Chapter 9 Controlling Microorganisms

Controlling Microbial Growth Sterilization destroy all microbial life Disinfection reduces number of pathogens pose no danger of disease Decontamination renders contaminated surface safe to handle Antisepsis kills microorganisms on living tissue Microbiocidal killing of microorganisms Microbiostatic inhibiting the growth of microorganisms

Microbial Death Rate Decimal reduction time Heat treatments D-value time required to kill 90% of cells Heat treatments temperature type of microorganism physiological state other substances Figure 9.1

Sterilization Thermal death point Thermal death time lowest temperature kill all microorganisms 10 minutes Thermal death time minimal time liquid suspension at given temperature

Physical Controls Heat Moist Dry boiling autoclave Pasteurization kills most vegetative cells, not sterile autoclave steam under pressure: 15 lbs./in2 at 121oC for 15 min. kills endospores Pasteurization kills certain microorganisms Dry long periods 160oC for 2 hours 171oC for 1 hour

Physical controls Cold Radiation does not kill many die slowly useful in slowing growth Radiation UV light sterilizes by damaging DNA Ionizing: X-rays and gamma rays sterilizes by stripping electrons

Physical controls Filtration Drying Osmotic strength physically removes microorganisms heat-sensitive liquids media, vitamins, antibiotics, hormones air filters (HEPA) Drying not sterilization evaporation lyophilization Osmotic strength salts and sugars damage cells by plasmolysis preserving foods

Chemical controls Antimicrobial agents 1200 germicides Selection chemotherapeutic agents used to treat disease germicides and germistats disinfectants and antiseptics 1200 germicides sold by 330 manufacturers in U.S. Selection damage the tissue or object control the target microorganism(s) purpose of treatment

Testing germicides Paper disc method germicide placed on paper disc disc placed on agar seeded with microorganism after incubation, observe zone of inhibition Figure 9.2

Classes of germicides Phenols and phenolics denature proteins act on lipids disrupt cytoplasmic membrane Examples: Paracresol Lysol Hexachlorophene prescription use Figure 9.3

Classes of germicides Alcohols disrupt lipids denature proteins attack cytoplasmic membrane denature proteins 50-70% solutions more effective increase plasmolysis after membrane is damaged Figure 9.1

Classes of germicides Oxidizing agents Halogens inactivate proteins oxidizes functional groups Halogens iodine: antiseptic betadine, tincture of iodine chlorine: disinfectant Hydrogen peroxide (H2O2) 3% solution-weak antiseptic broken down to water and oxygen bubbling when applied to skin

Classes of germicides Surfactants hydrophobic and hydrophilic parts penetrate cytoplasmic membrane quaternary ammonium salts charged nitrogen with four hydrophobic groups Cepacol Zephiran Figure 9.5

Classes of germicides Alkylating agents inactive proteins Formalin attach short chains of carbons (alkyl groups) Formalin 37% solution of formaldehyde Ethylene oxide gas sterilization Figure 9.1

Food preservation Temperature pH Water Chemicals refrigeration canning Pasteurization 63oC for 30 minutes 72oC for 15 seconds pH vinegar lactic acid fermentation Water drying evaporation freeze-drying salting high osmotic strength Chemicals preservatives