1. Microbial control agents

2. Controlling Microorganisms
Many microorganisms are beneficial and necessary for human well-being.
However, microbial activities may have undesirable consequences, such as food spoilage and disease.
It is essential to be able to kill microorganisms or inhibit their growth to minimize their destructive effects.
Physical, chemical, and mechanical methods to destroy or reduce undesirable microbes in a given area
Primary targets are microorganisms capable of causing infection or spoilage:
vegetative bacterial cells and endospores
fungal hyphae and spores, yeast
protozoan trophozoites and cysts
worms
viruses
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3. Terminology of Microbial Control
Sterilization: Killing or removing all forms of microbial life (including endospores) in a material or an object.
Commercial sterilization: sufficient heat to kill Clostridium botulinum endospores (some non-pathogenic thermophilic bacteria may survive)
Disinfection: destruction of vegetative pathogens on inert substances
Antisepsis: destruction of vegetative pathogens on living tissue
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4. Biocide or germicide: kills microorganisms. Fungicide: kills fungi.
Bacteriostatic agent: stops growth of bacteria.
Aseptic technique minimizes contamination.
Degerming: mechanical removal of microbes from limited area.
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5. Microbes die at a constant rate
Microbial Death
Microbes die at a constant rate
Factors affecting how long it takes to kill bacteria
number of microbes
environment (slowed by organic materials, biofilms - hastened by prior cleaning, heat).
time of exposure
characteristics of microbes: most resistant are
spores
thick lipid coats
protozoan cysts
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6. Actions of Microbial Control Agents
Cell wall.
Cell membrane.
Nucleic acid synthesis.
Protein synthesis.
Protein function.
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7. Cell wall Cell membrane Bacteria and fungi.
Block synthesis.
Degrade cellular components.
Destroy or reduce stability.
Agent: Penicillin, detergents, alcohols
Cell membrane
All microbes and enveloped viruses.
Bind and penetrate lipids.
Lose selective permeability (leakage).
Agent: Polymyxin B
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8. Nucleic acid synthesis
Irreversible bind to DNA.
Stop transcription and translation.
Mutations.
Agent:
Chemical agent – formaldehyde
Physical agent – radiation
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9. Agent: chloramphenicol Protein function Block protein active sites.
Protein synthesis
Binds to ribosome's.
Stops translation.
Prevents peptide bonds.
Agent: chloramphenicol
Protein function
Block protein active sites.
Prevent binding to substrate.
Denature protein.
Agent
Physical – Heat, pH change
Chemical – alcohols, acids, phenolics, metallic ions
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10. Physical Methods of Microbial Control
Heat – moist and dry
Cold temperatures
Radiation
Filtration
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11. Mode of Action and Relative Effectiveness of Heat
Moist heat: lower temperatures and shorter exposure time; coagulation and denaturation of proteins
Dry heat: moderate to high temperatures; dehydration, alters protein structure.
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12. Pasteurization
Pasteurization: heat is applied to kill potential agents of infection and spoilage without destroying the food value.
63°C - 66°C for 30 minutes.
71.6°C for 15 seconds.
Not sterilization - kills non-spore-forming pathogens and lowers overall microbe count; does not kill endospores or many nonpathogenic microbes.
Dry heat: using higher temperatures than moist heat
Dry ovens – oC- coagulate proteins
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13. Radiation
Ionizing radiation: deep penetrating power that has sufficient energy to cause electrons to leave their orbit, breaks DNA,
gamma rays, X-rays, cathode rays
used to sterilize medical supplies and food products
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14. UV light creates thymine dimers, which interfere with replication.
Radiation
Nonionizing radiation: little penetrating power must be directly exposed
UV light creates thymine dimers, which interfere with replication.
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15. ANTIMICROBIAL CHEMOTHERAPY TO CONTROL MICROORGANISMS
USING
ANTIMICROBIAL CHEMOTHERAPY
TO CONTROL MICROORGANISMS
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16. Antimicrobial chemotherapeutic chemicals
Antimicrobial Drugs
Chemotherapy
Antibiotics
Antimicrobial chemotherapeutic chemicals
Selective toxicity
The use of drugs to treat a
disease
produced by a microbe that inhibits another microbe
chemicals synthesized in the laboratory which can be used therapeutically on microorganisms.
kills harmful microbes
without damaging the host
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17. In fact, only 3 major groups of microorganisms have yielded useful antibiotics: the actinomycetes (filamentous, branching soil bacteria such as Streptomyces), bacteria of the genus Bacillus, and the saprophytic molds Penicillium and Cephalosporium.
