as urinary tract, peritoneum and wounds Frequent causes of enteric infections: Salmonella, Shigella, Campylobacter Less frequent: Escherichia, Vibrio, Yersinia Frequent UT infections: Escherichia (especially female) Enterobacter, Klebsiella, Proteus, Pseudomonas

Enterobaceriaceae: many found in colon Enterobaceriaceae: many found in colon. Facultative anaerobes, ferment glucose, oxidase negative Escherichia coli: many serotypes, some part of natural flora of large intestine (Bacteroides primary inhabitant). Use tests for the presence of “coliforms” as a measure of fecal contamination in water since these organisms not free-living. Lactose positive. Antigens: O= wall (LPS); H= flagella; K= capsular; thousands of serotypes.

Mechanism of infection: Different pili help determine host specificity- UT or GI, neonate or adult. After attachment, two types of enterotoxins produced by certain toxigenic strains. Heat-labile toxin (LT) stimulates adenyl cyclase similar to cholera toxin- causes release of K, Cl, fluid. Heat-stable toxin (ST) stimulates guanylate cyclase. These strains don’t invade or cause inflammation- watery diarrhea. Invasive strains cause bloody diarrhea. E. coli O157 H7 acquired from contaminated food (especially undercooked hamburger)- bloody diarrhea with no inflammation (no white cells). Causes HUS – Hemolytic Uremic Syndrome: acute renal failure often fatal. Capsule and endotoxin: prominent role in systemic infections- neonatal meningitis, urinary tract infections, sepsis. Leading cause of UTI, both community and nosocomial.

EPEC Pedestal

Salmonella: most common cause of bacterial enterocolitis in US. Also Salmonella: most common cause of bacterial enterocolitis in US. Also causes typhoid fever S. enterica major species- Most previous species now serotypes- typhi, typhimurium, enteritidis. Enterocolitis: food contamination especially poultry and eggs (mayo). Pets (turtles). Req. ≥105 organisms. Incubation: 6-48 hr. nausea and vomiting, then abdominal pain and diarrhea. Self-limited to a few days. Most common cause S. enterica-typhimurium. invasion of tissues- inflammation and diarrhea. Typhoid: caused by S. enterica-typhi. Slow onset, fever. After 1 week get bacteremia- high fever, delirium, tender abdomen, enlarged spleen. Resolves in 3 weeks, may cause intestinal damage. Can become chromic carrier. Vaccine available, but only moderately effective.

Intracellular life avoids immune response must turn on products that are required to survive inside cells In phagocytes, must block cell killing functions inhibit phago-lysosome fusion, inhibit respiratory burst produce toxins that lead to cell death

Bacterial Resistance to Macrophage Killing

Helicobacter pylori Urease positive. Causes gastritis and is major cause of peptic ulcers, risk factor for gastric carcinoma. Produces large amount of ammonia that protects it from stomach acid.