Chapter 7 The Control of Microbial Growth
Chapter 7
The Control of Microbial Growth
History
! Microbial growth control began about 100 yrs ago
! Before this time, nosocomial infections caused 10% of deaths involved with surgery and 25% of deaths involved with childbirth
Bubonic Plague or Black Death
! Epidemic that swept through Europe in the Middle Ages (13th & 14th centuries)
! 40 million people killed! 1/3 of the population of the continent
! Etiologic agent= Yersinia pestis! Two vectors
! Rat! Flea
Bubonic Plague Infection
! Flea bite with Yersinia pestis! Bacteria multiply in bloodstream! Bacteria localize in lymph nodes, especially
axillary and groin! 50 % mortality rate if no treatment! Hemorrhaging occurs in lymph nodes
! Hence the name Black Death (black and blue color of blood under skin)
Terminology
! Sterilization-destroys all life forms, including endopores
! Disinfection-destruction of vegetative pathogens by using chemical or physical methods (uv, chemicals, steam, etc)! Utilizes disinfectants and antiseptics
More terms…
! Cidal vs static! Bacteriocidal-kills bacteria! Bacteristatic-inhibits bacterial
growth/reproduction
! Fungicide:! Virucide:! Algacide:! Germicide:
Factors that Effect Antimicrobial Treatments
! Temperature & Time! Concentration of antimicrobial agent! Type of microorganism! Presence of organic matter! Number of microbes
Targets of Antimicrobial Agents
! Cell membrane! Alters membrane permeability due to lipid &
protein components! Enzymes and proteins
! H bonds broken and proteins/enzymes denatured or protein synthesis altered
! DNA and RNA! Interference with DNA and RNA replication
Methods to Control Microbial Growth
! Physical! Heat, filtration, low temp, desiccation, osmotic
pressure, and radiation
! Chemical! Disinfectants and antiseptics
Physical Methods to Control Microbial Growth
! Heat! Functions to denature enzymes & proteins! Two types=dry & moist! Thermal Death Time (TDT)
! Minimum length of time in which all microbes in a liquid culture are killed at a given temp
! Thermal Death Point (TDP)! Lowest temp at which all microbes in a liquid
culture are killed in 10 minutes
Moist Heat
! Boiling Water! Kills vegetative bacterial cells, fungi, and many
viruses! Not effective against endospores and a few
viruses! Hepatitis-needs 20 minutes to be killed! Spores-may live up to 20 hours in boiling water
Moist Heat
! Example-Autoclaving! Steam under pressure! Preferred method of sterilization! 121C at 15 psi will kill all microbes and their
spores within 15 minutes! Useful with media and instruments
Pasteurization
! Developed by Louis Pasteur! Mild heat kills pathogenic organisms
! Classic-63 C/30 minutes! Today-72 C/15 seconds! Ultra-140 C/5 seconds
Dry Heat
! Flaming! Inoculating loop and needle-100% effective
! Incineration! Disposable waste-paper cups, bags, etc.
! Hot Air Sterilization! 170 C for 2 hours! Used for items damaged by moist heat
! Wound dressings, powders
More physical methods…
! Filtration-passage of liquid through a screen-like pad (.45 um removes most microbes)! Sterilize water, media, vaccines, antibiotics! Removes microbes from solutions that can be
damages from heat
Filtration
More physical methods…
! Low temps-refrigerator temp (1-7 C) lowers metabolic rate of most microbes
! Desiccation-without water, microbes can’t grow/reproduce but may remain alive for years
! Osmotic pressure-high { } of salt or sugar causes microbes to undergo plasmolysis
More physical methods…
! Radiation-effectiveness depends on wavelength, intensity, and duration
! Two types:! Ionizing-gamma & x-rays
! Penetrates most substances! Useful on items damages by heat (plastic petri dishes, syringes,
catheters, surgical gloves)! Non-Ionizing-uv light
! Used in hospital & operating rooms! Won’t penetrate plastic and glass! Causes thymine dimers ! Causes burns and skin cancer
Radiant Energy Spectrum
Chemical Methods of Microbial Control
! Useful on living (antiseptics) and non-living (disinfectants) substances
! Rarely achieve sterilitity-just reduce the # of microbes to safe levels or just remove vegetative cells
! Examples: ! Phenol! Halogens! Alcohols! Heavy metals
Microbial Characteristics and Microbial Control
! Gram negative cells more resistant to disinfectants and antiseptics
! Endospores and protozoans very resistant! Some viruses (non enveloped) resistant