Built from GI Lecture 25 (LOs 1β3). Close notes. Fill from memory. Reveal only after a genuine attempt.
π Use alongside: GI Lecture 25 β GI Infections (HoyaDocs)
Focus on: viral vs. bacterial vs. protozoal diarrhea; invasive vs. toxigenic bacteria; E. coli pathotypes; preformed-toxin food poisoning; clinical syndromes (watery/dysentery/food poisoning/traveler's/C. diff); intraabdominal infections (H. pylori, liver abscess, SBP, secondary peritonitis, CAPD peritonitis).
Viral & Bacterial Bugs
E. coli, Vibrio, Toxins
Syndromes: Watery & Dysentery
Traveler's & C. difficile
Intraabdominal Infections
Practice Qs
LO 1
ON EXAM β clinical definition of diarrhea, viral vs bacterial vs protozoal buckets, Rotavirus/Norovirus triggers, invasive vs toxigenic bacterial framework, Campylobacter/Salmonella/Shigella/Yersinia/Listeria classic associations
Clinical definition of diarrhea (frequency / volume / form / acute)
Three big pathogen buckets β name representative bugs in each
Category
Representative Pathogens
Most common feature
Viruses
Bacteria
Protozoa/Parasites
Viral diarrhea β fill the pathogen profile
Feature
Rotavirus
Norovirus
Classic population
Classic setting
Mechanism (enterotoxin? malabsorption?)
Vaccine available?
Invasive vs. toxigenic bacteria β label each bug
Bug
Invasive or Toxigenic?
Signature feature
Campylobacter jejuni
Salmonella (non-typhoidal)
Salmonella Typhi
Shigella
Yersinia enterocolitica
Listeria monocytogenes
Vibrio cholerae
Campylobacter β deep dive
Salmonella β typhoidal vs non-typhoidal
Feature
Non-typhoidal
Typhoidal (S. Typhi)
Reservoir
Classic exposure
Key clinical features
Chronic carrier state site
Treatment approach (antibiotics?)
Shigella β classic associations
Listeria β signature groups
πͺ€ TRAP: "Most common" depends on what you're counting β overall gastroenteritis = viral (Norovirus/Rotavirus); most common bacterial = Campylobacter; most common traveler's = ETEC.
LO 1
ON EXAM β five E. coli pathotypes and their toxins/populations; Vibrio cholerae vs V. parahaemolyticus vs V. vulnificus; preformed-toxin bugs and their classic exposures/onsets
πͺ€ TRAP: "Preformed toxin" β infection. The toxin is already in the food. Reheating doesn't help (S. aureus toxin is heat-STABLE).
LO 2
ON EXAM β secretory vs invasive vs osmotic mechanism; cholera vs ETEC vs viral watery diarrhea; dysentery bugs (Shigella, EHEC, Campylobacter, Entamoeba, invasive Salmonella)
Secretory vs. invasive vs. osmotic β mechanism + WBCs in stool
Type
Mechanism
Blood/WBCs in stool?
Prototype bug
Secretory (non-inflammatory)
Invasive (inflammatory)
Osmotic
Cholera β classic features
ETEC (traveler's-style watery)
Viral watery diarrhea β pick the bug by stem
Dysentery (inflammatory bloody diarrhea) β match pathogen to stem
Entamoeba vs. E. coli O157:H7 β how to tell apart
πͺ€ TRAP: Bloody diarrhea + fever + Shigella β do NOT give anti-motility agents. Do NOT give antibiotics for EHEC (β HUS risk from Shiga toxin release).
LO 2
ON EXAM β traveler's diarrhea top pathogens (ETEC #1), prophylaxis/treatment; C. difficile risk factors (antibiotics), toxins A/B, diagnosis (PCR/EIA), treatment (PO vancomycin or fidaxomicin), fulminant features
Traveler's diarrhea β frequency ranking
Traveler's diarrhea β prevention + treatment
C. difficile β risk factors
C. difficile β toxins and damage
C. difficile β diagnosis
C. difficile β treatment
Scenario
Treatment
First episode, non-severe
Severe / fulminant
Recurrent (β₯2nd recurrence)
Fulminant w/ ileus
β οΈ Fulminant C. diff = hypotension/shock, ileus, toxic megacolon, perforation. Oral vancomycin + IV metronidazole; colectomy if deteriorating.
LO 3
ON EXAM β normal flora and empiric coverage; H. pylori disease associations and triple/quadruple therapy; pyogenic vs amebic liver abscess; SBP vs secondary vs CAPD peritonitis (organism, diagnosis, treatment)
Normal flora β potential pathogens
Empiric coverage for intraabdominal infections
H. pylori β disease associations
H. pylori β diagnosis + treatment
Test/Regimen
Details
Urea breath test
Stool antigen
CLO test
Triple therapy (classic)
Quadruple therapy (if macrolide resistance)
Eradication confirmation timing
Pyogenic vs. amebic liver abscess
Feature
Pyogenic
Amebic
Source
Typical organisms
Lobe predilection
Abscess contents description
Treatment
Peritonitis β three classifications
Type
Organism(s)
Diagnostic cutoff
Treatment
Primary (SBP) β cirrhosis/ascites
Secondary (perforation, surgery)
CAPD (peritoneal dialysis)
SBP β high-yield specifics
πͺ€ TRAP: SBP is usually MONOmicrobial; polymicrobial ascitic fluid or very high PMN/WBC β think secondary (perforation/visceral source) β needs surgery, not just antibiotics.
