Control of Microbial Growth

Chapter 5

5.1 Approaches to ControlPhysical methods

Heat

Irradiation

Filtration

Mechanical (e.g., washing)

Chemical methods

Disinfectants

Antibiotics

Principles of Control

Sterilization

Disinfectants

Germicides

Antiseptics (topical)

Pasteurization

Decontamination

Sanitation

Preservation

Situational Considerations

Daily life

Hospitals

Microbiology laboratories

Food production facilities

Water treatment facilities

Others

5.2 Selection of an Antimicrobial Procedure

Type of microorganism

Bacterial endospores and Mycobacterium species are heat and chemical resistant

Vegetative cells are susceptible to most disinfectants

Number of microbes initially present

“99.9% effective” - still a lot of live bacteria!

Environmental conditions (dirt, grease, etc.)

Potential risk of infection

Critical instruments - sharps

Semicritical instruments - mucous membranes

Noncritical instruments - intact skin

Composition of the item

Metals are damaged by some disinfectants

Plastics are damaged by irradiation

5.3 Using Heat to Destroy Microorganisms and Viruses

Moist heat

Boiling kills most bacteria and viruses

Pasteurization is effective for many food-borne microbes

Heat to 72° C for 15 sec for most liquids

Other objects can withstand higher temps and durations

Autoclave

High pressure increases boiling temp of water

Typically reach 120° C at 15 psi

Moist heat (cont.)

Commercial canning process

Most serious threat is Clostridium botulinum endospores

Canning facilities use retorts, which are large autoclaves

Prevents spores from germinating inside the can, thus prevents botulism toxin production

Dry heat

Heating items to 200° C or more

5.4 Using Other Physical Methods to Remove or Destroy Microbes

Filtration of fluids

Nylon membranes of defined pour sizes

0.45 µm

0.20 µm

Filters are sterilized by irradiation

Filter out bacteria

Will not filter viruses or prions

Filtration of air

High-efficiency particulate air filters (HEPA)

Used in a variety of settings

Room ventilation

Laminar flow hoods

Powered Air-Purifying Respirator (PAPR)

Radiation

Ionizing

Ultraviolet

Microwaves

High pressure (up to 130,000 psi)

5.5 Using Chemicals to Destroy Microorganisms and Viruses

Potency of germicidal chemical formulations

Sterilants - everything

High-level disinfectants - viruses and all vegetative bacteria

Intermediate-level disinfectants - all vegetative bacteria and most viruses

Low-level disinfectants - all vegetative bacteria and enveloped (membrane-bound) viruses

Selecting the appropriate germicidal chemical

Toxicity

Activity in the presence of organic matter

Residue

Cost and availability

Storage and stability

Environmental risk

Classes of germicidal chemicals

Alcohols - denature proteins, solubilize membranes

Aldehydes - chemically modify proteins and nucleic acids

Glutaraldehyde

Formaldehyde

Orthophthaladehyde

Classes of germicidal chemicals (cont.)

Biguanides adhere to skin and mucous membranes

Chlorhexidines

Surgical scrubbing

Mouthwash

Skin abrasions

Ethylene oxide

Highly oxidative

Used for surgical garbs

Classes of germicidal chemicals (cont.)

Halogens

React with organic molecules to form toxic substances

Chlorine

Iodine

Ozone

Oxidizing agent

Drinking water and wastewater

Classes of germicidal chemicals (cont.)

Peroxygens

Oxidizing agents

Hydrogen peroxide (H2O2)

Peracetic acid (CH3C[=O]OOH)

Phenolic compounds

Dissolve membranes

Denature proteins

Quaternary ammonia compounds

Cationic (+ charge) detergents

Bind to negatively-charged membrane lipids

5.6 Preservation of Perishable Products

Chemical preservatives

Benzoic and sorbic acids - breads, cheeses, juices

Nitrates - inhibit spore germination

Low temperature - reduces biochemical activities of microbes

Reduce availability of water - salting/drying