OIA1010 CONTROL OF MICROBIAL GROWTH

Terminology

Sanitization: lower microbial count on surface

Degerming: removing microbes from limited area

Cleaning: removing visible contaminants and microbes from surface, but not kill them

Disinfecting: kills or inactivates both bacteria and viruses from surface

Biocide/germicide: killing microbes

Bacteriostasis: inhibits w/o killing microbes

Effectiveness of Treatment

Number of microbes

Environment (organic matter, temperature, biofilms)

TIme of exposure

Microbial Characteristic

Biofilms

Reversible attachment to planktonic cells (secs)

First colonizers irreversibly attached (sec-mins)

Growth & cell division (hours-days)

Production of EPS & formation of water channels (hours-days)

Attachment of secondary colonizers & dispersion of microbes of new sites (days-months)

Microbial Death Curve

Logarithmic plotting (red lines) indicates where rate of kill is the same (different time needed to kill different populations)

Actions of Microbial Control Agents

Alteration of membrane permeability

Damage of proteins & nuclei acids

Physical Method of Microbial Control

Moist Heat Sterilization - denature proteins (autoclave: steam under pressure)

Pasteurization - reduces spoilage organism (63^C for 30 mins; high temp: 72^C for 15 sec; ultrua-high: 140^C <1sec) -> thermoduric organism could survive

Dry Heat Sterilization - kills by oxidation (dry heat, flaming, incineration)

Filtration - HEPA removes microbes > 0.3 micrometer; membrane filtration removes microbe > 0.22 micrometer

Biosafety cabinet

Low Temperature - inhibits microbial growth (lyophilization, high pressure denature protein, osmotic pressure cause plasmolysis, desiccation prevents metabolism)

Radiation - ionizing radiation (X-ray, gamma rays, electron beams) ionize water to release OH radical -> damages DNA //non-ionising radiation (UV)

Chemical Methods of Microbial Control

Principles of Effective Disinfection - concentration disinfectant increases, pH decreases, time increases and organic matter

Use Dilution Test

Disk Diffusion Method

Phenol & Phenolics - damage lipids & proteins of semi-permeable cytoplasmic membrane of microbes (e,g,, Enveloped virus)

Bisphenols - disrupt plasma membrane (e.g., hexachlorophene, triclosan)

Biguanides - disrupt plasma membrane (e.g., chlorhexidine - antiseptic for contact lenses)

Halogens - iodine (denature microbial proteins - alter protein synthesis & membrane -> topical antiseptic) and chlorine (bleach: hypochlorous acid, HOCl; denature microbial enzymes -> oxidising agents)

Alcohols (ethanol, isopropanol) - denature proteins & dissolve lipids (requires water)

Heavy Metals (Ag, Hg, Cu) - release ions that binds to plasma membrane & disrupt integrity of cells (e.g., AgNO3 prevent gonorrheal, ophthalmia)

Ammonium Ions

Surfactant - soap (less good disinfectant but good degerming), acid-anioinc detergents, quaternary ammonium compounds

Aldehydes - inactivate proteins by cross-linking with functional groups (-NH2, -OH, -COOH, -SH) -> for medical equipment

Gaseous Sterilant - ethylene oxide (for food: rice, oat, barley)

Plasma - free radicals destroy microbes (ions act as "bullet" to "shoot" target cells & destroy it)

Peroxygens (oxidising agents) - for contaminated surface (e.g., O3, H2O2, practice acid)

Decreasing Order of Resistance of Microorganism to Chemical Biocides