Intra-Abdominal Infections Flashcards
What is the main reservoir for introduction of bacteria into the GI tract?
Mouth, comprised of 99% anaerobes
Why are enterococcus different than other gram positive organisms?
They can live in the presence of bile salts -> highest concentration in duodenum
What is the purpose of M cells?
Aid in the presentation of antigens by phagocytosing bacteria and other antigens, and passing to underlying macrophages
What is the significance of crypts for enterocytes?
New enterocytes are produced in the crypt at a very high turnover rate, and are sloughed off at the tips of the villi
What is one major mechanistic difference between how bacterial flora are treated in the duodenum vs colon?
Small intestine: very fast movement of material which constantly washes bacteria out of it
Colon: Slow rate of flow allowing for dense bacterial population and protective flora
What are three major tests used in the identification of gram negative bacterial infections?
- Oxidase reaction - ability to oxidize cytochrome C
- Growth on MacConkey agar (gram negative and does it ferment lactose)
- Metabolism - Oxidative or fermentative growth based on oil / air above tubes of growth medium
What major family of bacteria is oxidase negative?
Enterobacteriacea - i.e. PEK + Salmonella, Shigella, Yersinia
What is secretory vs inflammatory diarrhea?
Secretory - watery, with no immune cells and no invasion
Inflammatory (dysentery) - Based on presence of leukocytes in the stool (inflammation)
What is invasive vs non-invasive?
Invasive - Organism can invade epithelial cells
Non-invasive - Bacteria stays in lumen but causes disease by secreting toxins
What is enterotoxin vs neurotoxin vs cytotoxin?
Enterotoxin - toxin released by microorganism in the intestine
Neurotoxin - acts directly on neurons
Cytotoxin - Has a specific toxic action on cells of special organs
What are the three classes of secretory diarrhea causes?
- Toxin-mediated - enterotoxin or neurotoxin
- Parasite-mediated - i.e. Giardia or Crypto
- Viral
What are the three types of invasive / tissue-damaging diarrhea, by etiology?
- Cytotoxin-mediated inflammation - i.e. C. difficile, EHEC
- Invasive-infection mediated - i.e. Shigella, EIEC, Salmonella
- Parasitic - i.e. E. histolytica
What two Vibrio cholerae serotypes cause pandemic cholera, and what is its morphology? Oxidase test?
O1 & O139 - gram negative (O antigens are on LPS), Comma-shaped motile rods. Oxidase positive
What is the mechanism of transmission of cholera? Can there be carriers?
Fecal-oral route, most commonly via water, fish, and shellfish
There can be carriers - important reservoir
What are the three main virulence factors of Vibrio cholera?
- Motility - Single polar flagellum to get close to mucosa
- Adherence - TCP (toxin co-regulated pili) which form bundles to adhere to mucosal epithelium of small intestine
- Cholera toxin - ADP-ribosylating AB type toxin
What is an AB toxin and how does cholera-toxin work?
AB = activity and binding.
B has 5 binding subunits, attaches to Ganglioside GM1. A is inactive until it enters the cell and becomes A1, enzymatically activating host adenylate cyclase.
Leads to chloride and fluid excretion via osmotic loss (up to 1 L / hr)
Why is the ID50 of cholera so high?
Need a large inoculum (>1 million) due to sensitivity to gastric acids
What special media is cholera grown on?
TCBS - Thiosulfate citrate bile sucrose
Think TCBY!~
What are the general virulence factors of E. coli?
- Bundle-forming pili- like TCP of Vibrio
2. Adhesins - bind to epithelial cells, “colonization factors” on pili
What are the two toxins produced by enterotoxigenic E. coli and how do you remember this?
“labile in the Air, stable on the Ground”
Heat labile toxin - raises cAmp levels, same mechanism as cholera-toxin (GM1 binding)
Heat stable toxin - raises cGmp levels, decreases the resorption of electrolytes and fluid
What does ETEC cause and how is it normally contracted?
