USMLE Step 1 First Aid – Microbiology with complete solution

USMLE Step 1 First Aid – Microbiology with complete solution Cell Wall Overall Picture - Compare *Gram (+) to Gram (-)* - answer *Flagellum* on bacteria is composed of what? What is the function? - answerComposition: Proteins Function: motility *Pilus/Fimbria* on bacteria is composed of what? What is the function? - answerComposition: Glycoprotein Function: Mediate adherence of bacteria to cell surface, sex plus during conjugation. *Spore* formation is ONLY possible is what type of bacteria? Why is the formation of spores important/beneficial? What is the composition of a spore? - answerGram Positive Only Function: Resistant to dehydration, heat, and chemicals. Composition: Keratin-like coat; dipicolinic acid; peptidoglycan. *Capsule* will be composed of what on bacteria? What is the benefit of a capsule? - answerFunction: Protects against phagocytosis Composition: Organized, discrete polysaccharide layer (except Bacillus anthracis, which contains d-glutamate) Glycocalyx - answerFunction: Mediates adherance to surfaces, especially foreign surfaces (e.g., indwelling catheters) Composition:Loose network of polysaccharides Periplasm - answerSpace between the cytoplasmic membrane and outer membrane in gram-negative bacteria (peptidoglycan in the middle) Gram Neg only Accumulates components exiting gram neg cells, including hydrolytic enzymes (β- lactamases) Outer membrane (gram negatives) - answerGram Neg Only Function: 1. Endotoxin: Lipid A induces TNF and IL-1; O polysaccharide is antigenic 2. Most OMPs are antigenic Porins: transport across outer membrane Composition: 1. Outer Leaflet: endotoxin lipopolysaccharide [LPS/LOS] 2. Embedded proteins: porins and other outer membrane proteins (OMPs) 3. Inner Leaflet: phospholipids Cell wall and Gram Stain - answerComposition: Peptidoglycan is a sugar backbone with peptide chains cross-linked by transpeptidase Function: Net-like structure give rigid support, protects against osmotic pressure damage. Gram Stain: Thick peptidoglycan layer retains crystal violet = gram + Thin peptidoglycan layer turns red or pink with counterstain = gram - Cytoplasmic Membrane - answerComposition: Phospholipid bilayer sac with embedded proteins (PBP's) and other enzymes. Lipoteichoic acids (gram Pos+ only) extend from membrane exterior. Function: Site of oxidative and transport enzymes PBPs involved in cell wall synthesis Lipoteichoic acids induce TNF and IL-1 Plasmid - answer-Contains a variety of genes for antibiotic resistance, enzymes, and tox