Control of Microbial Growth Chapter 5

5.1 Approaches to Control Physical 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 (H 2 O 2 ) Peracetic acid (CH 3 C[=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