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BTS CPI study materials
BTS CPI study resources

BTS CPI: Microbiology, Pathogenicity, and Transmission of Infectious Agents

BTS CPI · Work through your exam, one topic at a time.

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Microbiology, Pathogenicity, and Transmission of Infectious Agents

The microbiology and transmission domain tests whether you can connect a pathogen's biology to the precautions and outbreak controls it requires. Questions often ask about colonization versus infection, the chain of infection, mode of transmission, infectious dose, incubation, and why one organism needs soap and water while another needs an N95.

Core rules

  • Colonization means the organism is present without signs or symptoms; infection means tissue invasion with clinical findings. Colonized patients can still transmit (MRSA nares, C. difficile spores) and may need contact precautions.
  • The chain of infection: infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host. Break one link; hand hygiene and PPE break transmission.
  • Bacteria: gram-positive/negative, cocci/bacilli, anaerobes, spores. Virulence factors include adhesins, capsules, toxins (endotoxin, exotoxins), and enzymes. C. difficile produces toxins A and B; spores survive alcohol.
  • Viruses: obligate intracellular; RNA/DNA, enveloped or non-enveloped. Non-enveloped viruses (norovirus, rotavirus, poliovirus) are more resistant to alcohol and require soap and water plus bleach.
  • Fungi: yeasts (Candida), molds (Aspergillus), dimorphic. Parasites: protozoa, helminths; fecal-oral or vector transmission.
  • Prions are abnormal proteins, not organisms; resistant to standard sterilization; special reprocessing.
  • Modes: contact (direct/indirect), droplet (large droplets, usually up to 6 feet), airborne (small particles, remain suspended, need AIIR/N95), common vehicle, vector-borne.
  • Incubation and infectious dose inform exposure follow-up. Norovirus has low infectious dose and short incubation; TB has airborne transmission and long incubation.

Worked example

A patient with acute diarrhea and recent antibiotics has a positive C. difficile test. The patient is on contact precautions but the aide uses alcohol hand rub after touching the bed rail. Reasoning: C. difficile spores are not killed by alcohol; use soap and water after contact and when leaving the room, gown and gloves, and a sporicidal disinfectant. The test alone does not prove disease-assay and symptoms should be assessed-but precautions are warranted for symptomatic infection.

Traps

Do not use alcohol for C. difficile or norovirus. Do not apply airborne precautions to influenza (droplet). Do not assume a positive culture means infection.