USMLE (Fach) / Microbiology (Lektion)

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  • Staphylococcus aureus Gram-positive, β-hemolytic, catalase ⊕, coagulase ⊕, cocci in clusters.- Protein A (virulence factor) binds Fc-IgG, inhibiting complement activation and phagocytosis.- Forms fibrin clot around self → ...
  • Staphylococcus epidermidis Gram ⊕, catalase ⊕, coagulase ⊝, urease ⊕ cocci in clusters.- Novobiocin sensitive.- Does not ferment mannitol (vs S aureus).- Normal flora of skin; contaminates blood cultures. - Infects prosthetic ...
  • Staphylococcus saprophyticus - Gram ⊕, catalase ⊕, coagulase ⊝, urease ⊕, cocci in clusters.- Novobiocin resistant.- Normal flora of female genital tract and perineum. - Second most common cause of uncomplicated UTI in young ...
  • Streptococcus pneumoniae - Gram ⊕, lancet-shaped diplococci. Encapsulated. IgA protease. - Optochin sensitive.- Quellung-reaction-positive (capsule swelling on microscopy after exposure to capsular antigens) Most common cause ...
  • Viridans group streptococci Gram ⊕, α-hemolytic cocci.- Resistant to optochin (vs S pneumoniae).- Normal flora of the oropharynx. - Streptococcus mutans and S mitis cause dental caries. - S sanguinis makes dextrans that bind ...
  • Streptococcus pyogenes (group A streptococci) Gram ⊕ cocci in chains. - Bacitracin sensitive, β-hemolytic, pyrrolidonyl arylamidase (PYR) ⊕.- Hyaluronic acid capsule and M protein inhibit phagocytosis. Cause:- Pyogenic – pharyngitis, cellulitis, ...
  • Streptococcus agalactiae (group B streptococci) Gram ⊕ cocci, β-hemolytic, bacitracin resistant, colonizes vagina. - Causes pneumonia, meningitis, and sepsis, mainly in babies - Produces CAMP factor, which enlarges the area of hemolysis formed ...
  • Streptococcus bovis Gram ⊕ cocci, γ-hemolysis, grows in bile, but does no growth in 6.5% NaCl. - Colonizes the gut. - S gallolyticus (S bovis type 1) can cause bacteremia and subacute endocarditis and is associated with ...
  • Enterococci Gram ⊕ cocci. Catalase ⊝, PYR ⊕ (like group A streptococcus)- γ-hemolysis. Can grow in 6.5% NaCl and bile (vs Streptococcus bovis).- Enterococci (E faecalis and E faecium) are normal colonic ...
  • Bacillus anthracis Gram ⊕, spore-forming rod that produces anthrax toxin.- Only bacterium with an antiphagocytic polypeptide capsule (contains D-glutamate).- Colonies show halo of projections, sometimes referred to as ...
  • Bacillus cereus - Gram ⊕ rod. Spores are heat-stable.- Grows in heated food that cools down too slowly or is improperly refrigerated (also known as reheated rice syndrome).- Keeping rice warm results in germination ...
  • Corynebacterium diphtheriae Gram-⊕, aerobic rod.- Transmitted by respiratory droplets.  - Causes diphtheria via exotoxin encoded by β-prophage. Potent exotoxin inhibits protein synthesis via ADP-ribosylation of EF-2. Symptoms:- ...
  • Listeria monocytogenes Gram ⊕, catalase ⊕, motile, facultative intracellular rod.- Acquired by ingestion of unpasteurized dairy products and cold deli meats, via transplacental transmission, or by vaginal transmission during ...
  • Leprosy (Hansen disease) - Caused by Mycobacterium leprae, an obligate intracellular, acid-fast bacillus - Likes cold temperatures (infects skin and superficial nerves – "glove and stocking" sensation).- Cannot be cultured.- ...
  • Neisseria Gram ⊝ diplococci. Metabolize glucose and produce IgA proteases. Contain lipooligosaccharides (LOS) with strong endotoxin activity. N gonorrhoeae is often intracellular (within neutrophils). Meningococci- ...
  • Haemophilus influenzae Small gram ⊝ coccobacillary rod. - Aerosol transmission. - Produces IgA protease. - Culture on chocolate agar, which contains factors V (NAD+) and X (hematin) for growth. - Can also be grown with S ...
