(From Micro, 9 Oct 2000, by Brian Buschman)
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Enterics are gram-negative bacteria that are normal flora in our intestines. They include four families being the Enterobacteriaceae, Vibrionaceae, Pseudomonadaceae and Baterioidaceae. They are from all sorts of genus but are divided based on biochemical and antigenic properties.
Lactose fermentation is used to subclassify Enterobacteriaceae. All but Salmonella, Shigella, Proteus and Pseudomonas are able to ferment lactose.
Special media to grow enterics include:
1) EMB agar (eosin methylene blue) works to inhibit gram-positive and if they ferment lactose they grow deep purple.
2) MacConkey agar has bile salts to inhibit gram-positive bacteria while lactose fermenters turn pink/purple.
E. coli only exist in the GI tract and fecal material so they can be used to determine if water has been polluted with fecal material.
ID E. coli by:
1) A presumptive test involves growing them in a tube with an inverted tube that will catch gas that turns purple if they are lactose fermenters. E. coli a partial lactose fermenter turns green.
2) You confirm the test by growing the organism on EMB agar and look for green colonies that grow at both 37oC and 45.5oC. This helps differentiate E. coli from other similar organisms.
3) To complete the test you will once again want to take the colonies that grow at both temperatures and grow them in the tube as in the presumptive test.
Antigens are classified based on what type they are.
1) O antigens are on the outer part of the plasma membrane. It is part of the LPS.
2) K antigens are on the capsule that covers the O antigens.
3) H antigens are on the flagella. Obviously only motile organisms have H antigens.
The enterics cause:
1) Diarrhea from one of three mechanisms:
a. Without invading epithelial cells they release exotoxins that cause electrolyte loss like E. coli and Vibrio cholera.
b. Invasion of intestinal epithelial cells. The bacteria have virulence factors helping them to get inside the epithelial cells. This causes diarrhea and local inflammation.
c. Invasion of local LNs and the bloodstream. This causes bloody diarrhea and will present with systemic symptoms.
2) Other infections can be caused when the enterics gain entrance to the body. This is primarily true in the hospital with opportunistic (nosocomial) infections.
E. coli is a normal intestinal bacterial that can be deadly if it acquired virulence factors. Such factors include:
1) Ways to interact with the mucosa such as pili.
2) Heat-liable and heat-stable Shiga-like exotoxins.
3) Endotoxin – Lipid A (LPS)
4) The ability to bind Fe2+ from the human host.
E. coli diarrhea is the most common thing caused by E. coli and is usually seen in young children or travelers to less developed countries. Diarrhea is caused by:
1) Enteropathogenic E. coli (EPEC) which causes diarrhea in the young.
2) Enterotoxigenic E. coli (ETEC) which is the cause of traveler’s diarrhea which works by both heat stable and heat liable exotoxins. The heat liable one has an A subunit that enters the cell activating adenylate cyclase while it’s B subunits help it get in. The head liable toxin turns on guanylate cyclase.
These black Na+/Cl- reabsorption and secrete Cl- and HCO3- which as noted causes diarrhea.
3) Enterohemorrhagic E. coli (EHEC) colonizes the lower bowel and releases the Shigella-like toxin after binding to the enterocytes. The Shigella like toxin inactivates the 60s subunit killing the enterocytes and causing bloody diarrhea.
Hemolytic uremic syndrome is caused by E. coli O157:H7 from burgers.
It causes anemia, thrombocytopenia and renal failure.
4) Enteroinvasive E. coli (EIEC) has a shared virulence factor with Shigella and invades the enterocytes. This causes a systemic response including fever.
In this case the diarrhea has pus and blood.
Other E. coli infections include:
1) UTIs that can be caused by E. coli leading to frequent urination and pain.
2) Meningitis in newborns.
3) Cause of gram-negative nosocomial infections.
Klebsiella is a non-motile (no H antigen), lactose fermenter that uses O-antigen to cause opportunistic infections of pneumonia (bad and bloody) UTI and wound infections.
Proteus is a lactose-negative (see the four exceptions), urease positive organism that also causes UTI and nosocomial infections.
Enterobacter is a motile lactose-fermenter that is opportunistic.
Serratia is a slow lactose fermenter that makes a bright red pigment and causes opportunistic infections.
Shigella is another one on the list of non-lactose fermenters (dx from E. coli). It is a non-motile that does not make H2S (dx from Salmonella). It has a low inoculation dose.
Shigella is NOT NORMAL FLORA and is always pathogenic. It is only spread in humans and often causes dysentery in preschool age.
It works like EIEC hence EIEC has the Shigella-like toxin. Once again this toxin inhibits the 60s subunit. It, like EIEC, causes bloody diarrhea, abdominal pain and infection signs (fever). The diarrhea is caused by the inability of the damaged enterocytes to do their job.
Salmonella is another of the non-lactose fermenters. It is usually motile and (except for S. typhi) is carried by animals. It has an O antigen and the secret Vi antigen which is a type of K antigen. Salmonella has a high inoculation dose.
Typhoid fever is caused by S. typhi and is also called enteric fever. Like E. coli and Shigella it invades the enterocytes but S. typhi invades the regional LNs. It is a facultative intracellular parasite.
Patients will have a fever, headache, abdominal pain that mimics appendicitis and a light pink rash. You diagnose with a culture of blood, urine or stool and treat with ciprofloxacin (a fluoroquinolone antibiotic) or ceftriaxone (3rd generation cephalosporin).
After typhoid infection some become chronic carries that excrete S. typhi so they need to wash after every poop. The bacteria lie in the gal bladder and urinary tract so some need their gal bladder removed.
S. enteriditis has hundreds of strains of bacteria that cause watery diarrhea that sometimes has blood or mucous. Many have fever and it will be cured on it’s own so antibiotics do not help. (In general do not use antibiotics with diarrhea.)
Yersinia is a motile-gram negative rod causing gastroenteritis. It is transmitted by humans and animals via the fecal-oral route and patients develop diarrhea and signs of appendicitis but it is just terminal ileal ulceration.
One form causes the bubonic plague passed by flea bite. The one we care about is what causes the diarrhea which invades like Salmonella typhi and sepsis results after it gets into regional nodes. Treatment does not help diarrhea but improves the case of sepsis.
Vibrionaceae in general are motile (with 1 polar flagellum), gram-negative, curved rods. They are peroxidase-positive, ferment glucose and grow on MacConkey’s agar.
Vibrio chlora is a curved rod that transmits via fecally contaminated water in places with sewage disposal problems. Like ETEC it does not invade but only releases it’s toxin to cause diarrhea. In this case the toxin is called chloragen and leads to increased cAMP.
With chloragen there are b-subunits for binding and b subunits for binding and a subunits to activate adenylate-cyclase.
Eating raw shellfish can cause gastroenteritis. Both are halophiles and V. vulnificus is specifically associated with oysters.
Campylobacter jejuni is a curved gram-negative rod with a single polar flagellum like V. cholera. It is zoonotic and transmits in foods, epically poultry or fecal-oral. It is popular in unpasteurized milk. It has a heat-liable endotoxin and it causes mucosal invasion, fever, cramping, bloody and loose diarrhea.
Helicobacter is a very common cause of gastritis, gastric ulcer and duodenal ulcer. It is treated with bismuth salts, metronidazol and amp and/or tetracycline.
Pseudomonas is another motile gram-negative rod. It grows in most anything but does not ferment lactose. It prefers to be anaerobic if NO3 is present.
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