Control of Microbial Growth

A few terms• Bacteriostatic: inhibits bacterial growth• Bactericidal: something capable of killing bacteria

• Antiseptic: an agent that is used to inhibit/kill bacterial growth on skin and mucus membranes

• Disinfectant: an agent that is used to inhibit/kill bacterial growth on inanimate objects

History behind microbial control

• Joseph Lister was the first to introduce the use of carbolic acid to reduce bacterial infections in hospitals (1860s)

• Ignatz Semmelweis regarded as the “Father of Infection Control”, physicians used chlorinated lime to cleanse hands (1850s)

What factors influence the success of

microbial control?• What type of organism are you targeting?

• What type of environment are you treating?

Are all microbes equally sensitive?

What parts of a bacterial cell are

sensitive to physical treatments and

chemicals?• Plasma membrane

• DNA and proteins

Physical Methods

Physical Methods• Heat

– Dry: kills by oxidation, ex: incineration

– Moist: promotes coagulation of proteins•Boiling (100oC)•Autoclave(121oC, 15lbs/sq inch)•Pasteurization

– Classic-63oC for 30 minutes– HTST-72oC for 15 seconds– UHT-140oC for 3 seconds

Autoclave

Autoclaves work due to steam under pressure

Indicators used in autoclaving

What if the substance is heat sensitive?

• Filtration is the best choice

• Pore sizes can be either .45um or .22um

Physical Methods

• Low temperature: freezing does not kill bacteria, most cultures are stored at -80oC

• Dessication: remove the water and bacteria can remain viable

Physical Methods

• Radiation– Ionizing radiation (gamma rays, x-rays)

– Non-ionizing radiation (uv rays)– Microwaves

Electromagnetic Spectrum

Chemical Methods• Most are only able to reduce the numbers of organisms, not achieve sterility

• Types of chemicals– Phenol and phenolics– Halogens– Alcohols– Heavy metals– Soaps– Quaternary Ammonium compounds– Biguanides

Evaluation of chemicals: disc diffusion method

Chemical structure of phenols/phenolics

Chemical control• Halogens

1. Iodine (binds to tyrosine, alters cell membranes)• Tincture• Iodophore

Target: all bacteria, fungi, most endospores, and some viruses

-Are all organisms killed by iodine???

2. Chlorine (strong oxidizing agent)Target: all types of microbes and viruses

Chemical Control

• Alcohols– Mechanism of action is protein denaturation, can also dissolve lipids

– Most common are ethanol and isopropanol

Target: kill vegetative cells and fungi, do not kill endospores and some naked viruses

Chemical control• Heavy metals exert oligodynamic action

• Silver, copper and mercury

Chemical control

• Surfactants (soaps)– Little value as an antiseptic– Acid-anionic surface-active sanitizers are important in cleaning of dairy equipment and utensils

Chemical control• Quaternary Ammonium Compounds (Quats)– Group of positively charged detergents

Action: react with the cell membraneTarget: kill many vegetative bacteria and enveloped viruses, do NOT kill endospores, Mycobacteria, or naked viruses

Chemical control– Biguanides (chlorhexidine)

•Used in antiseptic products•Adheres to skin and mucus membranes, low toxicity

•Action: membrane disruption•Target: wide range of bacteria, fungi, and some enveloped viruses

Which antiseptic works the best?