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)Soap DegermingAcid-anionic detergents SanitizingQuarternary 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

– O3, H2O2, peracetic acid

Types of Disinfectants

• Aldehydes– Inactivate proteins by cross-linking with

functional groups (–NH2, –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