# Enterobacteriaceae > Enterobacteriaceae for FMGE Microbiology: oxidase-negative family traits, E. coli groups, typhoid cultures and Widal, Shigella, Klebsiella and Proteus. - Canonical URL: https://prepelephant.com/topics/fmge/microbiology/enterobacteriaceae-fmge - Exam / course: FMGE · Subject: Microbiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Enterobacteriaceae", PrepElephant, https://prepelephant.com/topics/fmge/microbiology/enterobacteriaceae-fmge ## Direct answer One family definition opens every answer: Enterobacteriaceae are gram-negative, non-sporing bacilli that are catalase-positive, oxidase-negative, nitrate-reducing and glucose-fermenting — and that oxidase negativity is the bench line separating them from vibrios and pseudomonads. Escherichia coli, the lactose-fermenting, indole-positive coliform, exists in diarrhoeagenic flavours (enterotoxigenic, enteropathogenic, enteroinvasive, enteroaggregative and the Shiga-toxin-producing O157:H7 that causes bloody diarrhoea and haemolytic uraemic syndrome); Salmonella enterica serotype Typhi causes enteric fever, best diagnosed by blood culture in the first week; Shigella, non-motile and lactose-negative, causes bacillary dysentery with Shigella dysenteriae type 1 carrying the deadliest Shiga toxin; Klebsiella pneumoniae, the encapsulated mucoid lactose fermenter, produces currant-jelly sputum and lung abscess; Proteus, urease-positive, swarms on agar and shares antigens with rickettsiae — the basis of the Weil–Felix reaction. ## What you must remember - Family test card: oxidase negative, glucose fermented, nitrates reduced — contrast Pseudomonas (oxidase positive, non-fermenter) and Vibrio (oxidase positive). - E. coli O157:H7 is sorbitol non-fermenting on sorbitol MacConkey agar; its Shiga toxin causes haemolytic uraemic syndrome — antibiotics and antimotility drugs worsen outcomes. - Enteric fever timeline: blood culture positive in week one (about 90 per cent when sampled early), stool culture from week two, Widal test meaningful only from the second week and always interpreted against the local baseline titre. - Widal reads four antigens: TO and TH for Salmonella Typhi, and AH/BH for Paratyphi; a single raised titre is weak evidence — a four-fold rise in paired sera is stronger. - Current Indian outpatient therapy for culture-proven typhoid: azithromycin or cefixime; hospitalised or severe disease: intravenous ceftriaxone; fluoroquinolone resistance is widespread in South Asia. - Shigella: non-motile (unlike Salmonella), lysine decarboxylase negative, manitol non-fermenting in S. dysenteriae type 1; ciprofloxacin or azithromycin treat it, and this is the classic cause of large-outbreak dysentery. - Klebsiella: large polysaccharide capsule, mucoid lactose-fermenting colonies, alcohol misuse and diabetes as risk factors, and a notorious carrier of extended-spectrum beta-lactamase and carbapenemase genes in hospitals. - Proteus mirabilis swarms in waves on blood agar, splits urea (urease positive) and underlies staghorn struvite calculi; its OX-2, OX-19 and OX-K antigens cross-react in the Weil–Felix test for rickettsial infection. ## A stool culture from start to report A five-day fever with abdominal discomfort in a resident of an endemic city goes to the laboratory, and the specimen choice decides yield: blood culture first — the organism circulates in the first week before gut shedding dominates — plus a Widal after a week if culture is unavailable. The bottle flags positive, subculture on MacConkey yields colourless non-lactose-fermenting colonies, biochemistry shows no gas, no H2S initially for Typhi, and specific antisera confirm Salmonella Typhi. A dysentery stool travels a different lane: macroscopic blood and mucus, direct microscopy excluding amoebae, enrichment in selenite F overnight, then MacConkey and deoxycholate citrate agar; non-lactose, non-motile biochemistry points to Shigella. If the story is bloody diarrhoea without fever after a burger meal, the laboratory switches to sorbitol MacConkey hunting the pale, sorbitol-negative O157 colony — and the clinical order is supportive care, not antibiotics. Every branch of the pathway exists because the family members look alike until biochemistry and antisera split them. ## Where students slip The motility line is drawn wrongly under pressure: Salmonella is motile, Shigella is not — a favourite one-mark discriminator. Widal interpretation causes the next loss: a single TO of 1:160 means nothing without the local cut-off, and early-course negativity is expected, so examinees who call a first-week negative Widal "excludes typhoid" fail the stem. Third, O157 questions bait a quinolone or antimotility prescription; the correct answer is supportive therapy, because antibiotic-induced toxin release worsens haemolytic uraemic syndrome. Finally, Klebsiella versus E. coli on a plate: both ferment lactose, but only Klebsiella gives large mucoid, stringy colonies from its capsule — and only E. coli is typically indole positive. ## Frequently asked questions ### Which single test separates Enterobacteriaceae from Vibrio and Pseudomonas? The oxidase test — negative in Enterobacteriaceae, positive in Vibrio and Pseudomonas; add glucose fermentation to exclude the non-fermenting pseudomonads. ### Which specimen gives the best yield in first-week enteric fever? Blood culture, positive in the large majority when sampled in week one; stool and urine cultures yield more in weeks two to three, and the Widal test only becomes interpretable after about a week. ### Why should antibiotics be avoided in E. coli O157:H7 infection? Shiga toxin release may be increased by antibiotics and antimotility agents, worsening haemolytic uraemic syndrome; management is supportive with hydration and monitoring. ### How does Klebsiella pneumoniae appear on culture and why? Large, mucoid, stringy lactose-fermenting colonies because of its thick polysaccharide capsule — the same capsule that underlies currant-jelly sputum and abscess formation in the alcohol-user and the person with diabetes. ### What is the basis of the Weil–Felix test? Proteus OX-2, OX-19 and OX-K strains share alkali-stable polysaccharide antigens with rickettsiae, so antibodies to rickettsiae agglutinate these Proteus strains — OX-19 positive in epidemic typhus, OX-K positive in scrub typhus.