Post on 16-Dec-2015
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MEDICAL MICROBIOLOGY I
Lesson 11 Lesson 11
Enterobacteriaceae Part I
Enterobacteriaceae
The family Enterobacteriaceae is the largest
most heterogeneous collection of medically
important Gram negative bacilli
A total of 32 genera and more than 130 A total of 32 genera and more than 130
species have been described
These genera have been classified based on
biochemical properties, antigenic structure,
and nucleic acid hybridisation and sequencing
Enterobacteriaceae
Enterobacteriaceae are ubiquitous organisms, being found worldwide in soil, water and vegetation, and are part of the normal intestinal flora of most animals, including human
Some organisms (e.g. Salmonella typhi, Shigella Some organisms (e.g. Salmonella typhi, Shigellasp., Yersinia pestis) are always associated with disease
Others (e.g. E. coli, Klebsiella pneumoniae, Proteus mirabilis) are members of the normal commensal flora that can cause opportunistic infections
Enterobacteriaceae
A third group of Enterobacteriaceae exists -those normally commensal organisms that become pathogenic when they acquire virulence factor genes on plasmids, bacteriophages, or pathogenicity islands (e.g. bacteriophages, or pathogenicity islands (e.g. E. coli associated with gastroenteritis)
Infections with the Enterobacteriaceae can originate from an animal reservoir, or through the endogenous organisms in a susceptible patient and can involve virtually all body sites
Enterobacteriaceae
Physiology and Structure
Moderately sized (0.3 - 1.0 x 1.0 - 6.0 m)
Gram negative bacilli
They are either non-motile or motile with
peritrichous flagella and do not form sporesperitrichous flagella and do not form spores
All members can grow rapidly aerobically and
anaerobically (facultative anaerobes) on a
variety of non-selective (e.g. blood agar) and
selective (e.g. MacConkey) media
Physiology and Structure
Simple nutritional requirements - ferment glucose, reduce nitrites, and are catalase-positive and oxidase-negative
The absence of cytochrome oxidase activity is The absence of cytochrome oxidase activity is an important characteristic, because it can be measured rapidly with a simple test and is used to distinguish the Enterobacteriaceae from many other fermentative and non-fermentative Gram negative bacilli
Physiology and Structure
Characteristics of the organisms colonies on
different media have been used to identify
common members of the family
EnterobacteriaceaeEnterobacteriaceae
Ability to ferment lactose: differentiate lactose-
fermenting strains (e.g. Escherichia, Klebsiella,
Enterobacter, Citrobacter and Serratia sp.)
from non-lactose fermenting strains (e.g.
Proteus, Salmonella and Yersinia sp.)
Physiology and Structure
Resistance to bile salt: separate enteric
pathogen (e.g. Shigella, Salmonella) from
commensal organisms inhibited by bile salt
Enterobacteriaceae have prominent capsules, Enterobacteriaceae have prominent capsules,
whereas other strains are surrounded by a
loose-fitting, diffusible slime layer
Physiology and Structure
The heat-stable lipopolysaccharide (LPS) is the
major cell wall antigen and consists of 3
components:components:
The somatic O polysaccharide
A core polysaccharide
Lipid A
Physiology and Structure
The serology classification of the
Enterobacteriaceae is based on 3 antigens:
somatic O polysaccharide
Capsular K antigens Capsular K antigens
Flagella H proteins
Specific O antigens are present in each genus,
although cross-reactions between closely
related genera are common (e.g. Salmonella
with Citrobacter, Escherichia with Shigella)
Physiology and Structure
Different genera both within and outside the family Enterobacteriaceae possess K antigens
e.g. E. coli K1 cross-reacts with Neisseria meningitidis and Haemophilus pneumoniae, meningitidis and Haemophilus pneumoniae, and Klebsiella pneumoniae cross-reacts with Streptococcus pneumoniae
H antigens are heat-labile, flagellar proteins
They may be absent from a cell, or they may undergo antigenic variation and be present in 2 phases
Pathogenesis and Immunity
Virulence factors:
1. Endotoxin
2. Capsule
3. Antigenic phase variation3. Antigenic phase variation
4. Sequestration of growth factors
5. Resistance to serum killing
6. Antimicrobial resistance
Virulence Factor - Endotoxin
A virulence factor shared among all aerobic
and some anaerobic Gram negative bacteria
The activity of this toxin depends on the lipid The activity of this toxin depends on the lipid
A component of lipopolysaccharide, which is
released at cell lysis
Virulence Factor - Endotoxin
Many of the systemic manifestations of Gram
negative bacterial infections are initiated by
endotoxin, including:endotoxin, including:
Activation of complement release of cytokines,
leukocytosis, thrombocytopenia, disseminated
