USMLE I Microbio – Flashcards
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| How does C. tenani's exotoxin work? |
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| Cleaves synaptobrevin, blocking vesicle formation & release of GABA & glycine (inhibitory). |
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| How does C. botulinum's exotoxin work? |
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| Cleaves synaptobrevin, blocking vesicle formation & release of ACh. |
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| What organisms make exotoxin A, shiga toxin, & shiga-like toxin, and how do these work? |
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| Exotoxin A: P. aeruginosa Shiga toxin: Shigella Shiga-like toxin (verotoxin): E. coli They all bind the 60S ribosome subunit & and inhibit protein synth. |
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| How does C. diphtheriae cause dz? |
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| It does not invade! It releases an exotoxin (diphtheria toxin) that binds a cell-surface receptor & inhibits protein synthesis. |
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| How does V. cholerae cause diarrhea? |
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| It does not invade! It causes secretory diarrhea by releasing an exotoxin (cholera toxin) that binds on the gut epithelium & keeps adenylate cyclase active, increasing [cAMP]. |
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| How does E. coli cause secretory diarrhea? |
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| In this case, it does not invade. It releases heat-labile (LT) & heat-stable (ST) exotoxins LT increases [cAMP] & causes crypt cells to secrete more Cl- & villous cells to absorb less Na+. ST increases [cGMP] & causes less cotransport of NaCl into cells. Both result in watery diarrhea. |
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| What are the most common causes of neonatal meningitis? |
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| S. agalactiae (Group B Strep), E. coli, & Listeria. |
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| What are the most common causes of neonatal pneumonia? |
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| Group B Strep, E. coli. |
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| How do you distinguish between bacterial, viral, & fungal meningitis? |
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| Bacterial & fungal will show increased glc & P; bacterial will show PMNs; viral & fungal will show lymphos (but in the former glc & P will be normal). |
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| Top 3 causes of Otitis media & Sinusitis? |
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| S. penumoniae, HiB, & M. catarrhalis. |
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| Describe 4 key features of Moraxella catarrhalis. |
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| 1.) Gram- 2.) Oxidase+ 3.) Diplococci 4.) Aerobic |
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| What is the main cause of Otitis externa? |
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| P. aeruginosa (GN, oxidase+). |
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| What is the main cause of dental caries? |
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| Streptococcus mutans (GN, catalase-) |
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| What is the main cause of biliary tract infection? |
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| E. faecalis (GP, gamma hemolytic, Lancefield Group D strep, hardy growth: in 6.5% NaCl and in bile. Biliary tract infection? Think the one that is hardy and can grow in bile. |
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| Describe the three types of diarrhea caused by protozoa. |
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| (1) Bloody diarrhea; Entamoeba histolytica (2) Fatty diarrhea; Giardia lamblia (3) Watery diarrhea; Cryptosporidium parvum |
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| What is the only protozoan to cause a urogenital infection? |
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| Trichomonas vaginalis! Lacks a cyst form. |
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| What are the two main agents responsible for rare cases of necrotizing fasciitis? |
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| S. pyogenes (Group A strep, beta-hemolytic), and C. perfingens (GN rod) |
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| How is the diagnosis of chlamydia made? |
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| PCR; visualize intracellular inclusions (reticular bodies); iodine stain + (inclusions contain glycogen); Giemsa stain + |
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| What is Fitz-Hugh-Curtis syndrome? |
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| That's the one when chlamydia, gonorrhea, etc. infects the liver capsule. |
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| What's Reiter's syndrome? |
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| Chlamydia can spread, esp. in males, to synovial joints & other organs, causing arthritis & conjunctivitis, esp. in HLA-B27 patients. |
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| What are the GP spore-forming bacteria? |
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| Rods: Clostridium (perfingens [non-motile], botulinum, tetani, difficile [all motile]) & Bacillus (cereus [motile], anthracis [nonmotile]) Beaded/branched filaments: Actinomyces israelii (not acid fast, anaerobe), Nocardia (aerobe, acid fast) (N. asteroides, N. brasiliensis) |
