Biology of common and important pathogens

Classification

Pathogen classification (bacteria, viruses, fungi, parasites) and, within bacteria, Gram stain characteristics and growth requirements, underpins rational empirical antimicrobial selection before organism-specific culture/susceptibility results are available
Foundational microbiology knowledge applied throughout clinical infectious diseases practice, from interpreting a Gram stain result to selecting empirical antibiotic cover based on the likely organism for a given clinical syndrome

Gram-positive organisms have a thick peptidoglycan cell wall retaining crystal violet stain (purple); Gram-negative organisms have a thinner peptidoglycan layer with an outer lipopolysaccharide membrane (pink/red counterstain) - this structural difference underlies both differing antibiotic susceptibility patterns and the specific endotoxin (lipopolysaccharide)-driven septic physiology seen with Gram-negative sepsis
Obligate intracellular organisms (e.g. Chlamydia, Rickettsia) cannot be cultured on standard media, requiring specific diagnostic approaches (cell culture, PCR, serology) rather than routine bacterial culture

Gram stain provides a same-day presumptive organism category, directing initial empirical therapy before culture/susceptibility results (which take longer) are available
Growth requirements (aerobic vs anaerobic, specific nutrient/temperature needs) determine appropriate culture technique and explain why some organisms require specific request/extended incubation (e.g. anaerobic culture, mycobacterial culture)

Empirical antibiotic selection is guided by the likely organism category for the clinical syndrome (e.g. Gram-negative cover for suspected urinary sepsis, anaerobic cover for suspected intra-abdominal source) pending culture confirmation, then rationalised once definitive results are available

Gram-negative sepsis and endotoxin-driven shock physiology, obligate intracellular organisms requiring non-culture diagnostic methods, anaerobic infection sources (intra-abdominal, dental, aspiration)
Gram-negative bacterial cell walls contain lipopolysaccharide (endotoxin), which directly contributes to the systemic inflammatory response and shock physiology characteristic of Gram-negative sepsis - this structural feature explains clinically important differences in presentation and severity compared with Gram-positive sepsis

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