USMLE (Fach) / Microbiology - Systems (Lektion)

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USMLE First Aid

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  • Normal flora Skin – S epidermidis Nose – S epidermidis; colonized by S aureus Oropharynx – Viridans group streptococci Dental plaque – S mutans Colon – B fragilis > E coli Vagina – Lactobacillus; ...
  • Bugs causing food-borne illness B cereus – Reheated rice C botulinum – Improperly canned foods (toxins), raw honey (spores) C perfringens – Reheated meat E coli O157:H7 – Undercooked meat L monocytogenes – Deli meats, soft ...
  • Bloody diarrhea Campylobacter – Comma- or S-shaped organisms; growth at 42°C E histolytica – Protozoan; amebic dysentery; liver abscess Enterohemorrhagic E coli – O157:H7; can cause HUS; makes Shiga-like toxin ...
  • Watery diarrhea C difficile – Pseudomembranous colitis; associated with antibiotics and PPIs, occasionally bloody diarrhea C perfringens – Also causes gas gangrene Enterotoxigenic E coli – Travelers' diarrhea; ...
  • Common causes of meningitis Newborn (0-6 months): Group B streptococci, E coli, Listeria Children (6 months-6 years): S pneumoniae, N meningitidis, H influenzae type B, enteroviruses 6-60 years: S pneumoniae, N meningitidis ...
  • Common causes of pneumonia Neonates (<4 weeks): Group B streptococci, E coli Children (4 weeks-18 years): RSV, Mycoplasma, C trachomatis (infants-3 years), C pneumoniae (school-aged children), S pneumoniae Adults (18-40 years): Mycoplasma, ...
  • Causes of pneumonia in special groups Alcoholics – Klebsiella, anaerobes usually due to aspiration (eg, Peptostreptococcus, Fusobacterium, Prevotella, Bacteroides) IV drug users – S pneumoniae, S aureus Aspiration – anaerobes Atypical ...
  • Osteomyelitis Assume if no other information is available – S aureus Sexually active – Neisseria gonorrhoeae (rare), septic arthritis more common Sickle cell disease – Salmonella and S aureus Prosthetic joint ...
  • Urinary tract infections Escherichia coli – Leading cause of UTI. Colonies show strong pink lactose-fermentation on MacConkey agar. Staphylococcus saprophyticus – 2nd leading cause of UTI in sexually active women. Klebsiella ...
  • Common vaginal infections Bacterial vaginosis:- No inflammation- Thin, white discharge with fishy odor- Lab findings: Clue cells, pH >4.5- Treatment: Metronidazole or clindamycin Trichomonas vaginitis:- Inflammation ("strawberry ...
  • Toxoplasma gondii Mode of maternal transmission:- Cat feces- Ingestion of undercooked meat Maternal manifestations:- Usually asymptomatic- Lymphadenopathy (rarely) Neonatal manifestations:- Classic triad: Chorioretinitis, ...
  • Rubella Mode of maternal transmission:- Respiratory droplets Maternal manifestations:- Rash, lymphadenopathy, polyarthritis, polyarthralgia Neonatal manifestations:- Classic triad: Cataract, deafness, congenital ...
  • Cytomegalovirus Mode of maternal transmission:- Sexual contact- Organ transplants Maternal manifestations:- Usually asymptomatic- Mononucleosis-like illness Neonatal manifestations:- Hearing loss- Seizures- Petechial ...
  • HIV Mode of maternal transmission:- Sexual contact- Needlestick Maternal manifestations:- Variable depending on CD4+ cell count Neonatal manifestations:- Recurrent infections- Chronic diarrhea
  • Herpes simplex virus-2 Mode of maternal transmission:- Skin or mucous membrane contact Maternal manifestations:- Usually asymptomatic- Herpetic (vesicular) lesions Neonatal manifestations:- Meningoencephalitis- Herpetic (vesicular) ...
  • Syphilis Mode of maternal transmission:- Sexual contact Maternal manifestations:- Chancre (1°) and disseminated rash (2°) are the two stages likely to result in fetal infection Neonatal manifestations:- Often ...
  • Sexually transmitted infections Chancroid (Haemophilus ducreyi) – Painful genital ulcer with gray to yellow exudate, inguinal adenopathy (often unilateral); can rupture and drain pus. Organisms clump in parallel strands ("school ...
  • Pelvic inflammatory disease Chlamydia trachomatis (subacute, often undiagnosed), Neisseria gonorrhoeae (acute). C trachomatis – most common bacterial STI in the US. Signs:- Cervical motion tenderness- Adnexal tenderness- Purulent ...
  • Nosocomial infections E coli (UTI) and S aureus (wound infection) are the two most common causes. Decubitus ulcers, surgical woulds, drains – S aureus (including MRSA), gram Θ anaerobes (Bacteroides, Prevotella, Fusobacterium) ...
  • Organisms associated with bites Eikenella corrodens: Gram-negative rods - Bleach-like odor- Transmission: Human oropharynx, human bites, or fight injuries- Cellulitis Capnocytophaga canimorsus: Gram-negative, filamentous rods- Transmission: ...
  • HACEK group infections Group of gram-negative fastidious rods- Aggregatibacter aphrophilus (formerly Haemophilus aphrophilus)- Aggregatibacter actinomcetemcomitans- Cardiobacterium hominis- Eikenella corrodens- Kingella kingae ...
