Chapter 11


Controlling Microorganisms



can use heat, chemicals, irradiation

generally applies to inanimate objects such as surgical instruments, syringes, and packaged foods

Antisepsis involves disinfection on exposed body surfaces

Can use washing, heat, or disinfectants

Includes washing dishes and clothes



Microbial Death

bacteriostatic - inhibit growth

bacteriocidal - kill bacteria

also microbicidal, germicidal, fungicidal, virucidal, sproricidal (or static)

Highest resistance prions and endospores

Moderate resistance protozoan cysts, fungal spores, naked viruses, some bacteria

Ex. Mycobacterium tuberculosis, Staphylococcus aureus, Pseudomonas

Least resistance most bacterial vegetative cells, fungal hyphae, enveloped viruses, protozoans



Methods of Physical Control: Heat




Methods of Physical Control: Moist Heat

Steam can only reach 100 C under normal atmospheric pressure

Increasing pressure allows for higher temps and better microbe killing

Destroys vegetative cells and spores

Autoclave commercial device used for steam-heat sterilization

Effective at sterilizing heat-resistant materials

glassware, cloth, metallic instruments, liquids, and any material that will be discarded (plastic Petri dishes and pipettes)

Not effective at sterilizing substances that repel or absorb moisture

waxes, oils, powders

Relied on for disinfection not sterilization

10-30 minutes will kill non-spore-forming pathogens & most viruses

Used to decontaminate suspect drinking water

Brief heat treatment to reduce the number of spoilage organisms and kill disease-causing pathogens

Increases the shelf life of food without altering quality

Does not kill thermoduric organisms or endospores



Methods of Physical Control: Dry Heat

Uses extreme heat to reduce the microbe to gas and ash

Flaming the wire loop in lab is an example

Hospitals use incineration to eliminate infectious waste materials

Electric coils radiate heat within an enclosed compartment

Sterilization occurs at 150-180 degrees for 1-4 hours

Used for heat-resistant materials that cannot be sterilized with moist heat

some glassware, powders, oils, and metallic instruments



Methods of Physical Control: Cold Treatment

Slow microbial growth by slowing enzyme activity

Common in food processing and storage

Neither is considered a viable method of sterilization or disinfection



Methods of Physical Control: Drying

Inhibits growth by removing water

Inhibits the growth of most pathogens

Lyophilization mixture of freezing & drying

used to preserve food

Not considered a viable method of sterilization or disinfection



Methods of Physical Control: Radiation

Causes damage to DNA and proteins that repair DNA

Used to sterilize materials that are sensitive to heat or chemicals 

commercial food products and medical equipment

Potential problems include changing flavor and nutritional value, and introducing undesirable chemical reactions

Causes covalent bonds to form between adjacent thymine/cytosine molecules in DNA

DNA again is very susceptible

Germicidal lamps are used for disinfecting air

found in hospitals, operating rooms, schools, nursing homes, cafeterias, and military housing

Does not pass through glass or plastic but is used on exposed microbes

Poses threat to human tissue if it is overexposed



Methods of Physical Control: Filtration

Used to remove microbes from air and liquids

involves the passage of air or water through filters containing precise, uniform pores

Pore size varies depending on what needs to be trapped but can be small enough to trap viruses

Used to prepare heat-sensitive liquids

Serum and other blood products, vaccines, IV fluids, and enzymes

Has also been used to sterilize beer and milk without altering flavor

Also used to capture airborne contaminants in hospitals and laboratories



Methods of Physical Control: Salt

involves the use of salt or sugar to create a hypertonic environment

results in water leaving the cell, inhibiting cellular processes

Used as preservative for jams, jellies, meat, and fish




Germicidal Chemicals



Low to Intermediate Activity

Chemicals derived from phenol that have additional functional groups added

Denature proteins and disrupt cell walls and membranes

Used in drain cleaners and Lysol

Often have a nasty odor and some side effects (skin irritation)

Cationic (positively charged) detergents

Bind to negatively charged bacterial surface to disrupt cell membrane permeability

Ex. soaps, detergents, and mouthwash

Bind to and inactivate proteins which shuts down metabolism

Silver and  mercury used as antiseptics and germicidal ointments

Most  are too toxic to be used medically



Intermediate Activity

Denature enzymes and inhibit metabolic reactions

Ex. iodine, chlorine, bromine, and fluorine

Chlorine - used to disinfect inanimate objects, drinking water, and wastewater

Iodine used as disinfectant and topical antiseptic

Isopropanol and ethanol most commonly used in 60-80% mixtures because they evaporate quickly

Denature proteins and disrupt cell membranes

Used as disinfectants and in hand sanitizers

Less prone to staining or irritating tissues

Include Betadine, Providone, and Isodine

Targets cell membranes and enzymes

Used as antiseptic with low toxicity



High Activity

Oxygen produced forms free radicals that are toxic to cells  

Hydrogen peroxide used as antiseptic for anaerobic organisms in deep wounds and as disinfectant for air and medical items

Ozone - used to disinfect air, water, and cooling towers

Denature proteins and inactivate nucleic acids

2% glutaraldehyde and 37% formaldehyde (formalin) are used for sterilization

  Cidex and Sporocidin used for medical and dental devices

Irritating to the respiratory tract, skin, and eyes, suspected carcinogen

Gaseous agent that destroys all microbes by reacting with proteins and nucleic acids

Chemiclave chamber - used to sterilize medical devices

It is toxic, explosive, and potentially carcinogenic

Propylene oxide is less toxic and used to sterilize foods