Control of Microbial Growth

Control of Microbial Growth

Common terminology for microbial control methods

1. Sterilization – destruction of all life forms, including viruses (by steam under pressure or sterilizing gas)
2. Disinfection – generally refers to destruction of vegetative (non-endospore forming) pathogens using physical or chemical methods, usually chemical, i.e. “disinfectants”; associated with treating inert substance or fomite (e.g., disinfect respiratory equipment in hospitals) so “harsher” than antiseptics
3. Antisepsis – disinfection of living tissue using chemical antimicrobials, i.e., “antiseptics”, milder than “disinfectants”
4. Degermation (process of degerming) – reduction of microbes on human skin usually by mechanical removal rather than killing, e.g, degerm around an injection site using alcohol soaked swab
5. Sanitization – lower microbial counts to safe public health levels, minimizing chance of disease transmission; usually done by high-temperature washing (restaurant glassware, china, tableware), washing in a sink followed by a dip in a disinfectant (glassware in a bar) or using antiseptic such as hand sanitizer
6. Biocide or Germicide – antimicrobial agents that aim to destroy microbes, e.g., bactericide, fungicide, virucide
7. Bacteriostasis – prevent bacterial growth
8. Asepsis – absence of significant contamination

Microbial Death Curve – when plotting the number of surviving cells logarithmically against time when heated or treated with antimicrobial chemicals, death rate is usually constant

Physical Methods of Microbial Control

1. Heat – usually denatures microbes’ enzymes

– heat resistance measured by thermal death point (TDP) and thermal death time (TDT)

– may use moist heat (as in an autoclave), pasteurization process (mild heating, e.g., 63o for 30 min or 72o for 15 sec equivalent treatment) or dry (as in direct flaming, hot-air oven sterilization (170o for about 2 hours) or incineration)

2. Low temperature – refrigeration slows down or stops bacterial growth (bacteriostatic effect) whereas slow freezing may kill and is most harmful
3. Filtration – using screen-like material with pores small enough to retain microbes; for heat-sensitive materials
4. Dessication (dehydration) – bacteriostatic effect
5. High osmotic pressure (using high concentrations of salts/sugars) – similar to dessication effect
6. Radiation – use of short-wave high energy radiation to kill microbes; may be ionizing (X-ray, gamma rays) or nonionizing (UV); may damage or destroy DNA

Chemical Methods of Microbial Control

1. use of chemical agents generally either damage cell wall (detergents, alcohol), disrupts plasma membrane (surfactants), denatures/coagulates proteins (alcohols, strong acids, metallic ions) and interfere with cellular processes
2. factors affecting germicidal activity:
3. nature of microbe & material being treated
4. degree of contamination
5. strength & chemical action of germicide and appropriate concentration
6. length of exposure affected by possible interference by presence of organic matter
7. degree of contact with microbes
8. proper pH and temperature conditions

3. grouped based on chemical composition- “disinfectants”
4. phenols & phenolics, bisphenols – disrupts lipidcontaining membranes resulting in leakage of cell contents, e.g., cresol in lysol, triclosan in antibacterials
5. biguanides – disrupts bacterial cell membrane, especially against gram-positive bacteria, e.g., chlorhexidine
6. halogens (F, Br, Cl, I) – strong oxidizing agents
7. alcohols – denatures proteins, e.g., ethyl & isopropyl
8. heavy metals – bind to & inactivate proteins
9. surfactants – e.g., detergents, soaps, acid-anionic sanitizers, quaternary ammonium compounds (“quats” are cationic detergents, note that fibers of gauze & bandages tend to neutralize quats so may not be as effective) – disrupt cell membranes or mechanical removal of microbes
10. chemical food preservatives – many are organic acids, e.g., sorbic acid or may inhibit bacterial enzyme, e.g., sodium nitrite h. aldehydes – strong reducing group, e.g., formaldehyde i. acids & alkalis – extreme pH denatures proteins
11. chemical sterilants – gaseous (ethylene oxide), plasma, supercritical fluids, peroxygens & other forms of oxygen (strong oxidizing agents, highly reactive)

5. antimicrobial drugs (from Chapter 20 of text)– administer drug to destroy infective agent without harming host, i.e., selectively toxic
6. antibacterial drugs (mostly antibiotics)

– block synthesis & repair of cell wall: penicillin, vancomycin, bacitracin, cephalosporins

– block protein synthesis: chloramphenicol, erythromycin, tetracycline, streptomycin

– enzyme inhibitor: sulfa drugs

1. antifungal & antiparasitic drugs – similarities among eukaryotic cells (pathogen & human cells) mean that drugs toxic to pathogens may also harm human hosts
2. antiviral drugs – may block penetration to host cell, block transcription or translation of viral molecules or inhibit assembly & release of mature viral particles

6. many bacterial species have developed resistance to chemical antimicrobials
7. physiological mechanisms: synthesis of enzymes that inactivate the drugs, blocking entry of drugs or modifying some metabolic pathway to minimize harmful effects of drugs
8. genetic recombination events that provided genetic diversity on which natural selection acts & humans acting as natural selection agents resulted in the evolution of microbial resistance to drugs
9. strategies to limit drug resistance by microbes:

– appropriate prescription, completing dosage, administer 2 or more drugs or rotate their use

– develop shorter term, higher dose antimicrobics

– reduce abuse of antimicrobic use, stop use of antibiotics to animal feeds, use vaccines as alternative

Microbial resistance of different microbes to antimicrobial agents

1. Many biocides (such as disinfectants) are more effective against gram-positive bacteria than gram-negative bacteria as a result of the external lipopolysaccharide layer (in the outer membrane of gram-negative cell wall), e.g., Pseudomonas, may even grow in quats; also resistant to many antibiotics due to “porins” in outer membrane of cell wall
2. Mycobacteria (non-endospore forming) exhibit greater than normal resistance to biocides due to presence of waxy, lipidrich component in their gram-positive cell wall (acid-fast bacteria), e.g., Mycobacterium tuberculosis
3. Bacterial endospores as well as cysts of protozoans are affected by relatively few biocides and antibiotics because of their protective coats & by being metabolically inactive.
4. Viruses’ resistance to biocides depend on whether they have envelopes (“plasma membrane-like material” derived from host cell) or not. Lipid-soluble antimicrobials such as alcohols in hand sanitizers will be effective against enveloped viruses but not as effective against non-enveloped viruses; note that SARS-CoV-2 is enveloped.
5. Destruction (“killing”) of prions is problematic still; animals with prions are incarcerated; autoclaved surgical equipment may still have intact prions, so treatment with NaOH is added.