Gram-negative Enteric Bacteria

The Enterobacteriaceae are a large family of Gram-negative, facultatively anaerobic bacilli that inhabit the gut and cause a huge spectrum of disease — from watery diarrhoea to typhoid, dysentery, UTI and sepsis. For NEET PG, the family is "tested as a panel": culture characteristics on MacConkey, biochemical reactions, virulence toxins and the drug of choice are the recurring anchors.

Family Enterobacteriaceae — shared characteristics

All members share a defining checklist that is itself a favourite MCQ stem:

• Gram-negative bacilli, non-sporing, mostly motile by peritrichous flagella (Klebsiella, Shigella and Yersinia are non-motile).
• Ferment glucose with acid production.
• Reduce nitrates to nitrites (key differentiating test from non-fermenters like Pseudomonas).
• Oxidase negative (single most useful screening test to separate them from oxidase-positive Pseudomonas/Vibrio/Aeromonas).
• Catalase positive.

High-yield: Oxidase negative + nitrate reduction + glucose fermentation = Enterobacteriaceae. An oxidase-positive Gram-negative bacillus is NOT a coliform — think Pseudomonas, Vibrio, Aeromonas, Plesiomonas, Campylobacter, Helicobacter.

Antigenic structure (Kauffmann–White scheme)

• O antigen — somatic, lipopolysaccharide, heat-stable. Used for serogrouping.
• H antigen — flagellar, protein, heat-labile. Most specific; basis of serotyping in Salmonella.
• K antigen — capsular, may mask O antigen agglutination (e.g., Vi antigen of S. Typhi, K1 of neonatal meningitis E. coli).

MacConkey agar — the master differentiator

MacConkey contains lactose + neutral red indicator + bile salts (selective for Gram-negatives).

Lactose fermenters (PINK colonies)	Non-lactose fermenters (PALE/colourless)
Escherichia coli	Salmonella
Klebsiella	Shigella
Enterobacter	Proteus
Citrobacter	Yersinia
Serratia (late/variable)	Edwardsiella

Mnemonic: Lactose fermenters = "KEE(C)" pink; non-fermenters = "Sh-Sal-Pro-Yer" pale. Late lactose fermenters = Citrobacter, Serratia, Shigella sonnei, some Klebsiella ("Citrobacter & Serratia ferment Slowly").

Escherichia coli

E. coli is the commonest cause of UTI and a major cause of diarrhoea, neonatal meningitis and Gram-negative sepsis. It is a lactose fermenter, indole positive, and classically gives a metallic green sheen on EMB (eosin–methylene blue) agar.

High-yield: IMViC reaction of E. coli = ++ – – (Indole +, Methyl red +, Voges-Proskauer –, Citrate –). Klebsiella is the mirror image – – + +.

Diarrhoeagenic E. coli — the six virotypes

This is the single most frequently asked E. coli table.

Virotype	Mechanism / toxin	Clinical picture	Key clue
ETEC (enterotoxigenic)	Heat-labile (LT, cAMP↑, cholera-like) + heat-stable (ST, cGMP↑) toxins	Watery; traveller's diarrhoea, infant diarrhoea in developing world	Non-invasive, no fever
EPEC (enteropathogenic)	Bundle-forming pilus + attaching–effacing lesion (intimin), no toxin	Infantile diarrhoea, outbreaks in nurseries	Loss of microvilli
EIEC (enteroinvasive)	Invades colonic mucosa (Shigella-like, no Shiga toxin)	Dysentery-like, bloody	Sereny test positive
EHEC/STEC (O157:H7)	Shiga-like toxin (verotoxin)	Bloody diarrhoea → HUS	Sorbitol non-fermenter on SMAC
EAEC (enteroaggregative)	"Stacked-brick" aggregative adherence	Persistent diarrhoea in children/HIV	Aggregative adherence
DAEC (diffusely adherent)	Diffuse adherence	Watery diarrhoea in young children	—

High-yield: EHEC O157:H7 does NOT ferment sorbitol — grown on sorbitol-MacConkey (SMAC) it forms colourless colonies. It causes haemolytic uraemic syndrome (HUS) = microangiopathic haemolytic anaemia + thrombocytopenia + acute kidney injury. Antibiotics are contraindicated in EHEC as they increase Shiga toxin release and HUS risk.

Approach to bloody diarrhoea in a child → think: Shigella → EHEC O157:H7 → EIEC → Salmonella → Campylobacter → Entamoeba.

Extraintestinal E. coli

• UTI — uropathogenic E. coli (UPEC) with P fimbriae; commonest organism overall.
• Neonatal meningitis — K1 capsular antigen strains.
• Gram-negative sepsis — leading cause; endotoxin (lipid A) driven.

Klebsiella pneumoniae

A non-motile, heavily capsulated lactose fermenter producing large mucoid colonies.