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| Which ABX inhibit cell wall synthesis? |
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| Penicillins & cephalosporins |
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| What is teichoic acid? |
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| An important surface antigen in GP bacteria |
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| What occurs in the periplasmic space of which bacteria? |
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| In the periplasmic space of GN bacteria, beta-lactamases cleave beta-lactams, e.g., penicillin. |
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| What is Toxic Shock Syndrome (TSS)? |
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| What S. aureus releases TSST-1 and it diffuses systemically, this superantigen promotes excessive cytokine release (e.g., IL-1, TNF)-> acute fever, rash, desquamation on palms & soles, hypotensive shock, possible death. |
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| An ASO+ pharygitis can do what that an ASO+ skin infection can't? |
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| S. pyogenes can cause a glomerulonephritis either way, but only Rheumatic fever post-pharyngitis. |
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| What are the five most common pediatric diseases with rash? |
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| Measles (measles virus), Rubella (rubella virus), Scarlet fever (S. pyogenes), Roseola (HHV-6), Erythema infectosum (parvovirus B19) |
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| Evidence of prior infection and 2 of the 5 major Jones criteria are req'd for dx of RF. What are the 5? |
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| Carditis (endo-, myo-, or peri-), Polyarthritis (sensitization to antigen of joints and heart tissue), Sydenham's chorea, Erythema marginatum rash, & SubQ nodules |
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| Which Strep grow in 40% bile? |
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| Group D: E. faecalis (grow in 6.5% NaCl) & S. bovis (susceptible to 6.5% NaCl). |
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| Which bacteria of the group Bacillus are motile? |
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| Bacillus cereus is motile & Bacillus anthracis is nonmotile. Both are spore-forming, aerobic GP rods. |
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| What is the only medically-important bacterium with a protein rather than a polysaccharide capsule? |
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| Bacillus anthracis |
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| What does Clostridium tetani look like and how does its toxin work? |
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| It looks like a tennis racket (GP rod w/ a large terminal spore) & its tetanospasmin blocks the release of the inhibitory NTs GABA & glycine from Renshaw interneurons of SC. |
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| How does botulinum toxin work? |
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| It blocks ACh release at CNs (diplopia, dysarthria, dysphagia), NMJ (symmetric descending paralysis from head to extremities), & postganglionic parasymp nerve endings/peripheral ganglia (dizziness, dry throat, ptosis) |
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| Why shouldn't infants consume honey? |
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| Because it might contain Clostridium botulinum spores. |
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| How does pseudomembranous colitis typically evolve? |
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| Tx w/ amoxicillin/clindamycin --> C. difficile survives by forming spores --> Germinate in an environment with plenty of room --> release toxin A (alters fluid secretion-->watery diarrhea); release toxin B (cytotoxic to epithelial cells-->pseudomembranes. Toxin B present in feces. |
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| What three pathologies can C. perfringens cause, and what makes it special? |
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| It is the only non-motile Clostrium family member. It can cause (1) cellulitis, (2) gas gangrene, & (3) food poisoning (enterotoxin inhibits glc transport & damages epithelium)-- diarrhea but no fever/vomiting! |
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| What are the exceptions to the GP/GN endotoxin thing? |
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| Listeria monocytogenes is the only GP WITH an endotoxin & Bacteroides are the only GN WITHOUT. |
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| I have tumbling motility, I can survive intracellularly AND extracellularly, & I am catalase+. Who am I? |
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| Listeria monocytogenes, & I can cause listeriosis (meningitis & sepsis in immunocompromised neonates). I also survive in poorly-pasteurized/unpasteurized milk. |
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| I have a "Chinese letters" appearance on potassium tellurite culture. Who am I? |
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| Corynebacterium diphtheriae. Don't try to scrape away my pseudomembranes (b/c bleeding & toxin spread might result). I can cause myocarditis, arrhythmia, & polyneuritis. |