Travelers diarrhea
Normally contracted via contaminated food and beverages in th e tropics
How is ETEC identified?
ELISA or agglutination tests for presence of toxins, or DNA probes for the LT or ST (toxin) genes.
What is enteropathogenic E. coli known for (EPEC)
Causing a fatal, watery diarrhea in infants / children in developing countries (due to loss of absorptive capacity of mucosal surface)
EPEC = Pediatric (look at the P)
What is the major virulence mechanism of EPEC? What type of secretion?
Attachment and “effacement” of cell membrane by destroying the microvilli and sitting on an actin-like pedestal
Also uses Type 3 secretion to inject bacterial proteins
What are the three major bacterial species that create pre-formed toxins?
- Clostridium botulinum
- S. aureus
- Bacillus cereus
What is the structure of botulinum toxin, and what usually causes death?
AB toxin which binds to peripheral neurons. B portion binds the ganglioside receptors on nerve cells, and A toxin is internalized, blocking Ach release at neuromuscular junction = flaccid paralysis
Why is botulism an even bigger concern for children?
Floppy baby syndrome = infant botulism, can cause SIDS because honey often contains C. botulinum spores which cannot be fought off due to immature microbiota.
How is C. botulinum identified?
Gram + spore-forming bacillus, identified typically by immunoassay for toxin in food substance or gastric contents or blood
What is the most common bacterial form of food poisoning and what is its mechanism?
S. aureus, due to unrefrigerated foods. Uses enterotoxin B, not destroyed via reheating. (heat stable)
Stimulates neural receptors - leading to vomiting center stimulation in CNS = project vomiting
What are the two forms of Bacillus cereus food poisoning, and which one is associated with pre-formed toxin?
- Emetic / vomiting type - preformed toxin - reheated rice syndrome (from sitting at Chinese restaurant). Similar to S. aureus, short onset
- Diarrheal form - longer incubation, slower onset. Heat LABILE toxin which is formed in vivo, activates cAMP. Associated with GI pain and non-blood diarrhea from foods sitting above room temp.
Both resolve in <24 hours
What is one final bacterium that can release a heat-labile enterotoxin after ingestion?
Clostridium perfringens - much like B. cereus diarrheal form, produced in GI tract. From undercooked food
What is the virulence factor of Giardia?
Adhere to upper small intestine via ventral surface disk on trophozoite “sucking disk”
Although they have flagella for motility as well
What is the mechanism of Giardia infection?
Oral ingestion of cysts, commonly from drinking unfiltered water. Incubation is about 1 week, before acute onset of watery diarrhea.
Chronic giardia can lead to bloating, and fat-rich stool “Ghirardelli chocolate fat” stool
How is Giardia identified?
Antigens for it in stool, duodenal biopsy shows “old man face” or cysts in stool
What is a non-inflammatory protozoan which is of major concern to immunocompromised patients?
Cryptosporidium parvum - causes severe, watery, prolonged diarrhea in aids
Immunocompetent is a mild watery diarrhea
How does Cryptosporidium reproduce?
Invades the microvilli of the intestine, releases oocysts in the feces, and these are eaten (fecal-oral)
What is a protozoal diarrhea more severe in AIDS patients that can persist up to 7 weeks?
Cyclospora cayetanesis
What is an obligate intracellular spore-forming protist causing watery diarrhea?
Microsporidia
What is another protist causing watery diarrhea which is pretty much indistinguishable from the others?
Isospora belli - infects entire intestines
When does C. difficile infection typically occur?
In the 5% of healthy adults who carry it, follows clindamycin use
What are the two toxins produced by C. difficile and what does this cause overall?
Toxin A - Enterotoxin binding mucosa, causes altered membrane permeability, necrosis, and inflammation
Toxin B - Cytotoxin like diphtheria, disrupts actin cytoskeletal structure and decreases cellular protein synthesis
Results in pseudomembranous colitis