LOs 1β3
Q1: Traveler's diarrhea pathogen
A 28-year-old medical student returns from a 2-week trip to Mexico with 3 days of profuse watery diarrhea, abdominal cramps, and no blood. Stool studies show a non-inflammatory profile. Which pathogen is most likely?
A) EHEC (O157:H7)
B) Shigella
C) ETEC
D) Campylobacter jejuni
E) Norovirus
Correct Answer: C
ETEC is the #1 cause of traveler's diarrhea β watery, secretory, non-inflammatory (LT toxin β cAMP; ST toxin β cGMP). EHEC and Shigella cause dysentery (bloody). Campylobacter is possible but less common in Mexico-origin traveler's diarrhea. Norovirus is plausible but less likely in this classic traveler presentation.
Q2: HUS after undercooked beef
A 5-year-old develops bloody diarrhea 4 days after a barbecue. Five days later he is pale, lethargic, with petechiae, oliguria, and Cr 2.9. Smear shows schistocytes. Which organism and toxin?
A) Shigella + Shiga toxin
B) EHEC + Shiga-like toxin (Stx)
C) ETEC + heat-labile toxin
D) Salmonella + LPS
E) Campylobacter + cytolethal distending toxin
Correct Answer: B
EHEC O157:H7 produces Shiga-like toxin β HUS triad (microangiopathic hemolytic anemia, thrombocytopenia, AKI). Undercooked ground beef / unpasteurized dairy is classic. Avoid antibiotics and anti-motility agents (worsen HUS). Shigella also produces Shiga toxin but HUS is rare compared with EHEC.
Q3: Preformed toxin β fast onset
Six people eat at a picnic; 3 hours later all develop vomiting and cramps. They recover within 24 hours. Which pathogen?
A) Salmonella
B) Campylobacter
C) S. aureus (preformed enterotoxin)
D) EHEC
E) C. difficile
Correct Answer: C
S. aureus produces a heat-stable preformed enterotoxin. Onset is 2β6 hours (toxin is already in the food β no time needed for replication). Vomiting predominates; resolution within 24β48 hours. B. cereus emetic form (fried rice) is the other classic fast-onset option.
Q4: Invasive vs. toxigenic classification
A 34-year-old man has bloody diarrhea, fever, and leukocytosis after eating undercooked chicken. Stool WBCs positive; stool culture grows curved gram-negative rods with corkscrew motility at 42Β°C. Which organism + category?
A) ETEC β toxigenic
B) Campylobacter jejuni β invasive
C) Vibrio cholerae β toxigenic
D) Listeria β invasive
E) Norovirus β viral
A 67-year-old inpatient on IV ceftriaxone develops 10 watery stools/day. Stool PCR positive for C. difficile; toxin EIA positive. He is afebrile with normal vitals and WBC 14k. What is the first-line treatment?
A) IV metronidazole alone
B) Oral vancomycin OR oral fidaxomicin
C) Oral ciprofloxacin
D) Loperamide for symptom control
E) IV cefepime
Correct Answer: B
Current first-line for non-severe/initial C. diff is oral vancomycin OR oral fidaxomicin (metronidazole has fallen out of first-line due to inferior cure rates). Fidaxomicin has lower recurrence. Stop the inciting antibiotic if possible. Loperamide is contraindicated (toxin trapping).
Q6: Spontaneous bacterial peritonitis
A 58-year-old man with alcoholic cirrhosis and ascites presents with fever and abdominal pain. Paracentesis: PMN 320/ΞΌL, single organism on gram stain (gram-negative rods). What is the diagnosis and most likely organism?
A) Secondary peritonitis β polymicrobial
B) SBP β E. coli
C) CAPD peritonitis β S. epidermidis
D) Tuberculous peritonitis β M. tuberculosis
E) Pancreatitis β sterile
Correct Answer: B
SBP is defined by ascitic PMN β₯250/ΞΌL in the setting of cirrhosis without a surgical source. Most common organism = E. coli, then Klebsiella, then S. pneumoniae. Treat with ceftriaxone Γ 5β7 d; long-term ciprofloxacin or TMP-SMX prophylaxis after first episode or in high-risk patients.
Q7: Amebic liver abscess
A 33-year-old man returns from Mexico with 2 weeks of bloody diarrhea, RUQ pain, and fever. Ultrasound shows a single right-lobe liver lesion. Serology for E. histolytica is positive. Best initial treatment?
A) Percutaneous drainage alone
B) Metronidazole, followed by a luminal agent (paromomycin)
C) Ceftriaxone + metronidazole
D) Surgical resection of the abscess
E) Oral vancomycin
Correct Answer: B
Amebic liver abscess: metronidazole (tissue amebicide) FIRST, then a luminal agent (paromomycin or iodoquinol) to clear intraluminal cysts and prevent relapse. Drainage usually not required. Classic "anchovy paste" abscess contents, right lobe, history of travel to endemic area.
Q8: H. pylori eradication
A 45-year-old with a duodenal ulcer tests positive for H. pylori on urea breath testing. Which of the following is a standard first-line triple-therapy regimen?
A) Amoxicillin + metronidazole + bismuth (no PPI)
B) PPI + clarithromycin + amoxicillin
C) PPI alone for 8 weeks
D) PPI + tetracycline (no other antibiotic)
E) H2 blocker + clarithromycin monotherapy
Correct Answer: B
Classic triple therapy: PPI + clarithromycin + amoxicillin for 14 days. Quadruple therapy (PPI + bismuth + metronidazole + tetracycline) is preferred in areas of high clarithromycin resistance or penicillin allergy. Confirm eradication β₯4 weeks after finishing antibiotics with urea breath test or stool antigen.