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The Enterobacteriaceae are a large family (>20 genera and 100
species) of facultatively anaerobic Gram-negative bacilli that are
easily cultured, reduce nitrate and ferment glucose. The wide
diversity of named species is in part because they are easy to grow
and study in the laboratory, but also because they are capable of
causing a wide variety of clinical syndromes.
Habitat and transmission
The Enterobacteriaceae are almost ubiquitous organisms. They
are found in:
- the normal flora of animals and humans (forming a major component);
Transmission is both from other animals or humans and from
the inanimate environment. Many infections arise from the body's
normal flora when opportunities are provided by medical, surgical
or other therapy (see Innate immunity and normal flora
Many species produce extracellular capsular polysaccharides (e.g.
, Escherichia coli
and Salmonella typhi
). S. typhi
possesses a capsule or Vi (virulence) antigen and a vaccine containing
the Vi antigen is protective against typhoid. E. coli
K1 is the
most common type of E. coli
isolated from patients with neonatal
meningitis and septicaemia. There are biochemical and structural
similarities between E. coli
K1 and N. meningitidis
group B and
human central nervous system antigens that may give these Pathogens
The lipopolysaccharide (LPS) molecule consists of a central lipid
A and oligosaccharide core, and a long straight or branched
polysaccharide 'O' antigen. It is located in the bacterial outer
membrane and protects the organism against the bactericidal
activity of complement. Lipid A stimulates host macrophages to
produce cytokines, such as interleukin-1 and tumour necrosis
factor (TNF), which mediate the fever, shock and metabolic acidosis
associated with severe sepsis. Some clinical syndromes are
associated with particular O antigens: for example, E. coli
may produce verotoxin, which causes haemolytic uraemic syndrome
(HUS); other O types are associated with urinary tract
infection or diarrhoea. However, these are merely temporal relationships
between a variety of bacterial characteristics that include
an O antigen and a particular virulence determinant.
express a potent urease. In the urinary tract, urea
lowers the pH, therefore infection with Proteus spp.
and phosphates to precipitate, with the formation of renal stones
(see Urinary and genital infections
or pili are bacterial organelles that promote colonization,
for example in the ureter. E. coli
that express mannose-
binding fimbriae are associated with lower urinary tract infections
and cystitis, whereas those that express P fimbriae are associated
with pyelonephritis and septicaemia. In the intestine, E. coli
express different fimbriae (colonization factor antigens, [CFAs])
have been associated with diarrhoea.
Enterotoxigenic E. coli
Enterotoxigenic E. coli
(ETEC) produce LT and ST toxins that
act on the enterocyte to stimulate fluid secretion, resulting in diarrhoea.
LT toxin, which is heat labile, shares 70% homology with
cholera toxin and, like cholera toxin, increases local cyclic adenosine
monophosphate (cAMP) in the enteric cell. ST toxin is heat
stable and stimulates cyclic guanyl monophosphate (cGMP). E.
coli that possess these enterotoxins are associated with travellers'
diarrhoea, which is a short-lived, watery diarrhoeal disease.
Enteroaggregative E. coli
Some strains of E. coli
, which are known as enteroaggregative E.
coli (EAggEC), secrete plasmid-encoded toxin, a serine protease
that binds α-fodrin and causes disruption of the actin cytoskeleton,
and are able to cause chronic diarrhoea. Strains express an
ST-like toxin or a haemolysin-like toxin.
Enteropathogenic E. coli
Enteropathogenic E. coli
(EPEC) cause disease by colonizing the
epithelial lining of the small intestine and injecting effector proteins
that cause effacement of microvilli and intimate adherence.
Isolates with this characteristic were the first E. coli
primary pathogens, when they caused outbreaks of diarrhoea in
Enterohaemorrhagic E. coli
The enterohaemorrhagic E. coli
(EHEC) strains produce a verotoxin
named because of its in vitro activity on 'vero' cells. The
haemorrhagic diarrhoea that they cause can be complicated by
haemolysis and acute renal failure (HUS). This organism is
commensal in cattle and is transmitted to humans through
hygiene failure in abattoirs and food production. A similar toxin
(Shiga toxin) is a major virulence determinant in Shigella
The Enterobacteriaceae can gain DNA rapidly from other organisms
through transposons, integrons or plasmids. This enables
antibiotic-resistance genes to spread from one species to another.
In the hospital environment the survival of antibiotic-resistant
strains is favoured. In some hospitals there have been outbreaks
of multidrug-resistant Klebsiella pneumoniae in intensive care
units. The Enterobacteriaceae have also been able to gain Pathogenicity
determinants by genetic exchange. Acquisition of a series
of connected genes can occur and these are known as Pathogenicity
islands. In this way Salmonella
have gained a series of genes that
enable them to invade intestinal cells.