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
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Phenol and phenolics
Halogens
Alcohols
Heavy metals
Soaps
Quaternary Ammonium compounds
Biguanides
Evaluation of chemicals: disc
diffusion method
Chemical Control
Chemical structure of
phenols/phenolics
Chemical Control
• Phenols and phenolics
– Destroy cytoplasmic membranes and denature
proteins
– Kill most vegetative bacteria and in higher
concentrations kill Mycobacterium tuberculosis
– Reasonable cost to use, effective in presence of
organics
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 membrane
Target: kill many vegetative bacteria and enveloped
viruses, do NOT kill endospores, Mycobacteria, or
naked viruses
Chemical control
– Biguanides (chlorhexidine)
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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?