• Pneumonia (Friedländer bacillus): upper-lobe consolidation, bulging fissure sign on CXR, and "currant jelly" (red-brown, thick) sputum. Seen in alcoholics, diabetics, COPD.
• IMViC = – – + + (Voges-Proskauer & Citrate positive).
• Urease positive, lysine decarboxylase positive.
• Hypervirulent strains cause liver abscess (esp. in diabetics, K1/K2 capsule, Southeast Asia).
• Major nosocomial pathogen and a leading producer of ESBL and carbapenemase (KPC, NDM-1).

High-yield: "Currant jelly sputum + bulging fissure + alcoholic" = Klebsiella pneumoniae. NDM-1 (New Delhi metallo-β-lactamase) was first described in Klebsiella from India.

Salmonella

Non-lactose fermenters, motile, H₂S producing (black centres on XLD/Wilson-Blair bismuth sulphite agar; pink with black centre on XLD). Two clinical syndromes: enteric (typhoid) fever and gastroenteritis.

Enteric (typhoid) fever — S. Typhi / Paratyphi A

• Reservoir: humans only; faeco-oral transmission. Infective dose ~10⁵.
• Pathogenesis: ingestion → invade Peyer's patches → mesenteric nodes → primary bacteraemia → reticuloendothelial seeding (liver, spleen, gallbladder, bone marrow) → secondary bacteraemia (clinical fever).
• Clinical: stepladder fever, relative bradycardia (Faget sign), rose spots (trunk), hepatosplenomegaly, coated tongue, constipation early then pea-soup diarrhoea.

Investigation of choice by week (flow): Week 1 → Blood culture (most sensitive early) → Week 2 → Widal becomes positive → Week 3 → Stool & urine culture → throughout → Bone marrow culture = MOST sensitive overall (positive even after antibiotics).

Specimen	Best yield (week)	Sensitivity note
Blood culture	Week 1	~80% in week 1, falls later
Bone marrow culture	Any week	Highest sensitivity (~90%), unaffected by prior antibiotics
Stool culture	Week 2–3	Reflects gut shedding
Urine culture	Week 3	Lower yield
Widal test	Week 2 onward	Serology, not diagnostic alone

High-yield: Bone marrow culture is the single most sensitive test for typhoid and remains positive despite prior antibiotics. Blood culture is the investigation of choice in the first week.

Widal test detects agglutinins to O (somatic) and H (flagellar) antigens.

• O ≥ 1:160 and H ≥ 1:160 (in endemic India) suggest infection; a four-fold rise in paired sera is most reliable.
• TO/TH rise = S. Typhi; AH rise = paratyphi A.
• Limitations: false positives (prior vaccination, anamnestic reactions, other fevers), false negatives (early disease, carrier, prior antibiotics).

High-yield: Vi antigen (capsular) is used to detect chronic carriers — high anti-Vi antibody titre (≥1:160) marks a carrier (e.g., classic case "Typhoid Mary"). Vi is also the antigen in the Vi polysaccharide vaccine and typhoid conjugate vaccine.

Chronic carrier: persistent excretion >1 year, harboured in gallbladder (esp. with gallstones). Treatment: prolonged ciprofloxacin/ampicillin; cholecystectomy if gallstones present.

Drug of choice (typhoid):

• Uncomplicated, susceptible: fluoroquinolone (ciprofloxacin) OR azithromycin (excellent for fluoroquinolone-resistant strains).
• Severe/complicated or MDR: ceftriaxone (third-generation cephalosporin).
• XDR S. Typhi (Pakistan outbreak; resistant to first-line + fluoroquinolones + 3rd-gen cephalosporins): azithromycin or carbapenems.
• Nalidixic acid resistance (NARST) predicts fluoroquinolone failure.

Complications: intestinal perforation / haemorrhage (3rd week) at terminal ileum Peyer's patches, encephalopathy, myocarditis, cholecystitis.

Salmonella gastroenteritis (non-typhoidal — S. Typhimurium, S. Enteritidis)

• Zoonotic — poultry, eggs; large infective dose.
• Self-limiting watery/inflammatory diarrhoea; antibiotics not routinely given (prolong carriage) — reserved for extremes of age, immunocompromised, bacteraemia.
• Salmonella has predilection for osteomyelitis in sickle-cell disease.

Shigella

The cause of bacillary dysentery. Non-motile, non-lactose fermenting, non-H₂S producing, anaerogenic (no gas).

• Four species (subgroups A–D): S. dysenteriae (A), S. flexneri (B), S. boydii (C), S. sonnei (D).
• S. dysenteriae type 1 (Shiga bacillus) = most severe, produces Shiga toxin (inhibits 60S ribosomal subunit, AB toxin) → can cause HUS.
• S. sonnei = mildest, commonest in developed countries, late lactose fermenter, ornithine decarboxylase positive.

