History of bacterial chemical control:
II. Define Bacterial death and exponential death:Size of microbial population Time of exposure to the microbicidal Concentration of the agent Temperature- low T, more time; high T, shorter time Type of microbe - different species have different sensitivities. The following all make a difference: Spores, type of vegetative cell waxy cell wall of Mycobacteria cysts of protozoa type of virus (enveloped or just a capsid) Metabolic state of the organisms - actively growing cells are more sensitive to many chemical agents Environment of the microbe - organic matter neutralizes many disinfectants - this is especially a problem with chlorine compounds. pH, salt concentration, and the concentration of the disinfecting agent are also important considerations.
III. Mechanisms of action of chemical agents:
a) Effects on proteins - altering protein structure
(denaturation)
b) Cell wall - blocking its synthesis, or digesting
it.--->lysis
c) Cell membrane - by lowering selective permeability
and surface tension -->open up
d) Cellular processes (DNA, RNA) - by binding irreversible
to DNA, preventing both
transcription (formation
of RNA) and translation (protein synthesis). By mutagenesis
resulting in permanent inactivation
of DNA.
IV. Physical agents used to disinfect
1). Heat - Protein denaturant and
DNA denaturant.
Heat: a) dry
heat - hot air obtained by a flame or electric heating coil.
b) moist heat - occurs in the form
of steam, hot water or boiling.
a) Moist heat is more effective than dry heat
2) Low temperatures: refrigeration and freezing don't kill, they preserve (STATIC)
3) Radiation - Two
types
a) ionizing
radiation: high energy X rays, gamma rays have high penetrating
power
(formation of hydroxyl radicals)
b) nonionizing radiation: UV light has low penetrating power (forms thymine dimers)
4) Filtration - effective for sterilization
of heat sensitive liquids. Filters with pore size of 0.2 um.
V. Chemical disinfection and antiseptic agents:
1. Agents which damage cell membrane
a) Antimicrobial detergents:
(denature proteins, disrupt cell membrane permeability)
Quaternary ammonium compounds
(QUATS)- benzalkonium chloride (Zephiran),
cetylpyridium chloride (Cepacol).
Good against gram (+) not so good against gram (-).
Soap and organic matter
will inactivate QUATS.
b) Phenolic compounds
(carbolic acid):
Lysol, phenol (carbolic
acid), coal tar distillates (Ichthamol), hexachlorophene -- Staph.
aureus skin infections.
All are powerful disinfectants.
Phenol coefficient:
- potency of an agent against Salmonella typhi after 10 min.
- dilution of an agent vs dilution of phenol
- if >1 better than phenol; if
<1 worst than phenol
c) Alcohols:
(remove lipids and disrupt cell membrane).
Also a dehydration effect
(denatures proteins). Good for washing skin. Kills vegetative
bacterial cells and enveloped
viruses well. Not effective on endospores and non-enveloped
viruses. Isopropyl slightly
better than ethanol, 70% more effective than higher
concentrations.
2. Agents which denature proteins and nucleic acids
a) Acids: most
commonly used are organic acids such as:
Many are used in food preservation where they are useful in inhibiting fungal growth.proprionic benzoic lactic citric sorbic.
b) Heavy metals (denature
proteins ---SH groups)
Mercury-
merthiolate, mercurochrome; effective but toxic
Silver-
1% silver nitrate used in newborn's eyes and as antiseptic in burn patients.
Silver
sulfadiazine is also used on burn patients.
Copper-
copper sulfate used in agriculture to control bacterial and fungal diseases.
Used in
paint and used to treat swimming pools and fish tanks. Toxic to invertebrates.
Zinc-
zinc chloride in mouthwash
c) Oxidizing agents (affect enzyme funtion)
Halogens: (Iodine, chlorine)d) Cross-linking agents (Alkylating agents):
Chlorine widely used as a disinfectant to treat water and in dairy industry. A 10% solution of chlorox is a very effective all around strong clinical disinfectant. Two to four drops of chlorox per liter of water will disinfect the water after 30 min.Iodine is used as a skin disinfectant. Halogens have the disadvantage of being inactivated by organic materials. Tincture (in aqueous alcohol) and Iodophor (in combination with a detergent)
Hydrogen peroxide (3%): relatively weak disinfectant especially if body fluids are present; perhaps best used as a cleaning agent. Used as a contact lens disinfecting agent.
Formaldehyde and glutaraldehyde (37% formaldehyde is called formalin). Very effective agents, these kill everything, but they are highly toxic.NOTE: We will cover this topic later in the course!!!Ethylene oxide gas
3. Antibiotics and chemotherapeutic agents. (Chapter 20; Tortora et.al.).
Antibiotics are substances produced in a microorganism which inhibit the growth of other microbes. Mechanisms of Antibiotic action. -- Exploit the differences between eucaryotes and procaryotes:
Ideal antibiotics: there is no such thing but if there were, these would be the characteristics:a. Inhibit cell wall synthesis
- penicillins
- cephalosporins
- vancomycin
- bacitracin
b. Interfere with membrane functions
- polymyxin
- nystatin
- amphotericin B
c. Inhibit protein synthesis
- aminoglycosides
- erythromycin
- chloramphenicol
- tetracyclin
d. Interfere with nucleic acid metabolism
- nalidixic acid
- rifamycin
e. Interfere with enzyme activity
- sulfa drugs
What is antibiotic sensitivity (antibiotic susceptibility) testing?Selective toxicity Not immunogenic (No anaphylaxis) Minimal effects on normal flora Doesn't lead to resistance