intravascular coagulation, fever, decreased
peripheral circulation, shock, death
Virulence Factor - Capsule
Encapsulated Enterobacteriaceae are protected from phagocytosis by hydrophilic capsular antigens, which repel the hydrophobic phagocytic cell surface
These antigens interfere with the binding of antibodies to the bacteria and are poor immunogens or activators of complement
The protective role of capsule is diminished, however, if the patient develops specific anti-capsular antibodies
Virulence Factor - Antigenic Phase
Variation
The expression of capsular K and flagellar H
antigens is under the genetic control of the
organism
Each of these antigens can be alternately
expressed or not expressed (phase variation),
a feature that protects the bacteria from
antibody-mediated cell death
Virulence Factor - Type III Secretion
Systems
A variety of distinct bacteria (e.g. Yersinia, Salmonella, Shigella, Escherichia, Pseudomonas, Chlamydia) have a common effector system for delivering their virulence genes into targeted delivering their virulence genes into targeted eukaryotic cells
This system consists of approximately 20 proteins that facilitate secretion of bacterial virulence factor into host cells
In the absence of the type III secretion system the bacteria lose their virulence
Virulence factor - Sequestration of
Growth Factors
Nutrients are provided to the organisms in
enriched culture media, but the bacteria must
become nutritional scavengers when growing become nutritional scavengers when growing
in vivo
Iron is an important growth factor required by
bacteria, but it is bound in haeme proteins
(e.g. haemoglobin, myoglobin) or in iron-
chelating proteins (e.g. transferrin, lactoferrin)
Virulence factor - Sequestration of
Growth Factors
The bacteria counteract the binding by
producing their own competitive iron-
chelating compounds (e.g. siderophores chelating compounds (e.g. siderophores
enterobactin and aerobactin)
Virulence Factor - Resistance to Serum
Killing
Virulent organisms capable of producing
systemic infections are frequently resistant to
serum killing
Although the bacterial capsule can protect the
organism from serum killing, other factors
prevent the binding of complement
components to the bacteria and subsequent
complement-mediated clearance
Virulence Factor - Antimicrobial
Resistance
As rapidly as new antibiotics are introduced,
organisms can develop resistance to them
The resistance can be encoded on transferable The resistance can be encoded on transferable
plasmid and exchanged among species,
genera, and even families of bacteria
Escherichia coli
The genus Escherichia consists of 5 species, of which E. coli is the most common and clinically most important
This organism is associated with a variety of This organism is associated with a variety of disease, including sepsis, UTIs, meningitis, and gastroenteritis
Many O, H, and K antigens have been described, and they are used to classify the isolates for epidemiologic purposes
Escherichia coli
Escherichia coli
Pathogenesis and Immunity
Specialised virulence factors: Adhesins
Colonisation factor antigens CFA/I, CFA/II and CFA/III
Aggregative adherence fimbriae AFF/I and AFF/II
Bundle-forming protein (Bfp)
Intimin Intimin
P pili
Ipa protein
Dr fimbriae
Exotoxins Heat-stable toxins Sta and STb
Shiga toxins Stx-1 and Stx-2
Haemolysin HlyA
Heat-labile toxins LT-I and LT-II
Epidemiology
Large numbers of E. coli are present in the GI tract, and the bacteria are common causes of sepsis, neonatal meningitis, infections of the urinary tract, and gastroenteritis
The most common Gram negative bacilli isolated from patients with sepsis
Responsible for causing more than 80% of all community-acquired UTIs as well as hospital-acquired infections
Epidemiology
A prominent cause of gastroenteritis in
developing countries
Most infections (with the exception of
neonatal meningitis and gastroenteritis) are neonatal meningitis and gastroenteritis) are
endogenous; that is, E. coli that are part of the
patients normal flora are able to establish
infection when the patients defenses are
compromised
Clinical Diseases - Septicaemia
Originates from infections in the urinary or GI
tract (e.g. an intra-abdominal infection with
sepsis following intestinal perforation)
The mortality associated with E. coli The mortality associated with E. coli
septicaemia is high for patients in whom
immunity is compromised or the primary
infection is in the abdomen or central nervous
system
Clinical Diseases - Urinary Tract Infection
(UTI)
Originate in the colon, contaminate the urethra,
ascend into the bladder, and may migrate to the
kidney or prostate
Disease is more common with certain specific Disease is more common with certain specific
serogroups
These bacteria are particularly virulent because of
