CIC – Certification in Infection Control

Microbiology is the foundation of infection prevention practice. Knowing the structure, behavior, and vulnerabilities of each class of microorganism lets the infection preventionist (IP) predict how an organism spreads, how long it survives in the environment, and which disinfectant or precaution will control it.

Bacteria are single-celled prokaryotes without a true nucleus. Their peptidoglycan cell wall determines Gram-stain behavior. Gram-positive bacteria have a thick peptidoglycan layer that retains crystal violet and appear purple. Gram-negative bacteria have a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharide (endotoxin) and appear pink after counterstaining. The outer membrane limits antibiotic entry, which helps explain the intrinsic resistance of many gram-negative organisms.

Bacterial shapes include cocci (spheres), bacilli (rods), and spirilla (spirals). Cocci arranged in clusters suggest staphylococci, chains suggest streptococci, and pairs suggest diplococci such as Neisseria or Streptococcus pneumoniae. Bacteria reproduce by binary fission, with generation times from about 20 minutes to many hours.

Some bacteria form endospores under adverse conditions, including Clostridioides difficile, Clostridium perfringens, and Bacillus species. Spores resist heat, drying, radiation, and most chemical disinfectants and can remain viable on surfaces for months, which is why sporicidal disinfection and soap-and-water hand hygiene matter for C. difficile.

Viruses are obligate intracellular parasites made of DNA or RNA inside a protein capsid. Some carry an outer lipid envelope from the host cell membrane. Enveloped viruses (influenza, SARS-CoV-2, HIV, hepatitis B) are generally easier to inactivate with disinfectants and alcohol because damaging the envelope inactivates the virus. Non-enveloped viruses such as norovirus, adenovirus, and hepatitis A persist longer on surfaces and resist alcohol-based products.

Viral replication follows attachment to specific host receptors, penetration, uncoating, replication of genetic material, synthesis of viral proteins, assembly, and release. Receptor specificity explains tissue tropism, and each step is a potential target for antiviral therapy.

Fungi are eukaryotes that grow as yeasts (single cells) or molds (multicellular). Dimorphic fungi grow as yeast at body temperature and as mold in the environment. Candida species are the most common healthcare fungal pathogens, and Candida auris is a multidrug-resistant yeast that persists on surfaces and spreads in healthcare facilities. Aspergillus releases airborne spores that cause invasive disease in severely immunocompromised patients, especially during construction.

Parasites include protozoa and helminths. Protozoa are single-celled eukaryotes; Cryptosporidium and Giardia form environmentally resistant cysts and are waterborne concerns. Helminths are multicellular worms that are rarely transmitted in healthcare facilities.

Prions are misfolded infectious proteins without nucleic acid that cause transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease (CJD). They resist conventional sterilization, so instruments contacting high-infectivity tissue (brain, spinal cord, posterior eye) from a patient with known or suspected CJD require special reprocessing protocols.