Download The Enterics

The Enterics
The enterics: are gram-negative bacteria that
are part of the normal intestinal flora or cause
gastrointestinal disease.
 Many of these bacteria are referred to simply
by their genus name because there are so
many different species in some groups.
 The main groups are Enterobacteriaceae,
Vibrionaceae,
Pseudomonadaceae
and
Bateroidaceae.
 These organisms are also divided into
groups based upon biochemical and antigenic
properties.
Biochemical Classification
The ability to ferment lactose and convert it
into gas and acid. Escherichia coli and most of
the enterobacteriaceae ferment lactose while
Salmonella, Shigella and Pseudomonas
aeruginosa do not.
1)
2) The production of H2S, ability to hydrolyze
urea, liquefy gelatin, and decarboxylate specific
amino acids.
Biochemical Classification
Some growth media do 2 things at once:
1) They contain chemicals that inhibit the growth
of gram-positive bacteria.
2) They have indicators that change colour in the
presence of lactose fermentation.
A) EMB agar (Eosine Methylene Blue): Methylene
blue inhibits gram-positive bacteria, and colonies
of lactose fermenters become deep purple to
black in this medium. Escherichia coli colonies
take on a metallic green sheen in this medium.
2) MacConkey agar: Bile salts in the medium
inhibit gram-positive bacteria, and lactose
fermenters develop a pink-purple coloration.
Antigenic Classification
 The enterics have 3 major surface antigens:
1) O antigen: This is the most external component of
the lipopolysaccharide (LPS) of gram-negative
bacteria. The O antigen differs from organism to
organism, depending on different sugars and different
side-chain substitutions.
2) K antigen: This is a capsule that covers the O
antigen.
3) H antigen: This antigenic determinant makes up the
subunits of the bacterial flagella, so only bacteria that
are motile will possess this antigen. Shigella does not
have an H antigen. Salmonella has H antigens that
change periodically, protecting it from our antibodies.
Antigenic Classification
The O antigen forms
the outer part of the
cell membrane, the
K antigen wraps
around the cell like a
capsule, and the
arms of the H
antigen
become
wavy flagella
Pathogenesis
The organisms produce 2 types of disease:
1) Diarrhea with or without systemic invasion.
2) Various other infections including urinary tract
infections, pneumonia, bacteremia, and sepsis,
especially in debilitated hospitalized patients.
Diarrhea
1) No cell invasion:
 The bacteria bind to the intestinal epithelial cells but
do not enter the cell.
 Diarrhea is caused by the release of exotoxins which
causes electrolyte and fluid loss from intestinal
epithelial cells or epithelial cell death. Watery
diarrhea without systemic symptoms (such as fever).
Enterotoxigenic Escherichia coli and Vibrio cholera
are examples.
Pathogenesis
2) Invasion of the intestinal epithelial cells:
The bacteria invade the cells. Toxins are then released
that destroy the cells.
 The cell penetration results in leukocytes in the stool as
well as fever. The cell death results in red blood cell
leakage into the stool. Examples: Enteroinvasive
Escherichia coli, Shigella, and Salmonella enteritidis
3) Invasion of the lymph nodes and bloodstream:
 Along with abdominal pain and diarrhea containing white
and red cells, this deeper invasion results in systemic
symptoms of fever, headache, and white blood cell count
elevation.
 The deeper invasion can also result in mesenteric lymph
node enlargement, bacteremia, and sepsis. Examples:
Salmonella
typhi,
Yersinia
enterocolitica,
and
Campylobacter jejuni.
Pathogenesis
Various Other Infections
 The enterics are normal intestinal inhabitants and
usually live with us in peaceful harmony. In the hospital
and nursing homes, however, some bad things happen.
They acquire antibiotic resistance and can cause
disease in debilitated patients.
 They can invade the debilitated patients when Foley
catheters are in the urethra or when a patient aspirates
vomitus that has been colonized by the enterics.
 Because of hospital acquisition, they described as
the hospital-acquired gram-negatives or nosocomial
gram-negatives. Examples: Escherichia coli, Klebsiella
pneumoniae, Proteus mirabilis, Enterobacter, Serratia,
and Pseudomonas aeruginosa.
Family Entrobacteriaceae
Escherichia coli
 Escherichia coli normally resides in the colon without
causing disease.
