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Microbiology (210)
MICR 2420 (131)


6 Pages

Course Code
MICR 2420
Joseph Lam

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ANTIBIOTICS Physical, Chemical and Biological Control of Microbes  Improvements in sanitation procedures, and antiseptics and the development of antibiotics has played a major role in extending life expectancy and in contributing to the population explosion 1) Physical Antimicrobial Control  Sterilization – process by which all living cells spores and viruses are destroyed on an object  Disinfection – the killing or removal of disease-producing organisms from inanimate surfaces, not necessarily result in sterilization, pathogens = killed but microbes could survive  Antisepsis – similar to disinfection, but applies to removing pathogen from the surface of living tissues (ie. Skin), chemicals are usually not as toxic as disinfectants o Antiseptics stop microbes from growing but do not really kill them  Sanitation – closely related to disinfection, consists of reducing the microbial population to safe levels and usually involves both cleaning and disinfecting an object  Hand sanitizer – sanitizes but does not change the metabolism of bacteria  Bacteriostatic – inhibits growth (cannot kill, immune system removes infection)  Bacteriocidal – kills cells (antibiotic, only effective if organism is building a new cell wall)  Death Curve and Decimal Reduction Time: o Microbes die according to a negative exponential curve where cell number are reduced in equal fractions at constant intervals, efficacy of a given lethal agent/condition is measured as decimal reduction time (D-value) which is the length of time it takes that agent to kill 90% (1 log value) of the population (subtract time at 100% from time at 90%), time required to kill = independent of initial cell concentration  Factors affecting the ability of an antimicrobial agent to kill microbes: initial population size, population composition, agent concentration and duration of exposure Heat and Pressure Sterilization: o Max growth temperature beyond with macromolecules denature – loss of function o moist heat rather than dry (water penetrates cells), need high pressure to kill spores and thermophiles, use the steam autoclave to achieve this (121 Celsius and 15 psi for 20 min, not the pressure but the heat achieved under pressure that kills)  Mesophiles (grow at 25-40 degrees) are much less tolerant to heat and pressure than thermophiles  death from heating is exponential  Time required to kill defined as fractions and is independent of initial concentration  Clostridium is more difficult to kill because of endospores - endospores require 121degrees for 4-5 mins in an autoclave for decimal reduction vs. 0.1-0.5 min at 65 degrees for vegetative cells  Pasturization: heating food to certain temperature long enough to kill the most heat resistant non-spore forming bacteria  Autoclave: o sealed device, steam and heat killing o sterilization achieved in 10-15 minutes but longer time needed for larger volume o it is not the pressure but the heat achieved under pressure that killes the microorganisms Radiation Sterilization  Irradiation sterilization: the bombardment of foods with high-energy electromagnetic radiation strategy for sterilizing food after harvesting (NASA) o UV = only good for surface sterilization – poor penetrating power o gamma rays, electron beams, an X-rays = high penetrating power  used to irradiate food, flasks, tissue cultures, IV and other heat- sensitive items  Gray is the unit for D10 – the amount of radiation needed to reduce the initial population tenfold  higher Gy relates to the killing of spores necessary for C. botulinium o D. radiodurans is very high because it is resistant to radiation – Gy reates to killing of vegetative cell, not spore Filtration  Filtration: through micropore filter with pore size of 0.2 um can remove microbial cells but not viruses from solutions, advantage = avoids heat (does not damage material), but solutions are not sterile but good to remove precipitate, types: depth, membrane, and nucleopore filter  Biological safety cabinet – o air is drawn into chamber and continually passes through the HEPA (high efficiency particular air) filter to remove hazardous agents preventing the escape of aerosolized infectious agents, UV light sterilizes in 30min-1hour o Contaminant free workspace and protects investigator by restraining air flow (laminar) 2. Chemical Agents for Antimicrobial Control  Physical agents are effective but sometimes impossible (skin), chemical agents are a better choice sometimes,  factors affecting efficacy of chemical agent: o the presence of organic matter (affects disinfection time), o kinds or organisms present (pathogens) o corrosiveness (should not corrode or damage surface) o stability, odour and surface tension (should be neutral, pleasant odour, low surface tension to penetrate cracks)  triclosan (phenol) can cause antimicrobial resistance Commercial Disinfectants and Antiseptics  all damage proteins, lipids and DNA, reduce/eliminate microbial content from objects o ethanol o iodine (Wescodyne and Betadine) o chlorine  Phenol coefficient test o Compares the effectiveness of disinfectants against phenol as the benchmark o consists of inoculating a fixed number of bacteria into dilutions of the test agent and withdraw samples at timed intervals from each dilution and inoculate into fresh broth o phenol coefficient is based on the highest dilution (low conc.) that will kill all
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