Chapter 27-Control of growth

Sterilization is the killing of all organisms, including viruses. Heat is the most widely used method of sterilization.
The temperature must eliminate the most heat-resistant organisms, usually bacterial endospores

An autoclave permits application of steam heat under pressure at temperatures above the boiling point of water, killing endospores
121 C at 15 psi for 15 min

Pasteurization
Pasteurization does not sterilize liquids but reduces microbial load
Kills most pathogens and inhibiting the growth of spoilage microorganisms.

Radiation Sterilization
Controlled doses of electromagnetic radiation effectively inhibit microbial growth.
Ultraviolet radiation is used to decontaminate surfaces and materials that do not absorb light, such as air and water.
Ionizing radiation, necessary to penetrate solid or light-absorbing materials, is widely used for sterilization and decontamination in the medical and food industries

Filter Sterilization
Filters remove microorganisms from air or liquids. Depth filters, including HEPA filters, are used to remove microorganisms and other contaminants from liquids or air.
Membrane filters are used for sterilization of heat-sensitive liquids, and nucleation filters are used to isolate specimens for electron microscopy

Chemicals
Chemicals that kill organisms are called cidal agents. Thus, these agents are termed bacteriocidal, fungicidal, and viricidal agents, killing bacteria, fungi, and viruses, respectively.

Bacteriocidal

Bacteriocidal agents bind tightly to their cellular targets and are not removed by dilution; but lysis, the loss of cell integrity and release of contents, does not occur.
Bacteriostatic

Agents that do not kill but only inhibit growth are called static agents, and these include bacteriostatic, fungistatic, and viristatic agents.
MIC

Antimicrobial activity is measured by determining the smallest amount of agent needed to inhibit the growth of a test organism, a value called the minimum inhibitory concentration (MIC)
Chemical Antimicrobial Agents for External Use

Sterilants, disinfectants, and sanitizers are compounds used to decontaminate nonliving material.

Disinfection is the elimination of microorganisms from inanimate objects or surface

Antiseptics
Antiseptics and germicides are used to reduce microbial growth on living tissues.

antimicrobial agents

Agent Classification
Classification:
Structure
Mechanism of action
Spectrum of antimicrobial activity.

Antimicrobial Action
Selective toxicity
Inhibition of Cell Wall Synthesis
Inhibition of Protein Synthesis
Disruption of Cytoplasmic Membranes
Inhibition of Metabolic Pathways
Inhibition of Nucleic Acid Synthesis

Growth Analogs
Growth factor analogs such as sulfa drugs, isoniazid, and nucleic acid analogs are synthetic metabolic inhibitors.
Growth Factor Analogs
Growth Factor Analogs: synthetic compounds that are structurally similar to a growth factor in a pathway used by the organism.
Analogs: vitamins, amino acids, nucleic acid.

Inhibition of Metabolic Pathways
Trimethoprim, a chemical analog, binds to enzyme involved in conversion of dihydrofolic acid to THF

Quinolones
Quinolones inhibit the action of DNA gyrase in Bacteria.

B-Lactam Antibiotics: Penicillins and Cephalosporins,
B-lactams
These compounds target cell wall synthesis in Bacteria. They have low host toxicity and a broad spectrum of activity.
Inhibition of Cell Wall Synthesis

Antibiotics from Prokaryotes,
The aminoglycosides, macrolides, and tetracycline antibiotics are structurally complex molecules produced by Bacteria and are active against other Bacteria. All of these work by interfering with protein synthesis.

Antiviral Drugs
Clinically effective antiviral agents include nucleoside analogs and other drugs that inhibit nucleic acid polymerases and viral genome replication.
Anti-viral Drugs
Agents such as the protease inhibitors (PIs) interfere with viral maturation steps. Host cells also produce the antiviral interferon proteins that stop viral replication.

Antifungal Drugs
Antifungal agents fall into a wide variety of chemical categories. Because fungi are Eukarya, selective toxicity is hard to achieve, but some effective chemotherapeutic agents are available.

 Antimicrobial Drug Resistance,

How do microbes become resistant?
Preexisting Resistance - resistant before exposure to antibiotics
Intrinsic resistance - born bad
Genetic mutation - in the course of rapid multiplication genetic mutations occur
Transfer of genetic material
New mutations of chromosomal genes
Acquisition of R-plasmids
transformation, transduction, and conjugation

How testing is performedMIC
Minimum inhibitory concentration (MIC).

Antimicrobial activity is measured by determining the smallest amount of agent needed to inhibit the growth of a test organism

Performed in either tubes or microtiter plates
Measured as the last tube which shows no growth
Minimum Inhibitory Concentration (MIC)
Kirby Bauer Diffusion Test