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| I form beaded filaments within yellow sulfur granules. Who am I? |
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| Actinomyces israelii. I am part of the normal oral cavity, GI, and female GU flora. I like to form abscesses that spread into ANY contiguous tissue, forming sinus tracts. |
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| What is an oddball cause of a caseating granuloma? |
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| Nocardia asteroides/brasiliensis! Can cause abscesses in kidney & brain in the immunocompromised, & can cause a local foot infection called mycetoma. |
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| How does the lab tell the three GN diplococci apart? |
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| Neisseria meningitidis is a maltose AND glucose oxidizer; Neisseria gonorrhoeae is a glucose oxidizer; Morexella catarrhalis is oxidase+. |
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| Which bug requires factors V & X to grow? |
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| H. influenzae |
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| What is Waterhouse-Friderichsen syndrome? |
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| When N. meningitidis invades systemically, causing necrosis of the adrenals and subsequent adrenal insufficiency. |
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| What is the most common cause of septic arthritis in sexually active individuals? |
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| N. gonorrhea (grows on Thayer-Martin media)! Antigenic variation stymies vaccines & allows for recurrent infection. |
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| How do EHEC & EIEC cause illness? |
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| Shiga-like toxins 1 & 2 inactivate 60s ribosomal subunit --> blocked protein synthesis. |
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| Where do Salmonella typhi (encapsulated) live in an individual who is a carrier? |
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| In the gallbladder, often in gallstones, waiting to to reenter bowel lumen and spread via stool. |
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| What produces struvite kidney stones? |
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| Proteus mirabilis elaborates urease, which creates alkaline urine, a favorable environment for the precipitation of struvite. |
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| Who does "rice water" diarrhea? |
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| Vibrio cholerae (oral rehydration with Na+glucose, to capitalize on cotransporters in the SI). |
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| How P. aeruginosa (blue-green colonies, fruitdy odor) & C. diphtheriae's toxins work? |
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| Ribosylation of host EF2-->inhibition of protein synthesis. |
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| What is the second most common cause of bacterial pneumonia in pts w/ COPD? |
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| Moraxella catarrhalis (GN diplococci, oxidase+, tributyrin hydrolyzer), behind H. influenzae. |
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| A 2-yr-old girl returns to her pediatrician one week after being started on amoxicillin for an ear infection. She has new-onset yellow discharge from her right ear. What is the bug? |
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| M. catarrhalis, normal colonizer of nasopharynx. About 95% produce beta-lactamase, so tx is amoxicillin+clavulanate. |
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| Many cases of Guillain-Barre Syndrome are thought to be complications of infection by which bug? |
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| Campylobacter jejuni (curved GN bacillus, urease-, grows well at 42C) |
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| What are the three most common causes of diarrhea in the world? |
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| C. jejuni, rotavirus, & ETEC. |
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| What common GI flora member makes Vit K for the host? |
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| B. fragilis (only GN w/o typical LPS=no ability to cause DIC/shock!) |
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| What bug poses an especially big risk to asplenic patients? |
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| Hib, b/c encapsulated organisms are typically primarily destroyed in the spleen after opsonization. "Non-typable" H. influenzae (unencapsulated) most commonly cuases pneumonia in COPD pts. |
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| What are the four main causes of atypical pneumonia? |
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| Mycoplasma, Legionella, Chlamydia, & viruses. |
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| How does pertussis toxin work? |
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| It is an AB toxin that ADP-ribosylates & inactivates Gi protein-->uninhibited adenylate cyclase activity-->increased cAMP-->lymphocytosis & decreased phagocytosis. |
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| What can you get from unpasteurized goat cheese that will give you an undulating fever? |
