Control of microbial growth Some terms to start with.

terms Biocide: kills biologicals Germicide: Kills bacteria Bacteriostasis: stops growth Asepsis: absence of growth

Some terms. Sterilization: destruction of all life Disinfecting: destruction of vegetative pathogens Antisepsis: disinfection of living tissue Sepsis: refers to microbial contamination. Aseptic surgery techniques prevent microbial contamination of wounds.

Factors that influence effectiveness Number of microbes Environmental influences Time exposure Microbial characteristics

Bacterial populations die at a constant logarithmic rate. Figure 7.1a

Methods of Control Physical Chemical Biological Why is prevention best?

Physical Controls: Not for use on living organisms Somehow, alter membrane permeability and / or structure of proteins and nucleic acids

The autoclave: Moist heat and pressure 15psi, 121’c, 15 minutes Thermal death point (TDP): Lowest temperature at which all cells in a culture are killed in 10 min. Thermal death time (TDT): Time to kill all cells in a culture Decimal reduction time (DRT): Minutes to kill 90% of a population at a given temperature

Other controls Heat, dry heat takes longer Low temperature Desiccation Osmotic Pressure Radiation –Ionizing –No ionizing (UV)

Radiation damages DNA Ionizing radiation (X rays, gamma rays, electron beams) Nonionizing radiation (UV) (Microwaves kill by heat; not especially antimicrobial)

Filtration Removes Microbes

Chemical Methods of Control May be used on living organisms Factors that may effect disinfecting –Concentration of the disinfectant –What is to be disinfected pH Organic materials –Contact with microbe –time

Testing Use dilution test –Three microbes are used Salmonella choleraesuis Staphylococcus aureus Pseudomonas aeruginosa –Metal carriers dipped and dried –Standard conditions 10 min at 20’C

Disk Diffusion Method

Types of disinfectants Phenol and Phenolics Halogens (iodine, chlorine) bind to aa of enzymes –Betadine iodophor (iodine bound to organic molecules)

Types of Disinfectants Figure 7.7 Phenol Phenolics. Lysol Bisphenols. Hexachlorophene, Triclosan –Disrupt plasma membranes

Alcohol’s usually by protein denaturizing Heavy metals –Oligodynamic action –Denature proteins

Surface active Agents (detergents) SoapDegerming Acid-anionic detergentsSanitizing Quarternary ammonium compounds Cationic detergentsBactericidal, Denature proteins, disrupt plasma membrane Antibiotics –Worry about resistance –Not many used as preservative (Nisin, Natamycin)

Gaseous Chemostaerilizers –Ethylene oxide denatures proteins Resistance to these methods depends on the microbe.

Peroxygens –Oxidizing agents –O 3, H 2 O 2, peracetic acid Types of Disinfectants

Aldehydes –Inactivate proteins by cross-linking with functional groups (–NH 2, –OH, –COOH, — SH) –Glutaraldehyde, formaldehyde Types of Disinfectants

Chemical Food Preservatives –Organic Acids Inhibit metabolism Sorbic acid, benzoic acid, calcium propionate Control molds and bacteria in foods and cosmetics –Nitrite prevents endospore germination –Antibiotics. Nisin and natamycin prevent spoilage of cheese Types of Disinfectants

Resistance to chemical Biocides