Nonpathogenic Escherichia coli (normal
flora) + Virulence factors = DISEASE.
Virulence factors include the following:
1) Mucosal interaction:
a) Mucosal adherence with pili (colonization factor).
b) Ability to invade intestinal epithelial cells.
2) Exotoxin production:
a) Heat-labile and stable toxin (LT and ST).
b) Shiga-like toxin.
3) Endotoxin: Lipid A portion of lipopolysaccharide.
4) Iron-binding siderophore: obtains iron from human
transferrin or lactoferrin.
Diseases
 Diseases caused by Escherichia coli in the
presence of virulence factors include the
following:
1) Diarrhea.
2) Urinary tract infection.
3) Neonatal meningitis.
4) Gram-negative sepsis, occurring commonly
in debilitated hospitalized patients.
Escherichia coli Diarrhea
 Escherichia coli diarrhea may affect infants or adults, death
from Escherichia coli diarrhea is usually due to dehydration.
The severity of Escherichia coli diarrhea depends on virulence
factors of Escherichia coli. There are 3 groups of diarrhea
producing E. coli.
1) Enterotoxigenic Escherichia coli (ETEC):
 This Escherichia coli causes traveler's diarrhea. It has pili
(colonization factor) that help it bind to intestinal epithelial
cells, where it releases exotoxins that are similar to the
cholera exotoxins.
 The toxins are the heat labile toxin (LT), which is just like the
cholera toxin. These exotoxins inhibit the reabsorption of Na
and CL and stimulate the secretion of Cl - and HCO3- into the
intestinal lumen. Water follows the osmotic pull of these ions,
resulting in water and electrolyte loss. This produces a severe
watery diarrhea with up to 20 liters being lost a day!!! The
stool looks like rice water just like cholera!
2) Enterohemorrhagic Escherichia coli (EHEC):
 These Escherichia coli also have a pili colonization factor like
the ETEC but differ in that they secrete the powerful Shigalike toxin ( also called verotoxin) that has the same
mechanism of action as the Shigella toxin.
 They both inhibit protein synthesis by inhibiting the 60S
ribosome, which results in intestinal epithelial cell death.
 The diarrhea is bloody (hemorrhagic), accompanied by
severe abdominal cramps, and is called hemorrhagic colitis.
Hemolytic uremic syndrome (HUS) with anemia,
thrombocytopenia (decrease in platelets), and renal failure
(thus uremia), is associated with infection by a strain of
EHEC, called Escherichia coli 0157:H7.
 Numerous outbreaks have occurred secondary to infected
hamburger meat served at fast food chains, suggesting that
cattle may be a reservoir for EHEC.
3) Enteroinvasive Escherichia coli (EIEC):
 This disease is the same as that caused by Shigella.
 In fact, the main virulence factor is encoded in a plasmid
shared by Shigella and Escherichia coli.
 This plasmid gives the bacteria the ability to actually invade
the epithelial cells.
 EIEC also produces small amounts of Shiga-like toxin. The
host tries to get rid of the invading bacteria, and this results
in an immune-mediated inflammatory reaction with fever.
 White blood cells invade the intestinal wall, and the diarrhea
is bloody with white blood cells. Like shigellosis!
Escherichia coli Urinary Tract Infections (UTIs)
 The acquisition of a pili virulence factor allows E.
coli to travel up the urethra and infect the bladder
(cystitis) and sometimes move further up to infect
the kidney itself (pyelonephritis).
 E. coli is the most common cause of urinary tract
infections, which usually occur in women and
hospitalized patients with catheters in the urethra.
 Symptoms include burning on urination (dysuria),
having to pee frequently (frequency), and a feeling
of fullness over the bladder.
 Culture of greater than 100,000 colonies of
bacteria from the urine establishes the diagnosis of
a urinary tract infection.
Escherichia coli Meningitis
 E. coli is the second most common cause of
neonatal meningitis (group B streptococcus is first).
During the first month of life, the neonate is especially
susceptible.
Escherichia coli Sepsis
 E. coli is also the most common cause of gramnegative sepsis.
 This usually occurs in debilitated hospitalized
patients. Septic shock due to the lipid A component of
the LPS is usually the cause of death.
Escherichia coli Pneumonia
 E. coli is a common cause of hospital-acquired
pneumonia.