Chapter 8: Control of Microbes in Environment

sterilization

destruction or removal of all viable organisms (vegetative cells and endospores) from an object or from a particular environment

disinfection

killing, inhibition or removal of pathogenic microorganisms (typically on inanimate objects)

sanitization

reduction of microbial populations to a safe level as determined by public health standards → inanimate objects

antisepsis

prevention of infection of living tissue by microorganisms, chemicals applied to body surfaces

antimicrobial category: -cide

indicating that the agent will kill the kind of organism in question

antimicrobial category: -static

indicating that the agent will prevent the growth of the type of organism in question

chemotherapy

chemicals used internally to kill or inhibit growth of microorganisms within host tissues

pattern of microbial death

microorganisms are not killed, rather instantly when exposed to a lethal agent → the population generally decreases by a constant fraction at constant intervals

efficiency of a killing agent on a microbe

• decimal reduction time (DRT or D) is the time required to kill 90% of the microorganism or spores at a specific temperature

• Z value is the increase in temperature required to reduce D to 1/10 of its previous value

• considered dead when it is unable to grow in conditions that it would normally reproduce or grow in → issue is confounded by organisms that are in a viable but nonculturable state

factors that influence effectiveness of antimicrobial agents

• population size

• population composition

• concentration or intensity of the antimicrobial agent

• contact time

• temperature

• local environment

population size on effectiveness

more cells take longer to kill

population composition on effectiveness

different species or of cells at different developmental stages (endospores/vegetative cells or young/old cells) differ in their sensitivity to various agents

concentration or intensity of the antimicrobial agent on effectiveness

higher concentrations or intensities are generally more efficient but the relationship is not linear

contact time on effectiveness

longer exposure, the greater number of organisms killed

temperature on effectiveness

higher temperature will usually (not always) increase the effectiveness of killing

local environment on effectiveness

environmental factors (pH, viscosity, and concentration of organic matter) can profoundly influence the effectiveness of some antimicrobial agents

physical control methods of microbes

• heat (moist head or dry heat)

• filtration

• radiation

heat: killing with moist heat

• boiling water: effective against vegetative cells and eukaryotic spores, not bacterial spores

• autoclaving (steam and pressure): 121 C, 15 psi → effective against most vegetative cells and bacterial endospores

• pasteurization: brief exposure to temps below boiling point of water, reduces the total microbial population increasing the shelf life

• tyndallization: exposure to steam kills vegetative cells, time elapses for spores to grow and steam treatment is repeated → several cycles sterilizes

heat: killing with dry heat

• used to sterilize moisture sensitive materials

• less effective than moist heat

• requires higher temps (160 to 170) and longer exposure times (2 to 3 hours)

filtration

sterilizes heat sensitive liquids and gases by removing microorganisms rather than destorying them

• depth filters: thick fibrous or granular filters that remove microorganisms by physical screening, entrapment and/or adsorption

• membrane filters: thin filters with defined pore sizes that remove microorganisms, physical screening

• HEPA filters: air can be sterilized by passage through surgical masks, cotton plugs in culture vessels, HEPA filters are used in laminar flow biological safety cabinets to sterilize the air circulating the enclosure

radiation

• ultraviolet radiation is effective, but limited to surface sterilization

• UV does not penetrate glass, dirt films, water and other substances

• ionizing radiation (gamma radiation) is effective and penetrates the material

chemical control methods

• phenolics

• alcohols

• halogens

• heavy metals

• quaternary ammonium compounds

• aldehydes

• sterilizing gases

phenolics

• laboratory and hospital disinfectants

• denature proteins and disrupt cell membranes

• ex: lysol and triclosan

alcohols

• widely used disinfectants and antiseptics

• will not kill endospores

• ex: ethanol and isopropanol

halogens

• widely used antiseptics and disinfectants

• iodine oxidizes cell constituents and iodinates cell proteins

• chlorine oxidizes cell constituents

heavy metals

• effective but usually toxic

• bind and inactivate proteins

quaternary ammonium compounds

• cationic detergents used as disinfectants for food utensils and small instruments

• low toxicity, as antiseptics for skin

• disrupt biological membranes and may denature proteins

• benzalkonium chloride and cetylpyridinium chloride are widely used

aldehydes

• reactive molecules that can be used as chemical sterilants

• may irritate the skin

• bind and inactive nucleic acids and proteins

• formaldehyde and glutaraldehyde widely used

sterilizing gases

• ethylene oxide, betapropiolactone

• used to sterilize heat sensitive materials such as plastic culture dishes and siposable syringes

• bind and inactive proteins

• vaporized hydrogen peroxide has been used to decontaminate large facilities

biological control of microbes

• emerging group of control agents rely on one microbe to kill another

• include predation, viral mediated lysis, and toxin mediated killing

• phageguard: natural solution for food safety

  • E. Coli

  • Salmonella

  • Listeria

evaluation of antimicrobial effectiveness

• phenol coefficient test

• dilution test

phenol coefficient test

• compares the effectiveness of disinfectants → in which the potency of a disinfectant is compared to that of phenol

  • phenol coefficient = 1

  • antimicrobial agents stronger than phenol have a greater than 1 phenol coefficient

  • antimicrobial agents weaker than phenol have a less than 1 phenol coefficient

dilution test

• more reliable test

• stainless steel cylinders are contaminated with bacteria and then exposed to disinfectant under controlled conditions