Microbiology, Pathophysiology, and Immunology of Infectious Diseases
Core concepts
Candidates must connect organism biology to clinical behavior. Know the major bacterial classes (gram-positive cocci and bacilli, gram-negative rods and cocci, anaerobes, atypicals such as Mycoplasma, Chlamydia, Legionella), their typical niches, and their intrinsic resistance patterns (e.g., Enterococcus vs. cephalosporins, Stenotrophomonas vs. carbapenems, Klebsiella intrinsic ampicillin resistance). Understand virulence determinants: capsules (S. pneumoniae, Cryptococcus), toxins (TSST-1, endotoxin/LPS), biofilm formation on catheters and prosthetic material, and intracellular survival (mycobacteria, Listeria, Legionella). Fungal morphology (yeast vs. mold, dimorphism) and viral replication strategies matter for drug selection. Immunology essentials include innate vs. adaptive responses, opsonization, complement, and the specific defects that predict infection: neutropenia (gram-negative rods, Aspergillus), asplenia (encapsulated organisms), complement deficiency (meningococcus, encapsulated bacteria), T-cell defects (PJP, CMV, mycobacteria), and immunoglobulin deficiency (encapsulated bacteria, enteroviruses).
Traps
- Confusing colonization with infection (e.g., Candida in urine, MRSA nasal colonization).
- Assuming any gram-negative rod is a single entity; resistance differs sharply by species.
- Missing that anaerobes require anaerobic coverage (metronidazole, carbapenems, piperacillin-tazobactam) when typical flora are involved.
- Overlooking host immune status when predicting likely pathogens.
Rule of thumb
Match the organism's intrinsic resistance profile to the drug's spectrum before considering acquired resistance. If the species is intrinsically resistant, no susceptibility test changes that fact.
Worked example: A neutropenic patient with fever and a new pulmonary nodule. Likely pathogens include Pseudomonas aeruginosa and mold (e.g., Aspergillus); empiric therapy should cover both, not just gram-positive cocci.