GI Microbiology & Diagnostics

Flashcards for review of GI Microbiology & Diagnostics concepts including bacteriology, virology, mycology, and parasitology.

What agar produces a metallic green sheen with E. coli?

EMB (Eosin Methylene Blue) agar.

What color does E. coli turn on MacConkey agar and why?

Pink — E. coli ferments lactose, lowering pH, which turns the dye pink.

What is the mechanism of diarrhea for ETEC?

Heat-labile toxin (LT) increases cAMP; heat-stable toxin (ST) increases cGMP, resulting in watery traveler's diarrhea.

What are the buzzwords for EHEC and its key rule?

O157:H7. Shiga toxin leads to HUS; bloody diarrhea, no fever, hamburger exposure. Do NOT give antibiotics.

What is the HUS triad?

1. Microangiopathic hemolytic anemia, 2. Thrombocytopenia, 3. Renal failure.

How can Shigella be differentiated from Salmonella?

Shigella is non-motile, H2S negative, requires very low inoculum, and is colorless on MacConkey agar.

What is the treatment for Shigella and what should NOT be given?

Treat with fluoroquinolones or azithromycin. Do NOT give antimotility agents.

What are the key buzzwords for Salmonella typhi?

Rose spots on abdomen, relative bradycardia, high fever.

In a sickle cell patient with osteomyelitis, which organism is likely responsible?

Salmonella.

What are the key diagnostic features of Campylobacter jejuni?

Gram-negative curved rod, microaerophilic, grows at 42°C, oxidase positive.

What are the post-infectious complications of Campylobacter?

Guillain-Barré syndrome and reactive arthritis.

What key features define H. pylori?

Microaerophilic, gram-negative curved rod, urease positive.

What diagnostic tests are used for H. pylori?

Urea breath test, stool antigen test, CLO test, serology.

What is the triple therapy for H. pylori?

PPI + clarithromycin + amoxicillin; or if allergic, metronidazole instead of amoxicillin.

What is the mechanism of diarrhea for Vibrio cholerae?

Cholera toxin increases cAMP leading to massive Cl⁻ secretion.

What unique characteristic does C. difficile have?

Obligate anaerobe, spore-forming gram-positive rod.

What triggers C. difficile infections?

Antibiotics such as clindamycin, fluoroquinolones, and cephalosporins.

What is unique about Yersinia enterocolitica?

Causes pseudoappendicitis, grows at 4°C, and is associated with pork ingestion.

What is the most common anaerobe in the colon?

Bacteroides fragilis.

What distinguishes Proteus mirabilis from other organisms?

Urease positive and swarming behavior on agar.

What is the mechanism for S. aureus food poisoning?

Preformed heat-stable enterotoxin causes vomiting and diarrhea within 1-8 hours.

What are the two toxin forms of Bacillus cereus?

Emetic form (vomiting), and diarrheal form (diarrhea).

What triggers C. perfringens food poisoning?

Reheated meat or gravies due to alpha toxin.

What is a key characteristic of Listeria monocytogenes?

Grows at 4°C and is associated with neonatal meningitis.

What is the mechanism of C. botulinum toxin?

It cleaves SNARE proteins and blocks ACh release at the neuromuscular junction.

What are the characteristics of Rotavirus?

Double-stranded RNA, non-enveloped, and the leading cause of pediatric diarrhea.

What is Norovirus commonly associated with?

Outbreaks in cruise ships or college dormitories.

What is the potential complication of HAV infection?

IgM anti-HAV indicates acute infection with jaundice.

What are the features of enteric adenovirus?

Non-enveloped dsDNA, causes diarrhea in children, and can lead to intussusception.

What is the distinguishing test for Candida albicans?

Germ tube test: positive at 37°C in serum.

What is the diagnostic finding for Candida esophagitis?

White plaques/exudates on endoscopy.

What is the main risk factor for Histoplasma capsulatum?

Exposure to bird or bat droppings.

What defines Aspergillus in the GI context?

Septate hyphae with 45° angle branching, associated with invasive disease in immunocompromised.

What are the key features of Giardia lamblia?

Pear-shaped, binucleate trophozoites with 'falling leaf motility' and foul-smelling diarrhea.

What is the pathognomonic finding for Entamoeba histolytica?

Trophozoites with ingested RBCs.

How does Cryptosporidium parvum present, and what makes it unique?

Causes profuse watery diarrhea in AIDS patients; resistant to chlorine.

What are the differences between Cyclospora and Isospora?

Cyclospora is smaller (8-10 µm) and autofluorescent, whereas Isospora is larger (25-30 µm) and causes eosinophilia.

What is unique about Balantidium coli?

It is the only ciliated protozoan pathogen of humans.

What identifies Ascaris lumbricoides?

Mammillated eggs in stool and Löffler's syndrome during lung migration.

How does hookworm infection progress?

Filariform larvae penetrate skin and cause iron-deficiency anemia.

What unique danger does Strongyloides present?

Autoinfection can lead to hyperinfection in immunocompromised patients.

What unique feature is seen with Trichuris trichiura?

Barrel-shaped eggs with bipolar plugs.

How is Enterobius vermicularis diagnosed?

D-shaped eggs detected by Scotch tape test.

What is the classic presentation for Trichinella spiralis?

Periorbital edema and myalgias from undercooked pork.

What distinguishes Taenia solium from Taenia saginata?

T. solium has hooks and can cause cysticercosis; T. saginata has no hooks.

What is the defining symptom of neurocysticercosis?

Seizures and ring-enhancing lesions on imaging.

What characterizes Diphyllobothrium latum?

Longest tapeworm, associated with vitamin B12 deficiency.

What is the importance of Echinococcus disease identification?

Causes hydatid cysts; anaphylaxis risk on biopsy.

What unique feature is seen in Schistosoma mansoni?

Lateral spine on eggs and liver fibrosis.

How does Clonorchis sinensis present?

Operculated eggs in stool from raw fish exposure.

What is the treatment trap for Fasciola hepatica?

Treat with triclabendazole, not praziquantel.

What does MacConkey agar differentiate?

Selects gram-negatives and differentiates lactose fermenters.

What are TCBS agar results for Vibrio species?

V. cholerae produces yellow colonies, while V. parahaemolyticus produces blue-green.

What do XLD/Hektoen agar results show for Salmonella vs. Shigella?

Salmonella produces black colonies; Shigella is pink/transparent.

What does CCFA agar identify?

C. difficile with yellow colonies.

What does Sabouraud dextrose agar do?

Selects for fungi and reveals Candida colonies.

What are the effects of cAMP-increasing toxins?

Promote massive Cl⁻ secretion and lead to watery diarrhea.

What does the ETEC heat-stable toxin (ST) do?

Activates guanylate cyclase to increase cGMP.

What is the action of Shiga toxin?

Cleaves 60S rRNA, inhibiting protein synthesis and leading to bloody diarrhea.

How does the botulinum toxin affect the nervous system?

Blocks ACh release at the NMJ, leading to descending paralysis.

What distinguishes watery from bloody diarrhea pathogens?

Watery: Vibrio cholerae, Rotavirus; Bloody: Shigella, EHEC.

In cases of eosinophilia and GI symptoms, which infections should be considered?

Tissue-migrating helminths: Ascaris, Hookworm, Strongyloides.

What are rapid identifiers for food exposures?

EHEC from hamburger; Campylobacter from poultry; S. aureus from potato salad.

What is important about microaerophilic GI organisms?

Campylobacter jejuni and H. pylori need specific culture requests.

What are key obligate anaerobes in the GI tract?

B. fragilis, C. difficile, C. perfringens, and C. botulinum.