Microbial Control, Antimicrobial Activity, and Viruses: Study Guide

Chapter 9: Physical and Chemical Methods of Microbial Control

Overview of Microbial Control

Microbial control involves the use of physical and chemical methods to reduce or eliminate microorganisms from a given environment. These methods are essential in healthcare, laboratory, and industrial settings to prevent infection and contamination.

• Physical Methods: Utilize physical agents such as heat, radiation, and filtration to destroy or remove microbes.

• Chemical Methods: Employ chemical agents to disinfect, sterilize, or sanitize surfaces and materials.

Physical Methods of Microbial Control

• Heat:

  • Moist Heat: Includes boiling, autoclaving (steam under pressure), and pasteurization. Effective at denaturing proteins and destroying cell membranes.

  • Dry Heat: Involves incineration and hot-air ovens. Kills by oxidation of cellular components.

  • Example: Autoclaving at 121°C for 15 minutes is used to sterilize surgical instruments.

• Radiation:

  • Ionizing Radiation: (e.g., gamma rays, X-rays) causes DNA damage, leading to microbial death. Used for sterilizing medical equipment and food.

  • Non-ionizing Radiation: (e.g., UV light) causes thymine dimers in DNA, inhibiting replication. Used for surface and air disinfection.

• Filtration:

  • Physically removes microbes from liquids or air using membrane filters with defined pore sizes.

  • Example: HEPA filters in biosafety cabinets remove airborne microorganisms.

Chemical Methods of Microbial Control

• Disinfectants: Chemicals used on inanimate objects to destroy most microbes (e.g., bleach, phenolics).

• Antiseptics: Chemicals applied to living tissue to reduce infection risk (e.g., alcohol, iodine).

• Sterilants: Chemicals that destroy all forms of microbial life, including spores (e.g., ethylene oxide gas).

• Sanitizers: Reduce microbial numbers to safe levels (e.g., detergents in food industry).

• Example: 70% ethanol is commonly used as an antiseptic for skin disinfection before injections.

Chapter 10: Modes of Activity of Antimicrobials and Their Effects

Antimicrobial Agents: Mechanisms and Selectivity

Antimicrobials are substances that kill or inhibit the growth of microorganisms. Their effectiveness depends on their mode of action and the type of organism targeted.

• Cell Wall Synthesis Inhibitors: Block formation of peptidoglycan in bacteria (e.g., penicillins, cephalosporins). Ineffective against fungi and viruses, as they lack peptidoglycan.

• Protein Synthesis Inhibitors: Target bacterial ribosomes (e.g., tetracyclines, aminoglycosides). Selective for bacteria due to differences in ribosomal structure.

• Nucleic Acid Synthesis Inhibitors: Interfere with DNA/RNA synthesis (e.g., quinolones, rifampin). May affect both bacteria and some viruses.

• Cell Membrane Disruptors: Damage microbial membranes (e.g., polymyxins for bacteria, amphotericin B for fungi). Fungal membranes contain ergosterol, while bacterial membranes do not, explaining selective toxicity.

• Metabolic Pathway Inhibitors: Block essential enzymes (e.g., sulfonamides inhibit folic acid synthesis in bacteria).

Why Drugs Are Selective for Certain Organisms

• Structural Differences: Bacteria, fungi, and viruses have unique cellular structures and metabolic pathways.

• Example: Penicillin is effective against bacteria due to their peptidoglycan cell wall, but not against fungi (which have chitin) or viruses (which lack a cell wall).

• Antifungal Agents: Target ergosterol in fungal membranes (e.g., azoles, polyenes). Ineffective against bacteria and viruses.

• Antiviral Agents: Inhibit viral replication steps (e.g., reverse transcriptase inhibitors for HIV). Ineffective against bacteria and fungi.

Chapter 13: Viruses and Other Infectious Agents

General Characteristics and Structure of Viruses

• Viruses: Acellular infectious agents composed of genetic material (DNA or RNA) enclosed in a protein coat (capsid). Some have an additional lipid envelope.

• Obligate Intracellular Parasites: Require host cells for replication.

• Size: Typically much smaller than bacteria (20–300 nm).

• Example: Influenza virus is an enveloped RNA virus.

Viral Replication Steps

• Attachment: Virus binds to specific receptors on host cell surface.

• Penetration: Entry of viral genome into host cell.

• Synthesis: Host machinery synthesizes viral nucleic acids and proteins.

• Assembly: New viral particles are assembled from synthesized components.

• Release: Mature virions exit the host cell, often destroying it.

Lytic vs Lysogenic Cycles

• Lytic Cycle: Virus replicates rapidly, leading to host cell lysis and release of new virions.

• Lysogenic Cycle: Viral genome integrates into host DNA (prophage) and replicates with host cell without causing immediate lysis. Can later enter lytic cycle.

• Example: Bacteriophage lambda can undergo both cycles in Escherichia coli.

Prions vs Viruses vs Other Infectious Agents

Agent	Genetic Material	Structure	Replication	Diseases
Virus	DNA or RNA	Protein capsid (± envelope)	Requires host cell	Influenza, HIV, measles
Prion	None	Misfolded protein	Induces misfolding of normal proteins	Creutzfeldt-Jakob disease, mad cow disease
Bacteria	DNA	Cell wall, membrane, cytoplasm	Binary fission	Tuberculosis, strep throat
Fungi	DNA	Cell wall (chitin), membrane	Spores, budding	Ringworm, candidiasis

Additional info: Prions are unique in that they lack nucleic acids and cause disease by altering the conformation of normal host proteins, leading to neurodegenerative disorders.