Enteric Gram Negative Bacilli: Enterobacterales

Enterobacterales are a large family of Gram negative bacilli, facultative anaerobes, oxidase negative, the key test distinguishing this family from the nonfermenting Gram negative bacilli, which are typically oxidase positive, most possessing peritrichous flagella when motile.

Most members are normal gastrointestinal flora, causing disease predominantly when translocated to a normally sterile site, urinary tract, bloodstream, peritoneum, wound, rather than through primary invasive pathogenicity, in contrast to the true enteric pathogens, which cause primary intestinal disease.

Lactose fermentation is a key initial differentiating test, historically performed on MacConkey agar, selective for Gram negatives, differential for lactose fermentation, pink colonies indicating a fermenter, colourless indicating a nonfermenter.

Serratia species are variable or late lactose fermenters, a point that itself can be diagnostically useful.

Morphology. Gram negative bacilli, typically motile, peritrichous flagella.

Key biochemical features. Lactose fermenting, indole positive, a classic distinguishing test within the Enterobacterales, urease negative.

Reservoir. Normal colonic flora, the dominant facultative anaerobic organism in the human gut.

Disease patterns. The single most common cause of urinary tract infection overall, both community and healthcare-associated. A leading cause of Gram negative bacteraemia and sepsis, frequently originating from a urinary or intra-abdominal source. Neonatal meningitis, particularly strains bearing the K1 capsular antigen. Intra-abdominal infection, typically polymicrobial, reflecting its GI origin. Specific diarrhoeagenic pathotypes, distinct from the normal commensal strain, acquired exogenously, cause primary intestinal disease.

Diarrhoeagenic E. coli pathotypes. Enterotoxigenic E. coli, ETEC, produces heat-labile and/or heat-stable enterotoxins, the classic cause of traveller's diarrhoea, a watery, noninflammatory diarrhoeal illness.

Enterohaemorrhagic E. coli, EHEC, notably serotype O157:H7, produces Shiga toxin, also called verotoxin, causes bloody diarrhoea, haemorrhagic colitis, and, critically, haemolytic uraemic syndrome, the triad of microangiopathic haemolytic anaemia, thrombocytopenia, and acute kidney injury, particularly in children. Transmitted via undercooked ground beef, unpasteurised milk or juice, and contaminated produce.

Antibiotic treatment is generally avoided in suspected EHEC infection, since antibiotic-induced bacterial cell death can increase toxin release and the risk of haemolytic uraemic syndrome, an important management point.

Enteropathogenic E. coli, EPEC, causes infantile diarrhoea in developing settings via attachment and effacement of intestinal microvilli, without toxin production in the classic sense. Enteroinvasive E. coli, EIEC, invades colonic epithelium, producing a Shigella-like dysenteric illness. Enteroaggregative E. coli, EAEC, causes persistent watery diarrhoea via aggregative adherence to intestinal mucosa, increasingly recognised in both traveller's diarrhoea and paediatric chronic diarrhoea.

Virulence factors, extraintestinal disease. Type 1 pili and P pili mediate uroepithelial adhesion, the key virulence determinant in urinary tract infection. K1 capsule is associated with neonatal meningitis. Haemolysin and siderophores, iron acquisition, are also important.

Diagnosis. Routine culture, urine, blood, wound, stool as appropriate. Specific pathotype identification for diarrhoeagenic strains requires toxin detection or PCR rather than standard culture, since these pathotypes are not distinguishable from normal commensal E. coli by routine biochemical testing alone.

Resistance patterns. Increasing prevalence of extended-spectrum beta-lactamase, ESBL, production and, less commonly but of increasing concern, carbapenemase production.

Morphology. Gram negative bacilli, characteristically nonmotile, a distinguishing feature from most other Enterobacterales, with a prominent polysaccharide capsule producing markedly mucoid colonies on culture.

Key biochemical features. Lactose fermenting, urease positive, indole negative.

Reservoir. Gastrointestinal flora, in smaller numbers than E. coli in health, also colonises the oropharynx and skin, particularly in hospitalised patients.

Virulence factors. Prominent antiphagocytic capsule is the dominant virulence factor, producing the organism's characteristic mucoid colony appearance.

Disease patterns. Urinary tract infection and healthcare-associated bloodstream infection, similar in general pattern to E. coli.

Classically associated with community-acquired pneumonia in patients with alcohol use disorder or diabetes, producing a distinctive severe, necrotising pneumonia with currant jelly, thick, blood-tinged, mucoid, sputum.

This classic presentation is now a less common cause of community-acquired pneumonia overall than in historical teaching, with healthcare-associated infection representing the more common contemporary clinical scenario.

Diagnosis. Culture. The mucoid, nonmotile colony appearance is a useful preliminary clue.

Resistance patterns. A particularly important reservoir for multidrug resistance in contemporary practice, including ESBL production and, of major global concern, carbapenem-resistant Klebsiella pneumoniae via carbapenemase production, for example KPC enzymes, representing one of the most clinically significant antimicrobial resistance threats in modern healthcare.

These related genera are grouped together here, each causing a broadly similar pattern of opportunistic, healthcare-associated infection, urinary tract infection, bacteraemia, wound infection, pneumonia in hospitalised or ventilated patients, with specific distinguishing biochemical or clinical features worth noting individually.

Enterobacter species are lactose fermenting, motile, and clinically notable for inducible AmpC beta-lactamase production, capable of emerging during treatment with certain cephalosporins even when the initial isolate appears susceptible, a specific and clinically important resistance mechanism.

Serratia species, particularly Serratia marcescens, show variable or late lactose fermentation, are motile, and some strains produce a distinctive red pigment, prodigiosin, on culture at room temperature, a classic identifying feature. They are associated with healthcare-associated outbreaks, particularly linked to contaminated equipment, solutions, or hand hygiene lapses, given the organism's environmental persistence, and are also inducible AmpC beta-lactamase producers, similar in principle to Enterobacter.

Proteus species, particularly Proteus mirabilis, are lactose nonfermenting, highly motile, producing a characteristic swarming motility pattern on solid agar, a spreading, wave-like colony growth rather than discrete colonies, a classic and easily recognised laboratory feature.

They are urease positive, hydrolysing urea to ammonia, alkalinising the urine, a mechanism directly linked to struvite, staghorn, kidney stone formation, since the alkaline environment promotes precipitation of magnesium ammonium phosphate crystals. Proteus is a common cause of urinary tract infection, particularly in patients with structural urinary tract abnormalities or long-term catheterisation.

Morganella morganii is lactose nonfermenting, motile, urease positive, though generally producing a less pronounced alkalinising effect than Proteus. It is a less common but recognised cause of healthcare-associated urinary tract infection, bacteraemia, and wound infection, often in the context of prior antibiotic exposure or underlying comorbidity.

Providencia species are lactose nonfermenting, motile, urease variable, and are associated particularly with catheter-associated urinary tract infection in long-term catheterised patients, and, like Proteus and Morganella, are capable of contributing to struvite stone formation when urease-positive strains are involved.