ENTEROBACTERIACEAE
ENTERIC GRAM NEGATIVE RODS
The Enterobacteriaceae Family
Enterobacteriaceae - gram-negative rods whose
natural habitat is the intestinal tract of humans &
animals
 Some enteric bacteria are free-living
saprophytes while others are human
pathogens
 Large, heterogeneous group - facultative
anaerobes or aerobes that ferment a wide
variety of carbohydrates & possess a
complex antigenic structure
Coliforms (ferment lactose)

Escherichia coli (normal flora)

Klebsiella pneumoniae

Enterobacter aerogenes – free-living
saprophyte

Serratia marcescens

Hafnia alvei

Citrobacter freundii
Non-Coliforms (Do Not Ferment Lactose)

Edwardsiella tarda

Providencia stuartii

Morganella morganii

Proteus mirabilis

Salmonella spp. - cause diarrhea and
enteric fevers

Shigella spp. – cause bacillary dysentery
Other Enterobacteria
Yersinia pestis - causes bubonic &
pneumonic plague
 Yersinia enterocolitica - causes
febrile diarrhea or abdominal
symptoms mimicking appendicitis
 Erwinia spp. - plant pathogen
(saprophyte)
 Ewingella spp., Cedecea spp.,
Kluyvera spp., and others

Clinical Importance

Classical Pathogenic Infections
 Enteric
fevers and septicemias
 Gastroenteritis,
dysentery
enterocolitis,
Opportunistic Infections

Endogenous - from normal flora

Shock and severe malnutrition

Prolonged antibiotic therapy &
superinfection

Debilitation from other diseases

Predisposing lesions of unrelated
etiology
Nosocomial or Iatrogenic

Hospital acquired infections

Developed by more than 5% of
hospitalized patients which are primarily
caused by gram-negative enteric bacteria
Intestinal Profile

Gastrointestinal tract
 Esophagus
- contains bacteria swallowed
with saliva and food
 Stomach
- usually sterile

Small intestine - Duodenum -103 – 106
bacteria per gram; Jejunum - - 105 – 108

bacteria per gram; Ileum - enterococci
and lactobacilli predominantly found

Large intestine - 108 – 1010

bacteria/gram - up to 1011 per gram
The Enterobacteriaceae : The Coliforms

Escherichia coli

General characteristics
 Gram-negative
 Non-spore-forming
 Motile
 No
- peritrichous flagella
capsule
 Facultative
 Oxidase
anaerobe (aerobe)
negative
E. Coli

Resistant to the inhibition of certain
bacteriostatic dyes - selective media (EMB,
MacConkey, SS agar)

Ferments lactose producing acid + gas

Basis for differential media and screening media
(TSI agar)

Morphology - large, gray, smooth colonies on
blood agar; colonies on EMB display a green
metallic sheen which is very characteristic
Antigenic Structure

Serological typing - E. coli 075:K100:H5

Flagellar or “H” antigens - heat labile

Capsular or “K” antigens - heat stable

Somatic or “O” antigens - heat stable; 150 types

Fimbrial antigens (pili)

Common enterobacterial antigen (heterophile)
Genetic Relationships

GI tract - ideal environment for exchange
of DNA between various bacteria

Large population density, continuous
culture conditions, & fluctuation of
nutritional conditions

Genetic Transfer - Multiple Drug Resistance
(R-factors); coli cinogeny (production of
colicins coded by plasmids)
Resistance to Environmental Factors

Used as an index of fecal contamination
in U.S.A.

