Q1 LONG ANSWER 10M ★ High-yield

Describe the general characters of the Enterobacteriaceae. Describe Escherichia coli with its diarrhoeagenic types and laboratory diagnosis.

Microbiology · 2nd Professional — Para-Clinical

General Characters of the Enterobacteriaceae

Gram-negative, non-sporing bacilli, motile by peritrichous flagella or non-motile; facultative anaerobes growing readily on ordinary media
All ferment glucose, all reduce nitrate to nitrite, all are oxidase negative and catalase positive
The oxidase test is the crucial first discrimination — a gram-negative bacillus that is oxidase positive is not an enterobacterium, and is more likely Pseudomonas, Vibrio, Aeromonas or Campylobacter

Antigen	Nature	Significance
O (somatic)	The polysaccharide side chain of lipopolysaccharide; heat-stable	Basis of serogrouping; the O antibody is IgM and appears early in infection
H (flagellar)	Flagellin protein; heat-labile	Absent in non-motile organisms (Klebsiella, Shigella); H antibody is IgG, persists long, and rises after vaccination
K (capsular)	Polysaccharide capsule	Antiphagocytic; may mask the O antigen and prevent O agglutination until the organism is heated. The Vi antigen of S. Typhi is a K antigen, and the K1 capsule of E. Coli causes neonatal meningitis
Fimbrial	Protein	Adhesion

Group	Lactose on MacConkey	Members
Lactose fermenters	Pink colonies	Escherichia, klebsiella, Enterobacter, Citrobacter (late)
Non-lactose fermenters	Pale, colourless colonies	Salmonella, shigella, proteus, Yersinia, Serratia

CLINICAL PEARL

The pathogens are, with useful regularity, the non-lactose fermenters. Salmonella, Shigella, Yersinia and Proteus all form pale colonies on MacConkey agar, while the commensal coliforms turn pink. This is why the plate is inspected for colourless colonies in a stool culture — a single cheap medium that is simultaneously selective (bile salts suppress gram-positives) and differential, and which directs the whole subsequent investigation.

Escherichia Coli — General Features

The predominant facultative anaerobe of the normal large bowel, and therefore an indicator OF faecal contamination of water and food
IMViC reactions ++–– — indole positive, methyl red positive, Voges-Proskauer negative, citrate negative; this distinguishes it from Klebsiella, which is ––++
Ferments lactose with acid and gas; on EMB agar produces a characteristic green metallic sheen
Extra-intestinal disease — the commonest cause of urinary tract infection (uropathogenic strains adhering by P fimbriae); neonatal meningitis (K1 capsule); intra-abdominal sepsis and peritonitis; wound infection; and gram-negative bacteraemia

Diarrhoeagenic Escherichia Coli

Type	Mechanism	Clinical picture
ETEC — enterotoxigenic	Heat-labile toxin (LT), which acts like cholera toxin through cAMP, and heat-stable toxin (ST) acting through cGMP; non-invasive	Traveller'S diarrhoea and infantile diarrhoea in developing countries; profuse watery stool without blood or fever
EPEC — enteropathogenic	Attaching and effacing lesions destroying microvilli; NO toxin	Outbreaks of infantile diarrhoea in nurseries; may be prolonged
EIEC — enteroinvasive	Invades colonic epithelium exactly as Shigella does; positive SERENY test	Dysentery — fever, blood and mucus in the stool
EHEC / STEC — enterohaemorrhagic	SHIGA-like (vero) cytotoxin, phage-encoded; the classical serotype is O157:H7, which does not ferment sorbitol — the basis of its selective isolation	Haemorrhagic colitis with bloody stool but characteristically little fever; complicated by haemolytic uraemic syndrome in children. From undercooked beef, unpasteurised milk, contaminated produce
EAEC — enteroaggregative	"stacked-brick" adherence to epithelium with biofilm	Persistent diarrhoea in children and in HIV

Antibiotics are contraindicated IN EHEC — they increase Shiga toxin release by inducing the prophage, and raise the risk of haemolytic uraemic syndrome. Antimotility drugs are likewise avoided