  • Bordetella pertussis Gram ⊝, obligate aerobic coccobacillus.- Attachment to nasopharyngeal ciliated epithelial cells is via hemagglutinin; pertussis toxin aids in attachment.- Pertussis toxin (A and B component): ADP ribosylation ...
  • Legionella pneumophila Gram ⊝ rod, facultative intracellular, obligate aerobe.- Aerosol transmission from environmental water source habitat (eg, air conditioning systems, hot water tanks). No person-to-person transmission.- ...
  • Pseudomonas aeruginosa Gram ⊝, aerobic, motile. Non-lactose fermenter. Oxidase ⊕.- Grows in water and humid conditions, eg, hot tub, contaminated lens solution.- Has grape-like odor. - Produces pyoverdin and pyocyanin ...
  • E coli Gram-negative rod. Produces β-galactosidase, which breaks down lactose into glucose and galactose. EMB agar – lactose fermenters grow as purple/black colonies. E coli grows colonies with a green sheen. ...
  • Klebsiella Gram ⊝ rod. - Polysaccharide capsule.- Reservoir: human colon and upper respiratory tract. - Mucoid, lactose-fermenting colonies on MacConkey agar.- Dark red "current jelly" sputum (blood/mucus) - ...
  • Campylobacter jejuni Gram ⊝, oxidase ⊕. Comma-shaped rod with polar flagella. Grows at 42°C.- Reservoir: Intestinal tracts of humans, cattle, sheep, dogs, cats, poultry- Transmission: Fecal-oral transmission through ...
  • Clostridium tetani Gram-⊕, spore-forming, obligate anaerobic rods.- Produces tetanospasmin- Ubiquitous (especially animal feces and soil) - Wounds with compromising blood supply create anaerobic conditions that are ideal ...
  • Clostridium botulinum Gram-⊕, spore-forming, obligate anaerobic rod. - Produces a heat-labile toxin that inhibits ACh release at the neuromuscular junction, causing botulism. - In adults, disease is caused by ingestion of ...
  • Clostridium perfringens Gram-⊕, spore-forming, obligate anaerobic rod.- Ubiquitous - Wounds with compromised blood supply create an anaerobic environment → optimal for the proliferation of C perfringens → necrosis progresses ...
  • Clostridium difficile Gram-⊕, spore-forming, obligate anaerobic rod. Produces 2 toxins:- Toxin A, an enterotoxin, binds to the brush border of the gut and alters fluid secretion.- Toxin B, a cytotoxin, causes cytoskeletal ...
  • Vibrio cholerae Gram ⊝, flagellated, oxidase ⊕, grows in alkaline media.- Endemic to developing countries.- Transmitted via ingestion of contaminated water or uncooked food (eg, raw shellfish). - Fecal-oral spread; ...
  • Yersinia enterocolitica Gram ⊝ coccobaccili. Lactose non-fermenter. Oxidase ⊝. Reservoir: ZoonoticTransmission: Pet feces (eg, puppies), contaminated milk, pork- Prominent in cold, northern climates (Michigan, Scandinavia) ...
  • Helicobacter pylori Gram ⊝, curved, flagellated (motile), triple ⊕: catalase ⊕, oxidase ⊕, urease ⊕- Urease produces ammonia, creating an alkaline environment, which helps H pylori survive in acidic mucosa.- ...
  • Spirochetes Spiral-shaped bacteria with axial filaments. - Borrelia: only spirochete visible in light microscopy due to big size; aniline dyes (Wright or Giemsa stain) - Leptospira - Treponema: dark-field microscopy ...
  • Leptospira interrogans Spirochete with hook-shaped ends.- Zoonosis. - Found in water contaminated with animal urine.- Prevalent among surfers and in tropics (eg, Hawaii). Leptospirosis- Flu-like symptoms - Myalgias (classically ...
  • Lyme disease - Caused by Borellia burgdorferi, an anaerobe facultative intracellular spirochete, which is transmitted by the Ixodes deer tick- Natural reservoir is the mouse- Common in northeastern United States ...
  • Syphilis - Caused by spirochete Treponema pallidum Primary syphilis: Painless chancre. VDRL ⊕ in 80%, dark-field microscopy. Secondary syphilis: Maculopapular rash (including palms and soles), condylomata ...
