Control of Microbial Growth

Control of Microbial Growth
A few terms
• Bacteriostatic: inhibits bacterial growth
• Bactericidal: something capable of killing
• 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
What factors influence the
success of microbial control?
• What type of organism(s) 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
• 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
Autoclaves work due to steam
under pressure
**Indicators used in autoclaving**
What does a color change on the autoclave tape indicate? Why would it be necessary to also
place an indicator in the center of the pack? What type of biological indicator is used for
quality assurance in an autoclave monitoring program? Why? Where would the biological
indicator be placed during the autoclaving procedure?
What if the substance is heat
• Filtration is the best
• 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
Heavy metals
Quaternary Ammonium compounds
As a general rule: if you need to
achieve sterility then you should be
thinking about physical, not
chemical methods of microbial
Evaluation of chemicals: disc
diffusion method
Chemical control: choosing the
right germicidal chemical
• Goal: sterility vs. disinfection; level of disinfection
required dictates potency of chemical required
• Toxicity: risk benefit analysis
• Activity in presence of organic material: most are
diminished or inactivated
• Sensitivity of the material to be treated
• Residue: toxic or corrosive vs residual desired
antimicrobial effect
• Cost and availability
• Storage and stability: concentrate vs stock solution
• Environmental risk: antimicrobials in the environment
Chemical control
• Phenolics
Destroy cytoplasmic membranes, denature proteins (binds
to tyrosine, alters cell membranes)
• Triclosan
• Hexachlorophene
Target: vegetative bacteria +/- mycobacteria; not sporocidal;
inadequate if viricidal activity is required; leave an active
antimicrobial residue
Chemical control
• Halogens
1. Iodine (binds to tyrosine, alters cell membranes)
• Tincture
• Iodophore
Target: all bacteria, fungi, most endospores, and some viruses
2. Chlorine (strong oxidizing agent)
Target: all types of microbes and viruses; inactivated by organic
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” =
ability of very low
concentrations of metals
to kill microbes:
interfere with function
of enzymes by binding
to –SH
• Silver (and formerly,
copper, mercury, lead,
Chemical control
• Surfactants (soaps)
– Little value as an antiseptic
– Acid-anionic surface-active sanitizers are
important in cleaning of dairy equipment and
Chemical control
• Quaternary Ammonium Compounds (Quats)
– Group of positively charged detergents
Action: react with the cell membrane → reduced surface
tension, membrane damage
Target: kill many vegetative bacteria and enveloped
viruses, fungi; 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; does not kill endospores; will not
destroy naked viruses
Which antiseptic works the best?