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ENTEROBACTERIACEAE
ENTEROBACTERIACEAE
Objectives:
• Definitions
• Scientific Classification
• Common features
• Growth on differential media distinguishing
properties
• Strains specific virulence factors , diseases and
common treatment
ENTEROBACTERIACEAE
• Definition: A family of aerobic and anaerobic
Gram-negative bacteria that includes, both normal
and pathogenic enteric microorganisms they are
able to reduce nitrates to nitrites and to ferment
glucose with production of acid and gas
ie:Salmonella, Escherichia coli, Yersinia pestis,
Klebsiella and Shigella.
Identification features
Antigenic features (Serological):
They have complex antigenic structure grouping
into three antigens:(O somatic,H flagellar and K
capsule)
o O antigens: The most external part of the cell
wall lipopolysaccharides, O antingens are
resistant to heat and alcohol.
o H antigens: They are located on flagella they are
denatured or removed by heat or alcohol
o Capsular antigens: They are external to O
antigens but not all enterobacteriaceae some are
polysaccharides others are proteins in nature
Scientific Classification
•
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Domain: Bacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order:Enterobacteriales
Family: Enterobacteriaceae
Genera:
Identification features cont..
 Structure:
a. Are Gram-negative rods with lipopolysaccharides in
their cell envelope.
Most have flagella and motile except shigella
Some have capsules
Biochemical and growth properties:
All ferment glucose and are oxidase negative
Lactose fermenting genera are benign, non lactose
fermenting ones are more pathogenic
c. All are facultative anaerobes survive in GI tract
d. They grow well on MacConkey medium
b.
c.

a.
b.
Virulence Factors
Virulence determinants:
a. Endotoxin: Part of lipopolysaccharides are
produced by all Enterobacteriaceae responsible
for systemic infection establishment.
b. Exotoxins: Produced by some species leading to
diarrhoea
c. Adhesins and fimbriae: Found on some species
help in adhesion to the colon,bladder or on
other tissues.
d. Intracellular growth and enteroinvasive:
Protects them from host defenses i.e:
Shigella,Yersinia and E.coli respectively
Virulence Factors cont…
e. Antibiotic resistance: Develops rapidly
f. Capsule: On some spp. Which is antiphagocytic
ie:Klebsiella spp. and Salmonella spp.
Epidemiology
• The Enterobacteriaceae have Worldwide
distribution pattern found in the soil , water and
vegetation they are part of intestinal flora of
many animals and humans.
o Human pathogens: The invasive and that
associated with diseases in man are: Salmonella ,
Shigella and Yersinia.
o Opportunistics:
E.coli,Klebsiella,Enterobacter,Proteus generally
they are not invasasive and part of normal floraDisease is by compromised immunity or
introduced into sterile site.
Lactose-Fermenting Enterobacteriaceae
Escherichia coli (Normal flora of lower human
GIT) found in normal stool form pink colonies on
MacConkey medium
Pathogenesis:
a. Endotoxin: Caused by all strains
b. Noncytotoxic enterotoxins: Caused by
enterotoxigenic strains(ETEC)
 Heat labile enterotoxins leading to altered
electrolyte transport and diarrhoea.
 Heat stabile enterotoxins leading to fluid loss and
diarrhoea
Lactose-Fermenting Enterobacteriaceae
cont…
c. Verotoxin: a shiga like toxin –cytotoxic to
intestinal villi and colon epithelial cells
enterohemorrhagic strains.(EHEC)
d. Invasion and destruction of colonic epithelial
cells: Enteroinvasive strains(EIEC)
e. Adherence to mucosa of small intestine:(Plasmid
mediated): Enteropathogenic strains and
enteroaggregative strains.(EAggEC)
f. P fimbriae-mediated adherence to
uroepithelium: Uro-pathogenic strains(UPEC)
Diseases caused by E.coli
a. Gastroenteritis: Clinical manifestations are strain
specific.
E.coli strain
Site of
Action
Clinical Features
Pathogenesis
Enterotoxigenic
(ETEC)
Small
intestine
Waterly diarrhoea,cramp,
nausea,low grade fever
travelers& infants
Enterotoxins promote
cAMP hence electrolyte
loss
Enteroaggregative
(EAggEC)
Small
intestine
Persistent infant diarrhea,low
grade fever
Aggregative adherence to
mucosa prevents fluid
absorption.
Enteropathogenic
(EPEC)
Small
intestine
Copious watery infant
diarrhoea,vomiting mucoid
stool
Adherence and
destruction of epithelial
cell
Enteroinvasive
(EIEC)
Large
intestine
Fever,Cramps,watery
diarrhoea,dysentery
Invasion and destruction
of epithelial cells lining
the colon
Enterohemorrhagic
(EHEC)
Large
intestine
Severe Cramps,Blood
diarrhoea(hemorrhagic
Cytotoxic verotoxin
inhibits protein synthesis
Diseases caused by E.coli cont….
b. Hemolytic uremic syndrome(HUS):
Complication of gastroenteritis caused by EHEC which
occurs primarily in children associated with
undercooked beef products leads to acute renal
failure,thrombocytopenia
c. Urinary tract infection(UTI):
Cystitis and pyelonephritis are common conditions
d. Neonatal meningitis:
Associated with strains that have capsular antigen
e. Septicemia: The organisms enter circulation from GIT
or Urinary tract-Common cause of gram –ve
septicemia
Transmission
 Endogenous transmission(originating from within ):
• This originates from within the source ie: normal flora
from the bowel
 Nosocomial transmission: Transmission from
infected colleague ie: Hospital
 Ingestion of contaminated food or water
Treatment:
Antibiotic choice depends on the site of infection and
susceptibility of organism
Klebsiella pneumoniae
Pathogenesis:
Has prominent capsule that hinders phagocytosis
and important for virulence.