  • Congenital syphilis - facial abnormalities such as rhagades (linear scars at angle of mouth), snuffles (nasal discharge), notched (Hutchinson) teeth, deafness - To prevent, treat mother early in pregnancy, as placental transmission ...
  • VDRL false positives - VDRL detects nonspecific antibody that reacts with beef cardiolipin. - Widely available test for syphilis, quantitative, sensitive but not specific. False-positive results on VDRL with:V: Viral infection ...
  • Jarisch-Herxheimer reaction Flu-like syndrome (fever, chills, headache, myalgia) after antibiotics are started. Due to killed bacteria (usually spirochetes: Borrelia, Leptospira, Treponema) releasing toxins.
  • Gardnerella vaginalis Pleomorphic, gram-variable rods. Catalase and oxidase negative. - Involved in bacterial vaginosis (excessive growth of certain anaerobic bacteria in vagina). - Thin, gray vaginal discharge with a fishy ...
  • Q fever Zoonosis caused by Coxiella burnetti, no arthropod vector - Spores inhaled as aerosols from cattle/sheep amniotic fluid, animal waste Acute Q fever:- Nonfebrile illness with fever lasting >10 days- Fatigue, ...
  • Rickettsial diseases Rocky Mountain spotted fever- Rickettsia rickettsii, vector is Dermacentor (dog tick)- Occurs primarily on the East Coast (esp. North Carolina)- Rash typically starts at wrists and ankles and then spreads ...
  • Chlamydiae Obligate intracellular organisms (cannot make their own ATP) with lack of peptidoglycan (muramic acid) in cell wall. Chlamydia trachomatisTypes A, B, C: chronic infection, blindness due to follicular ...
  • Mycoplasma pneumoniae Pleomorphic, microaerophile, no cell wall (contains sterols for stability). Not seen on Gram stain.- Frequent outbreaks in military recruits and prisons, <30 years old. - Classic cause of "walking" atypical ...
  • Systemic mycoses All can cause pneumonia and can disseminate.All are caused by dimorphic fungi: cold (20°C) = mold; heat (37°C) = yeast. Only exception is Coccidioides, which is a spherule (not yeast in tissue).Systemic ...
  • Candida albicans Dimorphic; forms pseudohyphae and budding yeasts at 20°C, germ tubes (true hyphae) at 37°C serum. Forms pseudohyphae when it invades tissues. This test helps differentiate C albicans from other Candida ...
  • Aspergillus fumigatus Monomorphic septate hyphae that branch at 45°. Produces conidia in radiating chains at the end of conidiophore (like broccoli). - Causes invasive aspergillosis in pre-existing lung cavities, especially ...
  • Cryptococcus neoformans - Opportunistic infection in patients with CD4 <100/mm3.- 5-10 µm round/oval cells with narrow-based budding. Heavily encapsulated yeast with a polysaccharide capsule. Not dimorphic (grows as a yeast ...
  • Pneumocystis jirovecii Causes Pneumocystis pneumonia (PCP), a diffuse interstitial pneumonia.- Yeast-like fungus.- Most infections are asymptomatic.- Immunosuppression (eg, AIDS) predisposes to disease. - Diffuse, bilateral ...
  • Sporothrix schenckii Dimorphic, cigar-shaped budding yeast that grows in branching hyphae with rosettes of conidia; lives on vegetation.- Spores are traumatically introduced into the skin, typically by a thorn ("rose gardener's disease") ...
  • Toxoplasma gondii Protazoa. - Immunocompetent: mononucleosis-like symptoms, ⊝ heterophile antibody test.- Reactivation in AIDS → brain abscesses usually seen as multiple ring-enhancing lesions on MRI.- Congenital ...
  • Trypanosoma cruzi Chagas disease – dilated cardiomyopathy with apical atrophy, megacolon, megaesophagus, achalasia.- Acute stage: Romaña sign (unilateral periorbital swelling). - Predominantly in South America- ...
  • Giardia lamblia Protazoa.Intestinal flagellate that exists in 2 forms: a trophozoite (pathogenic stage) or a cyst (infective stage). - The trophozoite is a bilaterally symmetric, pear-shaped oragnism with multiple flagella ...