Microbiology: Classification, Growth, and Antimicrobial Targets
Microbiology questions reward a few organizing frameworks. First, classify: bacteria (prokaryotic, peptidoglycan cell wall), viruses (obligate intracellular, capsid plus genome, no own metabolism), fungi (eukaryotic, ergosterol membranes), and parasites. For bacteria, the Gram stain separates thick peptidoglycan, teichoic-acid gram-positives (purple) from thin peptidoglycan with an outer membrane and lipopolysaccharide gram-negatives (pink). Shape, oxygen requirement, spore formation, and hemolysis patterns add detail.
Growth follows lag, log, stationary, and death phases; antibiotics act best in log phase. Know bacteriostatic versus bactericidal, and the major targets: cell wall (beta-lactams, vancomycin), ribosome (aminoglycosides, macrolides, tetracyclines), DNA gyrase (fluoroquinolones), folate synthesis (sulfonamides). Sterilization kills all organisms including spores; disinfection reduces pathogens on surfaces; antisepsis applies to living tissue.
Traps: assuming antibiotics treat viruses; confusing colonization with infection; forgetting that gram-negative outer membrane blocks many drugs; and mixing up the replication steps of DNA and RNA viruses.
Worked example: a sputum Gram stain shows purple cocci in clusters. Purple means gram-positive; clusters suggest Staphylococcus. That narrows empiric choices toward a drug active against gram-positive cell walls, such as a beta-lactam, rather than an aminoglycoside.
Rule of thumb: first ask what kind of organism it is, then what structure or pathway the drug targets, then whether the drug is static or cidal. That sequence solves most microbiology items.