Chapter 5: Control of Microbial Growth

BSL-1

Procedures for work with microbes not known to cause disease in healthy people

BSL-2

Procedures for work with moderate-risk microbes that cause disease but have limited transmission potential

BSL-3

Procedures for work with pathogens that cause serious or potentially fatal disease through inhalation

BSL-4

Procedures for work with easily transmitted deadly pathogens; uncurable

Germicidal chemicals 

Disinfect and sterilize; react irreversibly with proteins, DNA, cytoplasmic membranes, or viral envelopes; less reliable than heat but useful for treating large surfaces and heat-sensitive items; some are sufficiently non-toxic to be used as antiseptics

High energy gamma rays

Used to sterilize heat-sensitive materials; generally used after packaging; approved for use on a variety of foods, but some consumers refuse irradiated products

Ultraviolet radiation

Damages DNA; used to destroy microbes in air, water, and on surfaces; poor penetrating power

Alchohol

Destroy vegetative bacteria and fungi as it denatures essential proteins, damages membranes; not reliable against endospores, non-enveloped viruses

Alcohol limitations 

Evaporates quickly, limiting contact time and can damage rubber, some plastics, and others

Aldehydes

potent chemical disinfectants and sterilants that function by inactivating proteins and nucleic acids, thereby effectively killing or neutralizing a broad spectrum of microbes (e.g. formaldehyde)

Formaldehyde

Type of aldehyde that is effective germicide that kills most microbes quickly; used to kill bacteria and inactivate viruses for vaccines and to preserve specimens; probable carcinogen

Ortho-phthalaldehyde

Type of aldehyde that is less toxic, but turns skin gray

Chlorhexidine (Biguanide)

Extensive use as antiseptics; stays on skin, mucous membranes; relatively low toxicity; destroys vegetative bacteria, fungi, some enveloped viruses; common in many products: skin cream, prescription mouthwashes

Ethylene oxide 

An explosive but extremely useful gaseous sterilizing agent that destroys all microbes and is often used to sterilize fabric, equipment, and implantable devices (toxic and a potential carcinogen) 

Hydrogen peroxide 

a more effective germicide on inanimate objects because aerobic cells, including tissue cells of the human body, produce catalase, the enzyme that breaks it down

Refrigeration 

inhibits the growth of many pathogens and spoilage microorganisms by slowing or stopping critical enzyme reactions although psychrotrophic and some psychrophilic microorganisms can grow at refrigeration temperatures

Halogens

highly reactive oxidizing agents that damage proteins and other essential cell components (e.g. chlorine)

Iodine

Kills vegetative cells, unreliable on endospores; used as tincture (in alcohol) and used as iodophor

Ozone 

an unstable, highly reactive form of oxygen (O₃) that acts as a powerful oxidizing agent. Due to its rapid decomposition, it is generated on-site and commonly used as an alternative to chlorine for disinfection and sterilization.

Metal compounds

antimicrobial agents that act by binding to sulfhydryl (–SH) groups in proteins, disrupting microbial function. While effective, their toxicity at high concentrations limits medical use. (Silver remains in topical applications like creams and bandages)

Water activity reduction

A preservation method that limits microbial growth by removing available water through salting, sugaring, or drying. It dehydrates cells, though some microbes like Staphylococcus aureus can still grow. Freeze-drying (lyophilization) is common, but drying alone doesn’t reliably kill pathogens

Peroxygens

powerful oxidizers used as sterilants; environmentally friendly chemical sterilants that act as powerful oxidizers, breaking down into harmless byproducts without leaving residue. Less toxic than agents like ethylene oxide or glutaraldehyde, they are especially effective on inanimate surfaces (e.g. hydrogen peroxide)

Phenolics

disinfectants that disrupt membranes and denature proteins, killing most vegetative bacteria. They remain active with organic matter and leave antimicrobial residue. Some, like triclosan, face safety-related restrictions

Quaternary Ammonium Compounds (Quats)

Cationic disinfectants that disrupt microbial membranes; effective against bacteria and enveloped viruses, but Pseudomonas resists. Common in personal care and surface cleaners

Chemical preservatives 

Substances added to prevent or slow microbial growth, extending shelf life; must be safe for ingestion.

Weak Organic Acids

Preservatives that disrupt microbial membranes; effective against molds and bacteria in foods (e.g., benzoic, sorbic, propionic)

Nitrates/Nitrites

Used in processed meats to inhibit endospore germination and vegetative growth, especially Clostridium botulinum; also impart pink color but may form carcinogenic nitrosamines during cooking or digestion

Microwaves

do not affect microorganisms, but the heat they generate can be lethal

High Pressure Processing (HPP)

Decreases number of microorganisms in commercial food products like fruit juice and guacamole

Autoclave

used to sterilize using pressurized steam; 121°C, 15-20 minutes, 1.5 atm pressure

Hot air oven

kill microbes by destroying cell components and denaturing proteins; requires higher temperature and longer times than moist heat because dry heat takes longer to penetrate and is less efficient at killing microbes

Critical instruments

Medical instruments such as needles and scalpels that come into direct contact with body tissue

Semicritical instruments

Medical instruments such as endoscopes that come into contact with mucous membranes, but do not penetrate body tissues.

Non-critical instruments & surfaces

Medical instruments and surfaces such as stethoscopes and countertops that come into contact only with unbroken skin.

Irradiation

provides an alternative to heat for sterilization and disinfection, but the process damages some types of plastics

Moist heat

irreversibly denatures proteins; boiling destroys most microorganisms and viruses; does not sterilize: endospores can survive

Incineration

The destruction of microbes by subjecting them to extremes of dry heat, reducing them to ashes

Antimicrobrial procedure factors

Type and number of microbes to be controlled, environmental conditions, risk for infection, and the composition of the treated item; temperature and pH influence the effectiveness of disinfection on microbial death rates

Germicide selction factors

Toxicity, Activity in the presence of organic matter, Compatibility with the material being treated, Residue, Cost and availability , Storage and stability, and Environmental risk

Filtration

The physical removal of microbes from samples; retains bacteria while allowing the fluid to pass through

Protozoan cysts & oocysts

microbes resistant to disinfectants; excreted in feces; causes diarrheal disease if ingested; destroyed by boiling

Mycobacterium species

A microbe species with waxy cell walls makes resistant to many chemical treatments

Pseudomonas species

A microbe species resistant to and can actually grow in some disinfectants

Non-enveloped viruses

Resistant microbes that lack lipid envelope; more resistant to disinfectants

D-value

Decimal reduction time; the time needed to kill 90% of the population

Disinfection

reduces or destroys microbial load on a non-living, or inanimate, object through heat or antimicrobial chemicals; kills most but endospores

Sanitization

a treatment to reduce the microbial population to a level that meets accepted health standards; reserved for inanimate items

Sterilization

eliminates all vegetative cells, endospores, and viruses from inanimate items through physical or chemical processes

Antisepsis

reduces microbial load from skin or tissue through antimicrobial chemicals

Degerming

reduces microbial loaf on skin or tissue through gentle to firm scrubbing and the use of mild chemicals

Decontamination

General reduction of pathogens to safe levels

Pasteurization

Brief heat treatment to reduce spoilage organisms and pathogens in food/liquids

Preservation

Slows microbial growth to extend shelf life using methods like refrigeration or chemical preservatives

Bacteriostatic

Inhibits bacterial growth without killing

Bactericidal

Kills bacteria directly