their ability to produce adhesins (primarily P pili,
AAF/I, AAF/II, and Dr.), which bind to cell lining the
bladder and upper UT, and haemolysin HlyA
Clinical Diseases - Neonatal Meningitis
E. coli and group B streptococci cause the
majority of central nervous systemic infections
in infants younger than 1 month
Approximately 75% of the E. coli strains Approximately 75% of the E. coli strains
possess the K1 capsular antigen, which is
commonly present in the GI tract of pregnant
women and newborn infants
Clinical Diseases - Gastroenteritis
Divided into 6 groups:
1. Enterotoxigenic E. coli (ETEC)
2. Enteropathogenic E. coli (EPEC)
3. Enteroinvasive E. coli (EIEC)3. Enteroinvasive E. coli (EIEC)
4. Enterohaemorrhagic E. coli (EHEC)
5. Enteroaggregative E. coli (EAEC)
6. Diffusely adherent E. coli (DAEC)
Clinical Diseases - Gastroenteritis1. ETEC
Most commonly in developing countries, most common cause of Travellers diarrhoea
The inoculum for disease is high, so infections are primarily acquired through consumption of faecally contaminated food or waterprimarily acquired through consumption of faecally contaminated food or water
Produce 2 classes of enterotoxins: heat-labile toxins (LT-I, LT-II) and heat-stable toxins (STa and STb
LT-I is structurally similar to cholera toxin
This toxin consists of one A subunit and 5 identical B subunits
Clinical Diseases - Gastroenteritis
2. EPEC
Major cause of infant diarrhoea in impoverished countries, rare in older children and adults, presumably because they have developed protective immunitydeveloped protective immunity
Disease is characterised by bacterial attachment to epithelial cells of the small intestine with subsequent effacement (destruction) of the microvilli, which result in diarrhoea
Clinical Diseases - Gastroenteritis
Initially a loose attachment mediated by
bundle-forming pili (Bfp) occurs, followed by
active secretion of proteins by the bacterial
type III secretion system into host epithelial type III secretion system into host epithelial
cell
Translocated intimin receptor (Tir) is inserted
into the epithelial cell membrane and
functions as a receptor for an outer membrane
bacterial adhesin, intimin
Clinical Diseases - Gastroenteritis
3. EIEC
Closely related by phenotypic and pathogenic properties to Shigella
Produce disease similar to shigellosis
The bacteria are able to invade and destroy the The bacteria are able to invade and destroy the colonic epithelium, producing a disease characterised initially by watery diarrhoea
A series of bacterial genes carried on a plasmid mediate invasion (plnv genes) into the colonic epithelium
Clinical Diseases - Gastroenteritis
The bacteria then lyse the phagocytic vacuole
and replicate in the cell cytoplasm
Movement within the cytoplasm and into
adjacent epithelial cells is regulated by adjacent epithelial cells is regulated by
formation of actin tails
This process of epithelial cell destruction with
inflammatory infiltration can progress to
colonic ulceration
Clinical Diseases - Gastroenteritis
4. EHEC
The most common strains producing disease in
developed countries
The ingestion of fewer than 100 bacilli can The ingestion of fewer than 100 bacilli can
produce disease
Symptoms: uncomplicated diarrhoea to
haemorrhagic colitis with severe abdominal
pain, bloody diarrhoea, and little or no fever
Clinical Diseases - Gastroenteritis
Haemolytic uremic syndrome (HUS)
Characterised by acute renal failure,
thrombocytopenia, and microangiopathic
haemolytic anaemiahaemolytic anaemia
Most cases attributed to the consumption of
undercooked ground beef or other meat
products, water, unpasteurised milk or fruit
juices, uncooked vegetables, and fruits
Clinical Diseases - Gastroenteritis
Express a Shiga-like toxin, induce lesions on
epithelial cells, and possess a 60 MDa plasmid
that carries genes for other virulence factors
The Stx toxins also stimulate expression of The Stx toxins also stimulate expression of
inflammatory cytokines (e.g. TNF-, IL-6)
Death can occur in 3 - 5% of patients with HUS,
and severe sequelae (e.g. renal impairment,
hypertension, CNS manifestation) can occur in
as many as 30% of patients
Clinical Diseases - Gastroenteritis
5. EAEC
Implicated as a cause of persistent, watery diarrhoea with dehydration in infants in developing countries
The bacteria is characterised by their auto-agglutination in a stacked brick arrangement -mediated by bundle - forming fimbriae
Stimulate secretion of mucus, shortening of the microvilli, mononuclear infiltration, and haemorrhage are observed
Clinical Diseases - Gastroenteritis
6. DAEC
Adherence characteristic to cultured cells
Stimulate elongation of the microvilli with the
bacteria embedded in the cell membranebacteria embedded in the cell membrane
The resulting disease is watery diarrhoea
found primarily in infants between 1 - 5 years
of age