If present in water supply, then fecal
contamination

Look for E. coli not Salmonella or Shigella
Pathogenicity

E. coli is relatively non-pathogenic - mainly an
opportunist, especially outside of GI tract

Urinary tract infections - bacteruria, cystitis,
pyelitis (inflammation of renal pelvis), and
pyelonephritis (kidney & pelvis) caused by
certain serotypes

These uropathogens cause 80% of the
infections
Other Infections

Septicemia

Peritonitis

Meningitis

Endocarditis

Abscesses

Decubitis ulcers

Septic wounds

E. coli is the most frequently encountered
species in gram-negative sepsis leading to
severe shock of the patient
Pathogenic Groups

Causing gastrointestinal infections

Enteropathogenic E. coli (EPEC) - certain
serotypes (0111,055, 0127) have been
associated with diarrheal outbreaks among
neonates in nurseries (40% mortality)

Enterotoxigenic E. coli (ETEC) - traveler’s
diarrhea caused by toxin producing strains an LT toxin (heat labile) similar to cholera toxin
& ST (heat stable) toxin have been
demonstrated

Enteroinvasive E. coli (EIEC) - invasion and
destruction of colonic epithelium

Enterohemorrhagic E. coli (EHEC) - verotoxin
causing hemorrhagic colitis with severe
abdominal pain & hemolytic uremic
syndrome with acute renal failure, microangiopathic hemolytic anemia, &
thrombocytopenia
Laboratory Diagnosis

Plating of appropriate specimen (urine,
stool, blood, pus, etc.) on blood agar and
on differential media such as EMB (EosinMethylene Blue) and/or MacConkey’s agar

Isolation and subculture on blood agar for
obtaining a pure culture

Inoculation of various biochemical media
for identification (sugar fermentations, etc.)
Lab Diagnosis

Serological typing with specific antiserum

Colicin typing can also be performed in
reference labs
Differential & Selective Media
EMB 
MacConkeys 
Gram Stain
Biochemical Tests

API STRIP

ENTEROTUBE
Lab Tests

Oxidase = Negative

Lactose fermentation = Positive

Indole = Positive

Methyl red = Positive

Voges-Proskauer = Negative

Simmons citrate = Negative

Hydrogen sulfide production = Negative

Urease = Negative
Treatment

Antibiotic susceptibility testing is mandatory

Resistance is frequently encountered and
varied among strains
Source & Transmission of E.coli

May be part of the normal intestinal flora of
some people

Horizontal transmission via fecal-oral route
from child to child, when children are living,
eating, or playing together

Transmission also occurs from drinking
contaminated water
Virulence Factors
Exotoxin
 Endotoxin – (O-antigens) – provide
antigenic variation
 Capsule - (K-antigens) – provide antigenic
variation
 Flagella - (H-antigens) – provide antigenic
variation as well as motility
 Pili (Fimbria) - also known as EPEC adhesion
factor

Vaccine & Toxoid

None

HOST DEFENSE & IMMUNITY
 Secreted
 Previous
IgA antibodies to fimbriae
infection confers long lasting type-
specific immunity
 ETEC
remains in intestinal lumen - non-invasive
Klebsiella-Enterobacter-Serratia Group
General Characteristics
 Gram-negative
(GI flora)
 Facultative,
neg.
 Klebsiella
rods, free-living saprophytes
lactose-fermenters, oxidase
and Enterobacter have capsules
 Enterobacter
and Serratia are motile

Klebsiella is non-motile

Some strains of Serratia are pigmented red or
orange
Pathogenicity

Opportunists

Especially in debilitated or compromised
patients

Predisposing factors are also important
determinants in pathogenesis

Old age, early infancy, diabetes, broadspectrum antibiotics, corticosteroids,
indwelling catheters, postoperative status,
terminal diseases, immuno-suppression, focal
debilitation
Pathogenicity

Urinary Tract infections, gram-negative sepsis,
biliary tract infections, lung infections,
peritonitis, meningitis, and infections of wounds
and cutaneous ulcers

Klebsiella pneumoniae - implicated as
etiologic agent in 3% of all bacterial
pneumonias, especially involving alcoholics
and persons with compromised pulmonary
function
Treatment

