Enterobacteriaceae
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Direct answer
Enterobacteriaceae are gram-negative, non-sporing bacilli that are facultative anaerobes, grow on ordinary and MacConkey media, ferment glucose, reduce nitrates, and are catalase positive but oxidase negative — the oxidase test separates them from vibrios and pseudomonads. Important genera include Escherichia, Klebsiella, Proteus, Enterobacter, Citrobacter, Salmonella, Shigella and Yersinia, causing urinary infection, diarrhoea, enteric fever, pneumonia, septicaemia and hospital-acquired infection. They carry O, H and K antigens used in classification and are identified biochemically by tests such as the IMViC series.
What you must remember
- Family features: gram-negative rods, non-sporing, motile except shigellae and Klebsiella, facultative anaerobes growing well on MacConkey agar, fermenting glucose and reducing nitrates to nitrites.
- Antigens: the O somatic antigen of lipopolysaccharide, the H flagellar antigen of motile strains, the K capsular antigen, and the Vi antigen of Salmonella Typhi — the basis of the Kauffmann–White scheme.
- Lactose fermentation: on MacConkey agar, rapid fermenters such as Escherichia coli and Klebsiella form pink colonies, while non-lactose fermenters such as salmonellae, shigellae and Proteus form colourless colonies; Shigella sonnei is the classic late lactose fermenter.
- IMViC: indole, methyl red, Voges–Proskauer and citrate — Escherichia coli is positive, positive, negative, negative, and Klebsiella the reverse.
- Escherichia coli: metallic sheen on eosin methylene blue agar; the commonest urinary pathogen; the diarrhoeagenic strains include enterotoxigenic, enteropathogenic, enteroinvasive, enterohaemorrhagic and enteroaggregative types, the haemorrhagic type causing haemolytic uraemic syndrome.
- Klebsiella and Proteus: Klebsiella pneumoniae is heavily capsulated with mucoid colonies and causes pneumonia with currant-jelly sputum; Proteus swarms on blood agar, is urease positive and causes urinary stones.
- Resistance: extended-spectrum beta-lactamase production is widespread, making carbapenems the usual choice for severe infection, with colistin reserved for carbapenem-resistant strains — always treat by sensitivity report.
Common confusion
The family is confused with the non-fermenters: Enterobacteriaceae are oxidase negative and ferment glucose, whereas Pseudomonas is oxidase positive and non-fermentative — a distinction made before reporting an isolate. Within the family, pink versus colourless MacConkey colonies are mixed up, and Proteus swarming is dismissed as contamination. Keep Shigella and Salmonella separate: shigellae are non-motile and produce no gas from glucose; salmonellae are motile and usually gas-producing.
Exam-focused takeaway
In theory, give the family features, the important genera, the antigens and the biochemical scheme from MacConkey through IMViC and triple sugar iron agar. In viva, expect the oxidase test's role, the IMViC of Escherichia coli, and why it is the commonest urinary pathogen. At the practical, a colourless MacConkey colony worked through indole, citrate, urease and motility places the organism in a genus.
Frequently asked questions
What defines the family Enterobacteriaceae?
Gram-negative, non-sporing, facultatively anaerobic bacilli that ferment glucose, reduce nitrates, are catalase positive and oxidase negative, and grow on ordinary media including MacConkey agar.
How does MacConkey agar help in this family?
It separates pink, rapid lactose fermenters such as Escherichia coli and Klebsiella from colourless non-lactose fermenters such as Salmonella, Shigella and Proteus.
What is the IMViC reaction of Escherichia coli?
Indole positive, methyl red positive, Voges–Proskauer negative, citrate negative — the reverse pattern belongs to Klebsiella.
Which antigens classify these bacteria?
The O somatic antigen of the cell wall, the H flagellar antigen, the K capsular antigen and, in Salmonella Typhi, the Vi surface antigen.
Why is Escherichia coli clinically important?
It is the leading cause of urinary tract infection, a common cause of neonatal meningitis and septicaemia, and its strains cause traveller's diarrhoea and haemorrhagic colitis with haemolytic uraemic syndrome.
What is an extended-spectrum beta-lactamase producer?
A strain hydrolysing third-generation cephalosporins, treated with carbapenems; infection control limits its hospital spread.