Unit 5: Controlling microbial growth

Culturing methods

Planktonic growth

Biofilm

Bacteria and fungi live in organized communities

Hard to get rid of

Quorum sensing

Environmental factors

Temperature

pH

Oxygen

Water

Control of microbial growth - overview

Principles

Sterilization

Disinfection

Pasteurization

removal of all microorganisms

elimination of most or all pathogens

heating of food etc to limit pathogens

Decontamination

Sanitization

reduce pathogens to levels considered sfe to handle

reduced microbial population that meets accepted health standards

Preservation

delaying spoilage of foods and other products

Methods

Daily life

Water and soap

Cooking

Refrigeration

Healthcare facilities

Sterilized instruments

Nosocomial infections

Microbiology laboratories

Aseptic techniques

Biosafety levels - different laboratories

Food production facilities

Heat, pasteurization

Irradiation

Chemical additives

Water treatment facilities

Chlorine

Selection of an antimicrobial procedure

Microbes

Bacterial endospores

Protozoan cysts and oocysts

Mycobacterium species

resistant to heat

killed by heat

Pseudomonas

resistant to some disinfections

Naked viruses

Lack lipid envelope, susceptible to chlorine

Environmental conditions

pH, temperature

Risk for infection

Different levels of cleanness

Physical methods of controlling microbial growth

Pasteurization

Sterilization with pressured steam

Canning

Moist heat is more effective than dry heat

Autoclave

Industrial-sized autoclave called retort

Destroys botulism pathogens

dry heat required longer times and higher temperatures

Filtration

Membrane filters

Depth filters

0.2 micrometer pore size or smaller

Liquid is sucked or pushed through material

Thick filtering material

Electrical charges on filter walls trap cells

Radiation

Gamma irradiation - ionizing radiation

Sterilizes heat-sensitive materials and can be used after packing

Removes electrons from atoms

Effectiveness depends on dose applied

Used for foods

Ultraviolet radiation

Damages DNA

Does not deeply penetrate

Effective when used at close range against expected microbes

Chemical methods of controlling microbial growth

Chemical formulants

Sterilant

High-level disinfectant

Intermediate-level disinfectant

Low-level disinfectant

Destroys all microbes

Destroys all viruses, vegetative cells

Destroys vegetative bacteria, mycobacteria, fungi, most viruses

Destroys fungi, vegetative bacteria, enveloped virus

Factors to consider when choosing a germicidal chemical

Toxicity

Activity

Compatibility

Residues

Cost and availability

Storage and stability

Environmental risk

Germicide classes

Alcohols

Denaturates essential proteins and damage lipid membranes

More effective when mixed in water

Aldehyde

Sterilants, toxic

Damage DNA

Biguanides

Chlorhexidine - low toxicity

Ethylene oxide gas

Penetrates fabrics etc

Reacts with proteins

Halogens

Chlorine, iodine

Ozone

Peroxygens

Phenolic compounds

Unstable, used in drinking water

Leave no residue, ex hydrogen peroxide

Strong irritant, denature proteins

Quaternary ammonium compounds

Cationic detergents, disinfect food prep surfaces