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Download Control of Microorganisms Physical and Mechanical
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In order to control microorganisms, you must either: Kill them Inhibit their growth When various cell structures become dysfunctional and the entire cell sustains irreversible damage If a cell can no longer reproduce under ideal environmental conditions Death begins when a certain threshold of microbicidal agent is met, and continues in a logarithmic manner [INSERT TABLE 9.1] Figure 11.1 Alteration of cell walls and membranes Cell wall maintains integrity of cell When disrupted, cannot prevent cell from bursting due to osmotic effects Cytoplasmic membrane contains cytoplasm and controls passage of chemicals into and out of cell When damaged, cellular contents leak out Viral envelope responsible for attachment of virus to target cell Damage to envelope interrupts viral replication Nonenveloped viruses have greater tolerance of harsh Action of Antimicrobial Agents conditions ◦ Damage to proteins and nucleic acids Protein function depends on 3-D shape Extreme heat or certain chemicals denature proteins Chemicals, radiation, and heat can alter or destroy nucleic acids Can produce fatal mutants Can halt protein synthesis through action on RNA Action of Antimicrobial A Binding to ribosomes to stop translation Bind irreversibly to DNA preventing transcription and translation Mutagenic agents ◦ ◦ ◦ ◦ Inexpensive Fast-acting Stable during storage Capable of controlling all microbial growth while being harmless to humans, animals, and objects Ideally, agents should be ◦ Site to be treated Harsh chemicals and extreme heat cannot be used on humans, animals, and fragile objects Method and level of microbial control based on site of medical procedure Factors Affecting the Efficacy of Antimicrobial Methods The number of microorganisms The nature of the microorganisms in the population The temperature and pH of the environment The concentration of the agent The mechanism of action of the agent The presence of solvents, interfering organic matter, and inhibitors ◦ Relative susceptibility of microorganisms Effectiveness of germicides classified as high, intermediate, or low High-level kill all pathogens, including endospores Intermediate-level kill fungal spores, protozoan cysts, viruses, and pathogenic bacteria Factors Affecting Efficacy Low-level killthe vegetative bacteria, fungi, protozoa, of Antimicrobial Methods and some viruses ◦ Phenol coefficient Evaluating the efficacy of disinfectants and antiseptics by determining an agent’s ability to control microbes as a ratio to that of phenol Greater than 1.0 indicates that agent is more effective than phenol Methods for replaced Evaluating Has been by newer methods Disinfectants and Antiseptics Heat as an Agent of Microbial Control ◦ Generally, elevated temperatures are microbicidal and lower temperatures are microbistatic ◦ Can use moist heat or dry heat [INSERT TABLE 9.4] • • • • Temperature and length of exposure must be considered Higher temperatures generally allow shorter exposure times; lower temperatures generally require longer exposure times Thermal death time (TDT): the shortest length of time required to kill all test microbes at a specified temperature Thermal death point (TDP): the lowest temperature required to kill all microbes in a sample in 10 minutes Steam under pressure ◦ Pressure raises the temperature of steam ◦ Autoclave is used ◦ Most efficient pressuretemperature combination for sterilization: 15 psi which yields 121°C [INSERT FIGURE 9.5] • • • • Used to disinfect beverages Heat is applied to liquids to kill potential agents of infection and spoilage, while retaining the liquid’s flavor and food value Special heat exchangers – Flash method: expose to 71.6°C for 15 seconds – Batch method: expose to 63°C to 66°C for 30 minutes Does not kill endospores or thermoduric microbes [INSERT TABLE 9.2] For disinfection and not sterilization Expose materials to boiling water for 30 minutes Usually an electric oven Coils radiate heat within an enclosed compartment Exposure to 150°C to 180°C for 2 to 4 hours Used for heat-resistant items that do not sterilize well with moist heat effective against endotoxin [INSERT TABLE 9.4] Radiation: energy emitted from atomic activities and dispersed at high velocity through matter or space For microbial control: ◦ Gamma rays ◦ X rays ◦ Ultraviolet radiation Food products Medical products Wavelength approximately 100 nm to 400 nm Germicidal lamp: 254 nm Not as penetrating as ionizing radiation Powerful tool for destroying fungal cells and spores, bacterial vegetative cells, protozoa, and viruses Usually disinfection rather than sterilization Hospital rooms, operating rooms, schools, food prep areas, dental offices Treat drinking water or purify liquids ◦ Decrease microbial metabolism, growth, and reproduction Chemical reactions occur slower at low temperatures Liquid water not available ◦ Psychrophilic microbes can multiply in refrigerated foods ◦ Refrigeration halts growth of most pathogens ◦ Slow freezing more effective than quick freezing ◦ Organisms vary in susceptibility to freezing ◦ Drying inhibits growth due to removal of water; only microbiostatic ◦ Lyophilization used for long = term preservation of microbial cultures Prevents formation of damaging ice crystals Dessication and Lyophilization Osmotic Pressure ◦ High concentrations of salt or sugar in foods to inhibit growth ◦ Cells in a hypertonic solution of salt or sugar lose water; cell desiccates ◦ Fungi have greater ability than bacteria to survive hypertonic environments Approximately 10,000 different antimicrobial chemical agents are manufactured Approximately 1,000 used routinely in health care and the home Occur in liquid, gaseous, or solid state Tinctures: solutions dissolved in pure alcohol or water-alcohol mixtures • • • • Effective for removing microbes from air and liquids Fluid strained through a filter with openings large enough for fluid but too small for microorganisms Filters are usually thin membranes of cellulose acetate, polycarbonate, and a variety of plastic materials Pore size can be controlled and standardizes [INSERT FIGURE 9.9] Prepare liquids that can’t withstand heat Can decontaminate beverages without altering their flavor Water purification Removing airborne contaminants (HEPA filters) [INSERT TABLE 9.3]
