Chapter 11


Controlling Microorganisms



Sterilization process of removing all viable microoganisms and viruses (not prions)

can use heat, chemicals, irradiation

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

Disinfection - process that eliminates most or all disease-causing pathogens

Can be used on inanimate objects (disinfectants) or exposed body surfaces (antiseptics)

Decontamination treatment to reduce the number of pathogens to a safe level

Can use washing, heat, or disinfectants

Sanitization - reducing microbial levels to meet accepted health standards

Includes washing dishes and clothes



Microbial Death

The permanent loss of reproductive capability, even under optimum growth conditions

bacteriostatic - inhibit growth

bacteriocidal - kill bacteria

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


Factors Affecting Microbial Death Rate

Number of microorganisms

Type of microorganisms in the target population

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

Temperature and pH of the environment

Concentration of the agent

Mode of action of the agent

can target cell wall, cell membrane, protein and nucleic acid synthesis, or protein function

Presence of solvents, interfering organic matter, and inhibitors



Methods of Physical Control: Heat

Causes protein denaturation and coagulation

Works well on all types of organisms although exposure time and temperature vary from microbe to microbe                  

Thermal death time time required to kill all microbes at a specific temperature

Thermal death point temperature required to kill all microbes in 10 minutes




Methods of Physical Control: Moist Heat

Steam under pressure

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

Boiling Water

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

Dry oven

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

Refrigeration & freezing

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

Ionizing radiation (gamma and X-rays)

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

Nonionizing radiation (UV radiation)

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

Osmotic Pressure

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 kill vegetative bacteria, fungal cells, and some viruses

Intermediate activity kill fungal spores, resistant bacteria, and viruses

High Activity kill endospores



Low to Intermediate Activity


Chemicals derived from phenol that have additional functional groups added

Denature proteins and disrupt cell walls and membranes

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

Quaternary Ammonium Compounds (Quats)

Cationic (positively charged) detergents

Bind to negatively charged bacterial surface to disrupt cell membrane permeability

ex. soaps, detergents, and mouthwash

Metal Compounds

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


Isopropanol and ethanol most commonly used in 60-80% mixtures

Denature proteins and disrupt cell membranes

One disadvantage is that they evaporate quickly and may not be in contact with the microbe long enough to be effective

Iodophores complexes of iodine and alcohol

Less prone to staining or irritating tissues

Include Betadine, Providone, and Isodine

Chlorhexidine contains chlorine and two phenolic rings

Targets cell membranes and enzymes

Used as antiseptic with low toxicity



High Activity

Peroxygens (oxidizing agents)

Oxygen produced forms free radicals that are toxic to cells      

Especially useful against anaerobic organisms in deep wounds

Also used to disinfect medical instruments, air, and water

Ex. peroxides, ozone, paracetic acid


Denature proteins and inactivate nucleic acids

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

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

Cidex and Sporocidin used for medical devices

Ethylene Oxide

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