Antibiotic susceptibility testing mandatory

Surgical intervention if focal infections are
sources of sepsis
Mucoid Klebsiella
Proteus-Morganella-Providencia
Group (Non-Coliforms)
General Characteristics - Gram-negative
rods, motile, lactose non-fermenters, oxidase
negative.
 Facultative, free-living in soil, sewage, and
water, but also common among normal
fecal flora
 Urease + = P. vulgaris, P. mirabilis,
Morganella morganii
Pathogenicity

Opportunistic infections when accidentally
introduced in areas other than GI tract

Can also be secondary invaders in
debilitated areas

Urinary tract infections, cystitis, pyelonephritis

Pneumonia, other lung infections, peritonitis

Wound infections, burns, cutaneous ulcers

Abscesses

Intertriginous foot infections (secondary to
dermatophytes)

Diarrheal disorders (some strains)
Treatment

Antibiotic susceptibility tests mandatory
because of diverse variation of sensitivity
among strains
Salmonella Group (Non-coliforms)
General Characteristics
 Gram-negative rod, motile, do not ferment
lactose, intracellular
 Facultative, oxidase-negative,
fermentative with formation of acid and
gas (hydrogen sulfide)
 Resistant to freezing and to certain
chemicals – brilliant green and sodium
desoxycholate inhibit coliforms but not
Salmonella

This is used to isolate these pathogens from
fecal material (selective media)
Transmission

Fecal-oral transmission via contaminated food
or water
 Sources - milk & other dairy products, raw
eggs, dried or frozen eggs, meats, meat
products, poultry, roast beef, corned beef,
shellfish and undercooked whitefish, animal
dyes, dried cocoanut
 Origin - many animals are naturally
infected with various Salmonellae
(especially poultry)

These can be found in tissues, eggs

Household pets - turtles, dogs, & cats can
also transmit these bacteria

Human carriers, especially food handlers.
Pathogenicity

Acute enterocolitis - 8 hour to 3 day
incubation followed by symptoms of
headache, low grade fever, abdominal
discomfort, and diarrhea; nausea and
vomiting at times
 S.
typhimurium
 S.
heidelberg
 S.
agona
 S.
newport
Isolation, Diagnosis, & Prevention

Specimens - blood cultures, stools, urine,
serum etc. - Must be taken repeatedly

Media inoculation - enrichment broths
(selenite F, tetrathionate) used to stimulate
the growth of pathogens, followed by
isolation on selective & differential media

MacConkeys, HE, XLD, SS agar
Identification

Biochemical tests followed by serology to
identify serogroup of Salmonellae isolates
Control & Prevention

Sanitary measures

Thorough cooking of poultry, eggs, meat,
etc.

Identification of carriers

Vaccination during travel to foreign
countries
Shigella Group (Non-Coliforms
General characteristics
 Gram-negative
rod, non-motile, lactose
non-fermenter, extracellular
 Facultative
anaerobe, oxidase-negative,
unencapsulated, fermentative with
formation of acid without gas (no hydrogen
sulfide)
Pathogenicity

Bacillary Dysentery - limited to large intestine
 Pathogen
invades mucosal epithelium only
(septicemia extremely rare)
 Symptoms
- abdominal pain, cramps,
diarrhea, and fever develop after 1-4 days
 Stools
are initially profuse, but become low
volume with blood and mucous
 Passage
is accompanied by much straining
and tenesmus (rectal spasms)
Diagnosis, Treatment, & Prevention
Specimens - stools & rectal swabs ; culture on
HE, SS, & other selective media for isolation;
identification based on biochemical tests and
serologies with specific antisera
 Antibiotic Therapy - susceptibility testing has to
be ordered to determine sensitivity
 S. dysenteriae is among the most antibiotic
resistant pathogens
 Trimethoprim-sulfamethoxazole


Fluid and electrolyte replacement

Control and Prevention
 Transmission
by food, fingers, feces, and
flies from human to human (sole reservoir)
 Sanitary
measures, isolation of patients
with active and sub-clinical infections;
identification of carriers