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| Brucellosis (Brucella spp, GN pleomorphic coccobacillus, anaerobic, facultative intracellular). |
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| If you are raising rabbits in Arkansas, Oklahoma, or Missouri, what might you get infected with? |
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| Francisella tularensis (GN, pleomorphic coccobacilli, req's cysteine for growth) most commonly causes tularemia: an ulcer with a black base & LAD. The only 2 live-attenuated vaccines for bacteria are BCG & F. tularensis. |
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| What can cause either cellulitis or osteomyelitis following a cat or dog bite? |
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| Pasteurella multocida (GN pleomorphic coccobacilli, bi-polar staining, oxidase+, catalase+). |
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| What causes cat scratch disease? |
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| Bartonela henselae (GN, pleomorphic coccobacilli), which can cause a variety of symptoms, incl. regional LAD, fever, ocular involvement, HSM, & bacillary angiomatosis (granulomatous infl.). |
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| What TB's means for evading phagocytosis? |
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| Sulfatides in cell wall inhibit phagosome from fusing to lysosome. |
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| What does the Quantiferon Gold assay actually measure? |
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| It measures IFN-gamma released by lymphocytes exposed to M. tb antigens. |
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| What is an easy way to remember TB treatment? |
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| RIPES: Rifampin, INH, Pyrazinamide, Ethambutol, & Streptomycin. If the individual is PPD+, tx w/ INH. |
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| What is a classical clinical finding of tuberculoid leprosy? |
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| Palpable thickened nerves (e.g., posterior tibial, ulnar). Remember than M. leprae likes to grow at cooler sites of the body (e.g., face, extremities). |
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| What are the placenta-crossing ToRCHES organisms? |
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| TOxoplasma gondii, Rubella, CMV, HIV, hErpes, & Syphilis. |
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| What are three ways to diagnose syphilis? |
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| (1) Dark-field microscopy (spirochetes not visible on Gram stain); (2) VDRL (nonspecific) detects reagin antibodies against cardiolipin (false positives with SLE and infectious mono); (2) FTA-ABS (specific) detects anti-treponemal antibodies. |
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| How long does an Ixodes tick need to feed in order to transmit a sufficient inoculum for infection? |
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| To transmit Borrelia burgdorferi, it takes about 24 hours. Check ECG! Can affect heart weeks after initial infection. |
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| Leptospirosis can cause Weil's syndrome, which is...? |
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| Vasculitis w/ hemorrhagic complications, kidney damage w/ RF, & iver damage w/ jaundice. |
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| Pneumonia in a young person + cold hemagglutination='s? |
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| Mycoplasma pneumoniae. Also: "fried-egg appearance" on 2-3 week's culture on Eaton's agar. |
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| What are the two obligate intracellular parasites and what is different about them? |
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| Chlamydiae replicate within inclusion bodies, while Rickettsiae replicate freely in the cytoplasm. (Note that tx w/ doxycycline works b/c doxy enters cells!) |
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| What is the #1 STI in the US and why (probably)? |
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| Chlamydia trachomatis! Probably b/c infections in males are usually clinically silent (but could cause Reiter's syndrome=arthritis+conjunctivitis, esp. in HLA-B27 pts). |
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| What 3 infections should you think of with a palm & sole rash? |
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| (1) Rocky Mountain spotted fever; (2) Syphilis; (3) Coxsackievirus |
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| What is transmitted by the bite of the Dermacentor wood or dog tick? |
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| RMSF (Rickettsia rickettsii), which is more common on the East Coast. The inoculum is sufficient only after 6-10 hours of tick feeding. The rash spreads proximally from the palms & soles. |
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| A patient develops a rash that spreads distally, sparing the palms, soles, & face. What is this? |
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| Epidemic typhus (Rickettsia prowazekii). Louse-borne. Often epidemic during/following wars/natural disasters. |
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| What's special about Coxiella burnetti? |
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| It's actually a Rickettsia, it can survive extracellularly as a spore, & it causes Q fever in individuals exposed to animal products (sheep, cattle, goats). |
