7 Control of Microbial Populations

Sterile: Free of all living organisms and viruses.
Sterilize: Destroy or remove all viruses and life forms.
Bacteriocide/Fungicide: Substances that kill bacteria or fungi.
Bacteriostatic: Substances that prevent bacterial cell growth but do not kill the cells.

Physical and Chemical methods can be used in isolation or together.
Choice of control depends on several factors: the situation, environment, material characteristics, and health & safety requirements.

Mechanical Methods: Barriers prevent entry and/or exit (e.g., lids, cotton wool plugs, masks, gloves).

Membrane: Thin material with specific pore sizes that trap larger particles.
Types of Filtration Include:
- Microfiltration (MF): Pore Size 1.0-0.01 µm; removes bacteria, large viruses, and suspended solids.
- Ultrafiltration (UF): Pore Size 0.01-0.001 µm; removes viruses, proteins, and organic substances.
- Nanofiltration (NF): Pore Size 0.001-0.0001 µm; removes sugars, pesticides, and divalent cations.
- Reverse Osmosis (RO): Pore Size <0.0001 µm; for monovalent salts.

Defined Pore-Size Filters:
- Made of polymers (cellulose acetate, cellulose nitrate, polysulfone).
- Commonly used pore sizes: 0.22µm excludes bacteria and some larger viruses, whereas 0.45µm allows viruses to pass.

Ultraviolet Light: Limited penetration, damages DNA causing mutations and cell death, sterilizes hoods and cabinets, used for water sterilization.
Gamma Rays: From Cobalt-60 isotope, penetrates well, used for sterilizing food and medical devices.

Low Temperature:
- Refrigeration (0-4°C); psychrotrophic bacteria like Listeria may grow.
- Freezing below -18°C; mostly bacteriostatic.
High Temperature:
- Heat effectiveness depends on temperature, time, and moisture.
- Dry heat sterilizes items that should not be wetted (160°C for 1.5-2h).
- Incineration is used for complete oxidation of materials.

Kills microorganisms more effectively than dry heat by coagulating and inactivating cell proteins.
Pasteurization:
- Used for heat-sensitive liquids (63-66°C for 30 minutes or 71°C for 30 seconds).
- Not sterilization; reduces numbers to safe levels (targeting pathogens like E. coli).
- Boiling at 100°C kills vegetative cells but may not kill all endospores.
Autoclaving: 121°C for 15min under pressure; effective against endospores.

Microbial susceptibility varies; spores are more resistant than vegetative cells.
Concentration and presence of organic material affect effectiveness.
Cleaning:
- Combination of physical and chemical processes to reduce microbial load.
Disinfection:
- May be achieved by heat or chemicals targeted at reducing pathogens.

Attack on Proteins: Disrupts 3-D structure of proteins.
Membrane Damage: Leaky membranes cause loss of cytoplasmic components.
Nucleic Acid Damage: Denaturation of DNA and RNA impedes replication and function.

Use of Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) tests.
Time Kill Curve: Measures time needed to kill organisms at varying disinfectant concentrations.

Factors affecting death rate include temperature, pH, disinfectant concentration, and organic load.
Disinfection can achieve a 5-log reduction in microbial numbers (99.999% kill).

Cleaning is necessary before disinfection.
Industrial and hospital settings require appropriate surfaces and implements for effective cleaning and disinfection.

Sporicides (e.g., Ethylene oxide for heat-sensitive equipment) and antiseptics (e.g., Chlorhexidine, alcohol-based products).
Natural antimicrobial substances are modified for enhanced efficacy (e.g., penicillin derivatives).