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