Laboratory Diagnosis and Treatment

Urine — clean-catch midstream sample; significant bacteriuria is conventionally 105 colony-forming units per mL, though lower counts matter in a symptomatic young woman or a suprapubic specimen
Stool — culture on MacConkey and selective media; serotyping and toxin or gene detection are needed to identify a diarrhoeagenic type, since ordinary culture cannot distinguish them from commensal E. Coli
Blood, pus and cerebrospinal fluid as appropriate; identification by IMViC, urease, and automated systems or MALDI-TOF
Treatment — guided by sensitivity, since resistance is now extensive; nitrofurantoin or fosfomycin for uncomplicated cystitis; aminoglycosides, carbapenems or beta-lactam with inhibitor combinations for serious infection. ESBL and carbapenemase production are widespread in India
Most diarrhoea needs only oral rehydration, with zinc in children; antibiotics are reserved for dysentery, severe illness and specific organisms

Biochemical Identification of the Enterobacteriaceae

Test	What it shows	Discriminating value
IMViC	Indole, Methyl red, Voges-Proskauer, Citrate	E. Coli is ++––; klebsiella is ––++ — the classical opposition
Urease	Splits urea to ammonia	Strongly positive in proteus, Morganella and Providencia; negative in E. Coli, Salmonella and Shigella
H2S production	Black colonies on TSI or Wilson-Blair	Positive in Salmonella (except a trace in S. Typhi) and Proteus; negative in Shigella
Motility	Hanging drop or semisolid agar	Salmonella is motile, shigella is non-motile — a key distinction between two non-lactose fermenters
Gas from glucose	Durham's tube	S. Typhi produces acid but NO gas; other salmonellae produce both
Triple sugar iron (TSI)	Glucose, lactose and sucrose fermentation with H2S and gas	A single slope giving several answers at once; the standard screening medium
Lysine decarboxylase, ONPG	Further separation	Distinguishes Salmonella from Citrobacter

Applied Aspects

E. Coli in water indicates faecal contamination, and coliform counts are the standard of water safety; this remains a central public health measurement in India
Do not give antibiotics for bloody diarrhoea until EHEC is excluded in a child with an appropriate history, since they worsen the outcome
Uncomplicated cystitis in a young woman needs no culture in most settings; culture is reserved for treatment failure, pregnancy, men, children and recurrent or complicated infection
Asymptomatic bacteriuria is treated only in pregnancy and before urological instrumentation; treating it otherwise selects resistance without benefit
Hand hygiene, safe water and sanitation prevent far more E. Coli disease than any antibiotic, and the burden of childhood diarrhoea in India reflects this directly
Take a travel and food history in acute diarrhoea; the pattern of illness, incubation and the presence of blood narrow the likely organism before any laboratory result
Watery diarrhoea points to a toxin acting on the small bowel (ETEC, cholera), whereas blood and pus point to invasion of the colon (Shigella, EIEC, Campylobacter) — a distinction that guides both investigation and whether antibiotics are needed at all
Suspect haemolytic uraemic syndrome in a child with bloody diarrhoea who becomes pale or oliguric, and check the platelet count, film and creatinine rather than simply treating the diarrhoea
Serotyping is needed to identify diarrhoeagenic E. Coli, so routine stool culture reporting "E. Coli grown" from a diarrhoeal specimen means nothing — it is normal flora
Neonatal meningitis caused by E. Coli carries the K1 capsule, and the organism should be sought in blood and cerebrospinal fluid in any septic neonate
Zinc for 14 days with oral rehydration is standard in childhood diarrhoea in India, reducing both the current episode and further episodes over the following months
Give ORS by spoon or cup, not by bottle, and continue after every loose stool; the commonest reason oral rehydration "fails" is that too little is given, too slowly
Do not use antibiotics for watery diarrhoea in the absence of cholera, severe illness or a specific indication; the great majority are self-limiting and antibiotics select resistance and prolong carriage
Recurrent urinary infection in a child warrants imaging for reflux or structural abnormality, since repeated pyelonephritis causes permanent scarring

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