To produce antibiotics, manufacturers inoculate large quantities of medium with carefully selected strains of the appropriate species of antibiotic-producing microorganism. After incubation, the drug is extracted from the medium and purified. Its activity is standardized and it is put into a form suitable for administration.
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18. Some antimicrobial agents are:
cidal in action: they kill microorganisms (e.g., penicillins, cephalosporins, streptomycin, neomycin).
Others are static in action: they inhibit microbial growth long enough for the body's own defenses to remove the organisms (e.g., tetracyclines, erythromycin, sulfonamides).
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19. Antimicrobial agents also vary in their spectrum.
Drugs that are effective against a variety of both gram-positive and gram-negative bacteria are said to be broad-spectrum (e.g., tetracycline, streptomycin, cephalosporins, ampicillin, sulfonamides).
Those effective against just gram-positive bacteria, just gram negative bacteria, or only a few species are termed narrow-spectrum (e.g., penicillin G, erythromycin, clindamycin, gentamicin).
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20. Antibiotic Resistance: bacteria gain ability to grow.
Antimicrobial Drugs
Antibiotic Resistance: bacteria gain ability to grow.
Antiretroviral: act specifically against viruses
Combination of drugs:
• Synergism: action of two antibiotics greater
• Antagonism: action of drug is reduced;
less effective
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21. Five Modes of Antimicrobial Activity
1. Injury to Plasma Membrane
• polymixin B
2. Inhibition of Cell Wall Synthesis
• penicillins, bacitracin, vancomycin
3. Inhibition of Protein Synthesis (translation)
4. Inhibition of Nucleic Acid replication & transcription
5. Inhibition of essential metabolites
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22. For some microorganisms, susceptibility to chemotherapeutic agents is predictable. However, for many microorganisms (Pseudomonas, Staphylococcus aureus, and gram-negative enteric bacilli such as Escherichia coli, Serratia, Proteus, etc.) there is no reliable way of predicting which antimicrobial agent will be effective in a given case.
This is especially true with the emergence of many antibiotic-resistant strains of bacteria.
Because of this, antibiotic susceptibility testing is often essential in order to determine which antimicrobial agent to use against a specific strain of bacterium.
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23. Antibiotic Susceptibility Testing
Several tests may be used to tell a physician which antimicrobial agent is most likely to combat a specific pathogen:
Tube dilution tests
The agar diffusion test (Bauer-Kirby test)
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24. Tube dilution tests
In this test, a series of culture tubes are prepared, each containing a liquid medium and a different concentration of a chemotherapeutic agent. The tubes are then inoculated with the test organism and incubated for hours at 35C. After incubation, the tubes are examined for turbidity (growth). The lowest concentration of chemotherapeutic agent capable of preventing growth of the test organism is the minimum inhibitory concentration (MIC).
Sub culturing of tubes showing no turbidity into tubes containing medium but no chemotherapeutic agent can determine the minimum bactericidal concentration (MBC). MBC is the lowest concentration of the chemotherapeutic agent that results in no growth (turbidity) of the subcultures. These tests, however, are rather time consuming and expensive to perform.
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25. The agar diffusion test (Bauer-Kirby test)
A procedure commonly used in clinical labs to determine antimicrobial susceptibility is the Bauer-Kirby disc diffusion method. In this test, the in vitro response of bacteria to a standardized antibiotic-containing disc has been correlated with the clinical response of patients given that drug.
In the development of this method, a single high-potency disc of each chosen chemotherapeutic agent was used. Zones of growth inhibition surrounding each type of disc were correlated with the minimum inhibitory concentrations of each antimicrobial agent (as determined by the tube dilution test).
The MIC for each agent was then compared to the usually-attained blood level in the patient with adequate dosage. Categories of "Resistant," "Intermediate," and "Susceptible" were then established.
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26. End of lecture
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