Diseases:
Primary lobar pneumonia which results from
inhalation of organisms into the lungs(Alcoholic
and compromised individuals are at high risk of
the infection).
Bacteremia and UTI also Klebsiella are implicated
Enterobacter,Citrobacter and Serratia
• These species are rarely cause infection unless an
individual is immunocopromised
• Multiple antibiotic resistance is common in
Enterobacter
Medically important Enterobacteriaceae and
Diseases
ORGANISM
DISEASES
Lactose Positive:
E.Coli
-Diarrhoea ,sepsis,UTI Neonatal
meningitis
Klebsiella,Serratia,Citrobacter,Ente -Opportunistic infection (e.g
robacter
pneumonia, neonatal meningitis
and sepsis)
Lactose Negative:
Salmonella
Shigella
Proteus
Yersinia
-Diarrhoea,typhoid
fever,bacteremia,eustiomyelitis,
menigitis,liver disease
-Dysentery
-UTI
Colony Morphology- Some Enterobacteriaceae
LACTOSE
FERMENTERS
NON LACTOSE
FERMENTERS
E.coli
Klebsiella
Salmonella
Shigella
MacConkey agar Purple/pink
Pink
Colourless
Colourless
Eosinmethylene
blue(EMB)agar
Purple
Colourless
Colourless
Medium
Red/black
Hektoen enteric Yellow/orang Yellow/orange Blue green
(HE)Agar
e
with black
centre
SalmonellaShigella agar
Pink
Pink
Colourless
with black
center
Green/blue
green
Colourless
Non Lactose Fermenting Enterobacteriaceae
 Salmonella:
They are normally found in many animals GIT but not
among normal human flora many serotypes are
identified based on surface antigens.
 Pathogenesis:
They have antiphagocytic capsule
They growth and replicate intracellularly
Endotoxin and exotoxin(Produces by some serotypes
cause enteritis)
Non Lactose Fermenting Enterobacteriaceae
Diseases:
Enteritis(S.enteritidis and other serotypes)
Incubation period 6-48 hrs -Spontaneous
recovery takes place within 2-7days
Typhoid(enteric) fever (S.typhi) Incubation
period: 10-14days
Bacteremia
Localized infections in other sites(e.g
Osteomyelitis,Meningitis, Liver)
Non Lactose Fermenting Enterobacteriaceae
Transmission:
All serotypes are transmitted by Fecal oral route
Ingestion of large numbers organisms in
contaminated water and food(eg: raw eggs ,
improperly cooked chicken etc.)
Asymptomatic carriers(S.typhi) Some who recover
from typhoid fever may continue to carry S.typhi in
gallbladder for months
Non Lactose Fermenting Enterobacteriaceae
Shigella
All four species are pathogenic to human
Pathogenesis:
Invasion of colonic mucosa and replication takes
place in mucosa cells
S.dysenteriae produces Shiga toxin(typeA-B) that
inhibits protein synthesis
Bacillary dysentery(Shigellosis)
Causes profuse watery diarrhoea,abd cramps
and fever,Tenesmus and bloody stool
Shigella cont…..
Transmission:
Transmission is by Fecal – oral route
Ingestion of contaminated food or water from
human
Treatment:
Rehydration –necessary
Antimotility agents(e.g Clotrimazole) but may
prolong disease.
Non Lactose Fermenting Enterobacteriaceae
Yersinia pestis:
Found in numerous small animals like dogs , cats
and transmittable to human by fleas.
Pathogenesis:
It has antiphagocytic capsule,grow intracellularly
and produces exotoxin and able to disseminate
from infection site
Disease caused by Yersinia pestis:
 Bubonic plague-Acquired from bite of infected
flea
Non Lactose Fermenting Enterobacteriaceae
• Bubonic plague cont..
• IP: 7days
• High fever,painful bubo(Inflammed swollen lymph
node) in groin or axilla, bacteremia that can lead to
death if untreated(Mortality rate >75%)
Pneumonic plague: Acquired by inhalation of
infectious droplets from infected person.
• IP: 2-3 days
• Fever,malaise followed by respiratory problem
then death (Mortality rate >90%)
Non Lactose Fermenting Enterobacteriaceae
Pneumonic plague cont….:
Prevention and Treatment:
• Effective pest control , isolation of infected
patients and vaccination.
• Treatment: Streptomycin ,Gentamycin,
Chloramphenicol and Tetracycline
Non Lactose Fermenting Enterobacteriaceae
Yersinia enterocolitica:
Carried by livestock , rabbits and rodents
transmitted though contaminated food and water
or blood products.
Diseases:
• Enterocolitis( Common)-Blood diarrhoea,fever
and abdominal pains (Chronic-may last for a year).
• Mesenteric lymphadenitis:-Enlargement of
mesenteric lymph nodes can present like
appendicitis-Common in young children
Non Lactose Fermenting Enterobacteriaceae
• Yersinia enterocolitica cont…..:
• Transfusion related septicemia:
• Y.enterocolitica can grow at low temperatures and
multiply to toxic level in refrigerated blood stored
for several weeks.
Proteus mirabilis:
Normal flora of human GIT
Common cause of UTI marked by elevated urine pH
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