  • Subcutaneous granulomas/ulcers/cellulitis Tropical fish enthusiasts; granulomatous lesion – Mycobacterium marinum (fish tank granuloma) Cellulitis following contact with saltwater or oysters – Vibrio vulnificu
  • Malignant pustule Pustule → dark red fluid-filled, tumor-like lesion → necrosis → black eschar surrounded by red margin – Bacillus anthracis Ecthyma gangrenosum – Pseudomonas septicemia
  • Wounds Surgical wounds (clean) – S aureus Surgical wounds (dirty) – S aureus, Enterobacteriaceae, anaerobes Trauma – Clostridium, Enterobacteriaceae, Pseudomonas Shallow puncture wound through tennis ...
  • Animal bites Various – Pasteurella multocida Human bites, fist fight – Eikenella corrodens Dog bites – Capnocytophaga canimorsus Rat bites – Streptobacillus moniliformis, Spirillum minus Cat scratches ...
  • Acute otitis media Red, bulging tympanic membrane, fever 102-103°; pain goes away if drum ruptures or ear tubes are patent: - Streptococcus pneumoniae- H. influenzae (often nontypeable)- Moraxella catarrhalis- RSV- ...
  • Otitis externa Ear pain Often mixed infections:- Staph aureus- Candida albicans- Proteus (water organism)- Pseudomonas (water) Malignant otitis externa: Severe ear pain in diabetic, life threatening – Pseudomonas ...
  • Sore throat Inflammed tonsils/pharynx, which may be purulent and may develop abscesses; cervical lymphadenopathy, fever, stomach upset; sandpaper rash – Streptococcus pyogenes- Rash indicates presence of erythrogenic ...
  • Common cold Rhinitis, sneezing, coughing - Rhinovirus (summer-fall)- Coronavirus (winter-spring)- Adenovirus
  • Eyelid infection Bilateral eye lid swelling, >10% eosinophilia, fever, muscle pain, earlier GI symptoms – Trichinella Unilateral inflammation at bite site often around eye or mouth; travel to Mexico, Central or South ...
  • Conjunctivitis neonate Red itchy eye(s)/pus; onset 2-5 days – Neisseria gonorrhoeae Red itchy eye(s)/pus; onset 5-10 days – Chlamydia trachomatis (serotypes D-K) Neonate with "sticky eye" – Staphylococcus aureus
  • Conjunctivitis (other age groups) Red itchy eye(s), thin exudate; pain, photophobia – Viral pink eye: adenovirus Red eye, pus:- Staphylococcus aureus- Streptococcus pyogenes- Streptococcus pneumoniae- Haemophilus influenzae Red eye, ...
  • Chorioretinitis Neonate or AIDS - Toxoplasma - CMV 
  • Bronchitis Wheezy; infant or child ≤5 years – RSV >5 years – Haemophilus influenzae, Myocplasma pneumoniae, Chlamydophila pneumoniae With cough >2 weeks, afebrile, >9 years – Bordetella pertussis
  • Acute pneumonia with chronic cough with weight loss, ... Over 55, HIV+, or immigrant from developing countries – Mycobacterium tuberculosis Dusty environment with bird or bat fecal contamination  (Missouri chicken farmers), yeasts packed into phagocytic ...
  • Acid fast organisms - Mycobacterium - Nocardia (partially acid fast) - Cryptosporidium oocysts
  • Non-motile gram-positive rods - Corynebacterium diphtheriae - Nocardia - Clostridium perfringens (rest of the pathogenic Clostridia are motile) - Bacillus anthracis
  • Bacteria and fungi with capsules - Streptococcus pneumoniae- Klebsiella pneumoniae- Haemophilus influenzae- Neisseria meningitidis- Pseudomonas aeruginosa- Cryptococcus neoformans (only encapsulated fungal pathogen)- Bordetella pertussis ...
  • Unique morphology/staining Metachromic staining – Corynebacterium  Lancet-shaped diplococci – Pneumococcus Kidney bean-shaped diplococci – Neisseriae Bipolar staining – Yersinia pestis Gulls wings – Campylobacter
  • Metabolism Aerobes:- Mycobacterium- Corynebacterium- Pseudomonas- Bacillus- Nocardia Anaerobes:- Bacteroides- Actinomyces- Clostridium- Fusobacterium - Prevotella- Propionibacterium- Lactobacillus Microaerophilic:- ...
  • Urease positive - Proteus - Helicobacter - Ureaplasma - Nocardia - Cryptococcus
  • Parotitis Risk factors: Decreased salivary flow- Medications (eg, anticholinergics)- Obstruction (eg, calculi, neoplasm)- Dehydration, postsurgical/intubation Microbiology: Staphylococcus aureus, anaerobes Presentation:- ...
  • Stool findings in acute diarrhea Watery diarrhea: Noninflammatory (enterotoxin)- No fecal leukocytes, no red cells- Vibrio cholerae, ETEC, Bacillus cereus, S aureus, some viruses, Giardia lamblia, Cryptosporidium Dysentery or inflammatory ...
  • Gonococcal cervicitis Clinical features:- Purulent or mucopurulent discharge- Friable cervix with easy bleeding (eg, intermenstrual or postcoital bleeding) Diagnosis: - Nucleic acid amplification testing Empiric treatment: ...