Overview
This lecture covers key methods and terms involved in the control of microbial growth, including physical and chemical techniques, mechanisms of action, and factors affecting microbial control effectiveness.
Microbial Control Terminology
- Sepsis refers to bacterial contamination.
- Asepsis is the absence of significant contamination; aseptic technique prevents cross-contamination.
- Sterilization destroys all microbial life.
- Commercial sterilization targets botulinum endospores in canned goods.
- Disinfection destroys harmful microorganisms but not all life.
- Antisepsis destroys harmful microbes on living tissues.
- Degerming is the mechanical removal of microbes (e.g., hand washing).
- Sanitization lowers microbial counts to safe levels.
- Biocides/germicides completely kill microbes.
- Bacteriostasis inhibits, but does not kill, bacteria.
- Bactericidal treatments kill bacteria.
Effectiveness of Microbial Control
- Effectiveness depends on number of microbes, environmental conditions (organic matter, temperature, biofilms), time of exposure, and microbial characteristics.
- Microbial death occurs at an exponential rate, so proper contact time is crucial for disinfectants.
- Zone of inhibition in disk diffusion shows effectiveness of antimicrobial agents.
Mechanisms of Action
- Antimicrobials may damage the cell membrane, denature proteins/enzymes, or disrupt nucleic acids (DNA/RNA).
Physical Methods of Microbial Control
- Heat: Denatures enzymes; includes moist (autoclave: 121°C, 15 psi, 15 min) and dry heat (incineration).
- Thermal death point: Lowest temperature to kill all cells in 10 min.
- Thermal death time: Minimum time to kill all cells at a set temperature.
- Decimal reduction time (DRT): Time to kill 90% of a population at a given temperature.
- Pasteurization: 72°C for 15 seconds; reduces pathogens, does not sterilize.
- Filtration: Removes microbes from liquids/air; HEPA and membrane filters.
- Low temperature: Bacteriostatic effect (refrigeration, freezing).
- Desiccation: Removes water, inhibiting metabolism, mainly bacteriostatic.
- Osmotic pressure: High salt/sugar creates hypertonic environments causing plasmolysis.
- Radiation: Ionizing/non-ionizing (UV) damages DNA; microwaves kill via heat.
Chemical Methods of Control
- Effectiveness influenced by concentration, organic matter, pH, and contact time.
- Disk diffusion method: Evaluates chemical agent efficacy using zones of inhibition.
- Phenols/phenolics: Disrupt plasma membrane.
- Bisphenols: (e.g., triclosan) disrupt membranes.
- Biguanides (chlorhexidine): Disrupt membranes, used in surgical scrubs.
- Halogens (iodine, chlorine): Impair protein synthesis, disrupt membranes, and oxidize enzymes.
- Alcohols (ethanol, isopropanol): Denature proteins, dissolve membranes; most effective at 70%.
- Heavy metals (silver, copper, zinc): Antimicrobial via oligodynamic action; small amounts needed.
- Surfactants (soaps, detergents): Remove microbes by emulsification and mechanical action.
- Quaternary ammonium compounds (quats): Denature proteins, disrupt membranes.
- Aldehydes: Inactivate proteins, used for preserving specimens.
Microbial Resistance
- Resistance varies; lipid-enveloped viruses and Gram-positives are least resistant, endospores, mycobacteria, and prions are most resistant.
- Different agents have varying effectiveness depending on organism characteristics.
Key Terms & Definitions
- Sterilization — removal/destruction of all microbial life.
- Disinfection — removal of harmful organisms from inanimate objects.
- Antisepsis — removal of harmful organisms from living tissues.
- Bactericidal — kills bacteria.
- Bacteriostatic — inhibits bacterial growth.
- Zone of inhibition — area of no bacterial growth around an antimicrobial agent.
- Thermal death point — lowest temperature to kill all cells in 10 min.
- Decimal reduction time (DRT) — time to kill 90% of microbes at a set temperature.
Action Items / Next Steps
- Review vocabulary terms and their definitions.
- Read textbook sections on chemical and physical microbial control.
- Practice identifying appropriate control methods for different situations.
- Complete any assigned laboratory exercises on disk diffusion and antimicrobial effectiveness.