15: Enterobacteria

Chapter 15 Enterobacteria

Most of the commensal Gram-negative rods that inhabit the normal gastrointestinal tract, and sometimes cause disease, belong to the family Enterobacteriaceae. All species belonging to this family are Gram-negative, facultative anaerobes that ferment glucose. The major medically important species are listed in Table 15.1.

Table 15.1 Enterobacteria commonly causing human disease

Genus	Representative species (no. of species)	Disease
Escherichia	E. coli (5)	Gastroenteritis, wound and urinary tract infection
Shigella	S. dysenteriae	Dysentery
	S. flexneri
	S. boydii
	S. sonnei
Salmonella	S. typhi	Enteric fever (typhoid)
	S. typhimurium (7 subgroups)	Food poisoning
Klebsiella	K. pneumoniae (7)
Morganella	M. morganii (2)	Urinary tract infection and other types of sepsis
Proteus	P. mirabilis (4)
Providencia	P. stuartii (5)
Yersinia	Y. pestis (11)	Plague, septicaemia, enteritis, etc.
Citrobacter	C. freundii (4)	Low pathogenicity, opportunistic infections
Enterobacter	E. cloacae (13)
Serratia	S. marcescens (10)

General characteristics of enterobacteria

Habitat

Found in the human gut, at a density of approximately 10⁹ cells per gram of faeces. However, the predominant species in the gut is Bacteroides. Up to 15% of the population may harbour enterobacteria in the oral cavity, mostly as transient commensals. Their oral carriage rate may increase in old age, and in states leading to reduced salivary flow (xerostomia).

Characteristics

Rapidly growing cells 2 × 0.4 µm in size; may appear coccobacillary. Many species are motile and possess a capsule, especially on initial isolation. All species are endotoxigenic because of the lipopolysaccharide outer cell wall. They also possess pili and flagella, which mediate adhesion and locomotion, respectively (Fig. 15.1).

Fig. 15.1 A scanning electron micrograph of Escherichia coli showing fimbriae and flagella (×10 000).

Culture and identification

Grow well on ordinary media (e.g. blood agar, MacConkey’s agar), producing characteristic circular, convex and glistening/mucoid colonies. Some motile species form swarming patterns on agar cultures. Most species are non-pigmented; a few produce red, pink, yellow or blue pigments.

Enterobacteriaceae ferment a large number of carbohydrates. This property, together with other biochemical tests, is used to identify and differentiate species.

Lactose fermentation

Growth on indicator media is used for the initial categorization of Enterobacteriaceae into two groups: lactose fermenters and lactose non-fermenters. Several selective media, such as MacConkey’s and cystine-lactose-electrolyte-deficient (CLED) media, are available for this purpose. On MacConkey’s agar, the lactose fermenters appear as pink colonies, while on CLED medium, the colour of lactose fermentation is yellow.

Other biochemical tests

Commercially available kit systems are routinely used to identify species of enterobacteria. The commonly available test systems are based on 10 (API 10E) or 20 (API 20E, Rapid E) biochemical tests (Fig. 15.2).

Fig. 15.2 Commercial identification kit for Enterobacteriaceae. This plate illustrates a colour reaction profile obtained after overnight incubation of the organism. The identity of the organism is Klebsiella pneumoniae.

Serological tests

These are based on the antigens of the organisms. All species have the somatic (O) antigen, and most have the flagellar (H) antigen. The capsular (K) antigen is seen in some species. The antigens are useful in the classification of species and invaluable for epidemiological investigation of outbreaks of disease. Identification of strains within a species can also be done by bacteriophage typing, bacteriocin typing, plasmid analysis and polypeptide analysis.

Pathogenicity

All Enterobacteriaceae are potentially pathogenic. Patients who are immunosuppressed, undergoing mechanical or medical manipulation, and have underlying disease are most susceptible to infection.

Endotoxin shock

This can be precipitated in humans by the lipopolysaccharide, which all Enterobacteriaceae release when they are destroyed. Toxic lipopolysaccharide comprises lipid A, the core polysaccharide and the O antigen; the lipid A is responsible for most of the symptoms associated with endotoxic shock. The toxic effects of lipopolysaccharide are many and include fever, hypotension, intravascular coagulation and effects on the immune system. Large doses of endotoxin may cause death.

Treatment

The antibiotic sensitivity patterns of enterobacteria are complex as they readily acquire resistance-coding plasmids. A spectrum of antibiotics are used, including ampicillin/amoxicillin, cephalosporins, aminoglycosides, trimethoprim, chloramphenicol and ciprofloxacin.

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