Microbiology and Public Health
Core concepts
Microbiology and public health combines organism biology with prevention and epidemiology. Know bacterial structure, Gram stain behavior, growth requirements, culture characteristics, and major virulence factors. Gram-positive bacteria have thick peptidoglycan; Gram-negative bacteria have an outer membrane containing lipopolysaccharide. Understand viral replication, fungal morphology, and parasitic life cycles at a survey level. Sterilization destroys all microorganisms, including spores; disinfection reduces pathogens on inanimate surfaces. Public health questions test incidence, prevalence, sensitivity, specificity, predictive values, outbreak investigation, immunization, and herd immunity.
Traps
Incidence counts new cases; prevalence counts all existing cases. Sensitivity detects disease among diseased; specificity excludes disease among nondiseased. A highly sensitive test is useful for screening because few cases are missed. Do not confuse sterilization with disinfection. Gram-negative endotoxin is lipid A of lipopolysaccharide, not an exotoxin.
Worked example and rule of thumb
If a test is positive in 90 of 100 diseased people, sensitivity is 90 percent. If it is negative in 180 of 200 nondiseased people, specificity is 90 percent. Rule: screening favors high sensitivity; confirmatory testing favors high specificity.
Exam focus
Pair organism features with classic syndromes, and pair epidemiologic measures with their definitions. Public health questions reward precise denominator thinking.