Chapter 7The Control of Microbial Growth

TERMINOLOGY

• Sepsis refers to microbial contamination.

• Asepsis is the absence of significant contamination.

• Sterilization: Removal of all microbial life

• Commercial Sterilization:

• Disinfection: Killing of vegetative microbes on nonliving objects

•Antisepsis: Killing of vegetative microbes in/on living tissue

•Degerming: Removal of microbes from a limited area

•Biocide/Germicide: Kills microbes

• Bacteriostasis: Inhibits, does not kill microbes

• Factors influencing death rate are:

• Number of microbes

• Environmental influences (ie. Organic substances, biofilms, temperature, pH)

• Biofilms:

• Time of exposure

• Microbial characteristics

• [antimicrobial agent]

Microbe Death by Antimicrobial Agents

• Alteration of membrane permeability

• Lipids and proteins are altered

• Intracellular substances leak out of cell

• Damage to proteins

• Denaturation of enzymes, etc.

• Loose their structure and function

• Damage to nucleic acids

• Damaged DNA and RNA prevents production of vital proteins

Actions of Microbial Control Agents

• Heat denatures proteins

• Boiling water• (May not kill

spores)

• Autoclave: generates very high pressure and temperature via steam

• Steam must contact surface to kill microbes

Moist Heat

Figure 7.2

• Pasteurization reduces spoilage organisms and pathogens via heating

• Thermoduric (i.e. thermophiles) organisms survive

• Pasteurization treatments

• High-temperature short-time (HTST): 72°C for 15s

• Ultra-high-temperature (UHT): 140°C for 3s

Pasteurization

• Dry Heat Sterilization:

• Direct flaming

• Incineration

• Hot-air sterilization

Dry Heat

Hot-air Autoclave

Equivalent treatments 170˚C, 2 hr 121˚C, 15 min

• Filtration removes microbes

• Low temperature inhibits microbial growth:

• Refrigeration

• Deep freezing

• Desiccation prevents metabolism

• Hypertonicity causes plasmolysis

Other Physical Methods of Microbial Control

• Radiation damages DNA and other organic molecules

• Ionizing radiation (X rays, gamma rays, electron beams)

• Nonionizing radiation (UV)

• Microwaves kill by heat; not especially antimicrobial

Other Physical Methods of Microbial Control

• Evaluating a disinfectant

• Use-dilution test

• 1. Stainless steel cylinders are inoculated with Staphylococcus aureus, Salmonella choleraesuis, Pseudomonas aeruginosa

• 2. Cylinders are dried in an incubator

• 3. Cylinders are placed in test disinfectant

• 4. Cylinders are incubated in broth media

Chemical Methods of Microbial Control

• Disk-diffusion method

Types of Disinfectants/Antiseptics

Figure 7.7

• Phenol

• Phenolics: Modified phenol

• reduced irritation, enhanced antibacterial action

• ie. Lysol

• Bisphenols:

• Hexachlorophene

• Triclosan (2nd link)

• Found in many household items

• Chlorhexidine

• surgical hand scrub

• preoperative patient scrub

• Halogens:

• Iodine: skin disinfectant; water treatment

• Chlorine (ie. Bleach): water, sewage treatment; swimming pools; restaurants, etc.

Types of Disinfectants/Antiseptics

Types of Disinfectants/Antiseptics

Table 7.6

• Alcohols: Ethanol, isopropanol

• May not be appropriate for wound cleaning

• May “seal in” microbes

• Alcohol-based handrubs

• Heavy Metals: ie. Ag, Cu

• Silver compounds: impregnated dressings; burn creams; catheters

• Copper sulfate: kills green algae in reservoirs, pools, ponds

Types of Disinfectants/Antiseptics

• 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 in meats

• Antibiotics. Nisin and natamycin prevent spoilage of cheese

Types of Disinfectants

• Aldehydes:

• Glutaraldehyde:

• sterilizes*

• hospital instrument disinfection

• used for embalming

Types of Disinfectants

• Gaseous Sterilants

• Ethylene oxide:

• Peroxygens

• Ozone = O3: ie. water treament

• Hydrogen peroxide = H2O2: ie. Food packaging; NOT good for open wounds

Types of Disinfectants

• Surface-Active Agents or Surfactants

Types of Disinfectants/Antiseptics

Soap Degerming

Cationic detergents:

Quaternary ammonium compounds = “Quats”

ie. Pseudomonads can grow in Quat solutions; can Norovirus?

Relative Resistances to Antimicrobials

Figure 7.11