USMLE STEP 1 MICROBIOLOGY Questions and Answers with Complete Solutions 2024

USMLE STEP 1 MICROBIOLOGY Questions and Answers with Complete Solutions 2024 What is a special cellular component of fungal membrane targeted in anti-fungal medication? - ANSErgosterol A major sterol What is the special component of bacterial cell wall? - ANSPeptidoglycan Where are the normal flora found? Contributions to health? Causes of infection? - ANSLocation: - found on body surfaces contiguous with outside environment (GI, etc.) - Semipermanent (dependent on life-changes) Contributions to health: - Protective functions by maintaining a certain pH other microorganisms cannot proliferate - Nutritional: synthesizing vitamin K + B Causes of infection: - "Getting lost" (E. coli to urethra causing UTI) - Overgrowth due to immunocompromised state Are there normal flora in the blood or organs? - ANSNo Sterile environment What is a medically important normal flora found in the nose? - ANSStaphylococcus aureus What are the pathogenicity mechanisms? (8) - ANS1. Colonization (adherence) 2. Avoiding immediate destruction by host defense sys. 3. "Hunting + Gathering" needed nutrients 4. Antigenic variation 5. Ability to survive intracellularly 6. Type III secretion systems 7. Inflammation of Immune-mediated damage 8. Physical damage What are the types of colonization? - ANS1. Adherence to cell surface - *Pili*/fimbriae: primary mechanism in most G- bacteria (E. coli) - Teichoic acids: G+ bacteria (acts as an antigenic determinant) - Adhesins - IgA proteases 2. Partial adherence - biofilms (S. epidermidis, S. mutans) (Biofilms are densely packed microorganisms forming a protective 'film' by doing so; "cops standing outside of general admission; cannot get to people within general admission area doing illegal things") What are the types of mechanism used in avoiding immediate destruction by host defense system? - ANS1. Anti-phagocytic surface components (inhibit phagocytosis): - *Capsules*/slim layers: "bar of soap slippery" kj: "Some Killers Have Pretty Nice Capsules Bro" S: Streptococcus pneumoniae K: Klebsiella penumoniae H: Haemophilus influenzae type B P: Pseudomonas aeruginosa N: Neisseria meningitidis C: Cryptococcus neoformans B: Bordetella pertussis - Streptococcus pyogenes: M protein - Neisseria gonorrhoeae: Pili - Staphylococcus aureus: A protein (binds IgG-Fc portion) "MAP" 2. IgA proteases 3. Siderophores: steal (chelate) iron What are antigenic variation mechanisms and microorganisms? - ANS- Changing surface antigens to avoid immune destruction - N. gonorrhoeae: pili + outer membrane proteins - Trypanosoma brucei rhodesiense + T. b. gambiense: phase variation - Enterobacteria: capsular and flagellar antigens may or may not be expressed What are bacteria with abilities to survive intracellularly