High-yield: Shigella has the lowest infective dose of all enteric pathogens (10–100 organisms) because it is acid-resistant — hence highly contagious, person-to-person spread. It invades the colonic mucosa via M cells, causing shallow ulcers and bloody mucoid stool with tenesmus.

Diagnosis: stool microscopy shows pus cells, RBCs, no trophozoites (helps differentiate from amoebic dysentery). Culture on MacConkey/XLD/DCA. Sereny test (keratoconjunctivitis in guinea pig) demonstrates invasiveness (EIEC/Shigella).

Treatment: ciprofloxacin (DOC); ceftriaxone or azithromycin where resistant. Avoid antimotility agents (worsen disease/toxic megacolon).

Feature	Bacillary (Shigella)	Amoebic (E. histolytica)
Onset	Acute, febrile	Gradual, afebrile
Stool	Small volume, mucoid, blood + pus	Large, "anchovy sauce", blood-streaked
Microscopy	Pus cells, no motile forms	RBC-laden trophozoites, few pus cells
Tenesmus	Marked	Present
Toxic dilatation	Possible	Rare

Proteus, Yersinia and others (quick hits)

• Proteus mirabilis/vulgaris — non-lactose fermenter, swarming on blood agar, strongly urease positive → struvite (staghorn) renal stones, alkaline urine. Weil–Felix reaction uses Proteus OX antigens (OX-19, OX-2, OXK) to diagnose rickettsial infections by cross-reaction.
• Yersinia enterocolitica — cold enrichment, motile at 25°C not 37°C; causes mesenteric adenitis / pseudoappendicitis.
• Yersinia pestis — plague (separate topic), bipolar "safety-pin" staining.
• Citrobacter, Enterobacter, Serratia (red pigment, prodigiosin) — nosocomial, drug-resistant.

Resistance mechanisms (frequently asked)

• ESBL (extended-spectrum β-lactamase): hydrolyse 3rd-gen cephalosporins; detected by ceftazidime ± clavulanate double-disc test → treat with carbapenems.
• AmpC β-lactamases — chromosomal, inducible (Enterobacter, Citrobacter, Serratia — "ESCPM/SPACE" group).
• Carbapenemases: KPC, NDM-1, OXA-48 — treat with colistin, tigecycline, ceftazidime-avibactam.

Mnemonic: Inducible AmpC chromosomal β-lactamase organisms = "SPACE" = Serratia, Pseudomonas/Providencia, Acinetobacter/Aeromonas, Citrobacter, Enterobacter.

Recently asked / exam angle

• EHEC O157:H7 = sorbitol non-fermenter on SMAC → HUS; antibiotics contraindicated. Repeatedly tested image/clinical vignette.
• Bone marrow culture = most sensitive for typhoid; blood culture in week 1; Widal week 2. Sequence-matching MCQs.
• Vi antigen → carrier detection + vaccine antigen. "Typhoid Mary" classic.
• IMViC: E. coli ++ – – vs Klebsiella – – + + — direct one-liner.
• Shigella lowest infective dose; S. dysenteriae type 1 produces Shiga toxin.
• NDM-1 first reported from India in Klebsiella — public-health/AMR question.
• Currant jelly sputum + bulging fissure = Klebsiella; rusty sputum = Pneumococcus; red-currant ≠ red.
• XDR typhoid (Pakistan) → azithromycin/carbapenem. Newer addition to question banks.
• Weil–Felix uses Proteus OX strains for rickettsial serology.
• Lactose fermentation on MacConkey to bin an unknown isolate (pink vs pale) — recurring practical-style stem.

Rapid revision

1. Enterobacteriaceae = oxidase negative, ferment glucose, reduce nitrate to nitrite.
2. Pink on MacConkey = E. coli, Klebsiella, Enterobacter, Citrobacter; pale = Salmonella, Shigella, Proteus, Yersinia.
3. E. coli = EMB metallic green sheen, indole +, IMViC ++––.
4. ETEC = traveller's diarrhoea (LT/ST toxins); EPEC = infantile diarrhoea (attaching-effacing).
5. EHEC O157:H7 = sorbitol non-fermenter, Shiga toxin → HUS; do not give antibiotics.
6. Klebsiella = non-motile, capsulate, currant-jelly sputum, IMViC ––++, urease +, NDM-1.
7. Typhoid: blood culture wk1, Widal wk2, stool/urine wk3, bone marrow most sensitive overall.
8. Vi antigen = chronic carrier detection (gallbladder) + vaccine; carrier titre ≥1:160.
9. Typhoid complication = ileal perforation in 3rd week; DOC ceftriaxone/azithromycin (cipro if sensitive).
10. Shigella = lowest infective dose (10–100), acid-resistant; S. dysenteriae 1 makes Shiga toxin.
11. Proteus = swarming, urease +, staghorn stones, Weil–Felix OX antigens.
12. SPACE group = inducible AmpC; ESBL → carbapenem, carbapenemase (KPC/NDM) → colistin/avibactam.