Introduction to Fungi, Viruses, Disinfectants & Sterility Testing
This unit completes the microbiology landscape by covering Fungi and Viruses, then transitions into the practical pharmaceutical application: how we control microorganisms (disinfectants/antiseptics) and the regulatory sterility testing required for all sterile pharmaceutical products.
Syllabus & Topics
- 1Fungi – Morphology: Eukaryotic, non-photosynthetic, heterotrophic organisms. Cell wall: Chitin (not peptidoglycan). Two types: Yeasts (unicellular, oval, reproduce by budding, e.g., Saccharomyces cerevisiae, Candida albicans) and Molds/Filamentous fungi (multicellular, hyphae and mycelium, e.g., Aspergillus, Penicillium, Rhizopus).
- 2Fungi – Classification: Ascomycetes (Saccharomyces, Aspergillus), Basidiomycetes (Mushrooms), Zygomycetes (Rhizopus), Deuteromycetes/Fungi Imperfecti (Candida, Penicillium – no known sexual stage).
- 3Fungi – Reproduction: Asexual (budding, fission, spore formation: conidiospores, arthrospores, sporangiospores); Sexual (ascospores, basidiospores, zygospores).
- 4Fungi – Cultivation: Sabouraud’s Dextrose Agar (SDA) – selective for fungi (low pH 5.6, antibiotics to suppress bacteria); 25-30°C.
- 5Viruses – Morphology: Acellular; obligate intracellular parasites; contain only ONE type of nucleic acid (DNA or RNA) surrounded by Capsid (protein coat). Capsid + Nucleic acid = Nucleocapsid. Some viruses have Envelope (lipid bilayer from host membrane).
- 6Viruses – Classification: By nucleic acid type (DNA or RNA, ss or ds), by capsid symmetry (icosahedral, helical, complex), by presence of envelope.
- 7Viruses – Replication: Lytic cycle (5 steps: Attachment → Penetration → Uncoating → Biosynthesis → Assembly → Release/Lysis of host). Lysogenic cycle (viral DNA integrates into host genome as prophage; can be induced later).
- 8Disinfectants – Classification and Mode of Action:
- 9Phenolics (Phenol, Cresol, Lysol): Disrupt cell membrane, denature proteins.
- 10Alcohols (Ethanol 70%, Isopropanol 70%): Denature proteins, disrupt cell membrane. NOT sporicidal.
- 11Aldehydes (Formaldehyde, Glutaraldehyde): Alkylating agents, cross-link proteins and nucleic acids. Sporicidal.
- 12Halogens (Chlorine: sodium hypochlorite; Iodine: Povidone-iodine): Oxidizing agents, damage proteins and nucleic acids.
- 13Quaternary Ammonium Compounds (QACs – Benzalkonium chloride): Cationic surfactants, disrupt cell membrane. NOT effective against Pseudomonas, spores.
- 14Heavy Metals (Mercury, Silver): Denature proteins by binding to sulfhydryl groups.
- 15Biguanides (Chlorhexidine): Disrupts cell membrane.
- 16Factors Influencing Disinfection: Concentration, contact time, temperature, pH, organic matter (blood, pus – inactivates disinfectant), number and type of organisms (spores > vegetative, Mycobacteria > Gram+).
- 17Evaluation of Disinfectants: Phenol Coefficient (PC) = MIC of phenol / MIC of disinfectant. PC > 1 = more effective than phenol. Rideal-Walker test. Use-dilution test (CDC method).
- 18Antiseptics vs Disinfectants: Antiseptics used on LIVING TISSUE (skin). Disinfectants used on NON-LIVING surfaces.
- 19Bacteriostatic (inhibits growth) vs Bactericidal (kills bacteria). Minimum Inhibitory Concentration (MIC) – bacteriostatic.
- 20Sterility Testing of Pharmaceutical Products (IP/BP/USP): Two methods: (1) Membrane Filtration Method: For products that can be filtered (most liquids). Product filtered through 0.45 µm membrane; membrane incubated in Fluid Thioglycollate Medium (FTM) for bacteria and Soybean Casein Digest (SCDA) broth for fungi. (2) Direct Inoculation Method: For non-filterable products (oils, ointments). Direct addition of product to media. Incubation: FTM 30-35°C for 14 days; SCDA 20-25°C for 14 days. Interpretation: No turbidity = sterile.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What is the Difference Between Yeasts and Molds?
Yeasts are unicellular fungi, usually oval in shape, that reproduce by budding, and their colonies resemble bacteria on agar. Examples include Saccharomyces cerevisiae (used in fermentation) and Candida albicans (an opportunistic pathogen). Molds are multicellular fungi that grow as hyphae (filamentous structures) forming a mycelium and produce spores. Examples include Aspergillus niger and Penicillium chrysogenum (source of penicillin).
Q2. What is the Lytic vs Lysogenic Cycle of Viruses?
In the lytic cycle, the virus infects the host cell, replicates, and causes the host cell to burst (lysis), releasing new virions. In the lysogenic cycle, viral DNA integrates into the host genome as a prophage (or provirus), replicates along with the host without causing immediate harm, and can later be induced by stress to enter the lytic cycle. Examples include Human immunodeficiency virus (provirus stage) and Bacteriophage lambda.
Q3. What is the Phenol Coefficient?
Phenol Coefficient (PC) is defined as: PC = (Minimum dilution of phenol that kills in 10 minutes but not in 5 minutes) / (Minimum dilution of test disinfectant). If PC > 1, the test disinfectant is more effective than phenol; if PC < 1, it is less effective. For example, the PC of Lysol is 5, meaning it is five times more effective than phenol under test conditions.
Q4. What Media Are Used in Sterility Testing and Why?
Two media are used as per pharmacopoeial standards: (1) Fluid Thioglycollate Medium (FTM), incubated at 30–35°C, supports growth of aerobic and anaerobic bacteria because thioglycollate reduces oxygen; and (2) Soybean Casein Digest Agar (SCDA), also known as Tryptic Soy Broth, incubated at 20–25°C, supports aerobic bacteria and fungi. Both media are incubated for a minimum of 14 days.
Q5. Why is Chlorhexidine Preferred Over QACs for Surgical Hand Washing?
Chlorhexidine is preferred because it has broad-spectrum activity against Gram-positive, Gram-negative bacteria and yeasts, shows a persistent effect by binding to skin proteins and remaining active for hours, has low skin toxicity, and remains active in the presence of organic matter. In contrast, quaternary ammonium compounds (QACs) are less effective against Gram-negative bacteria such as Pseudomonas aeruginosa and are inactivated by organic matter and anionic soaps.