Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings LECTURE 9: CONTROL OF MICROBIAL GROWTH MICROBIOLOGY (MIB701S)

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Terminology of Microbial Control 1.Define the following terms (aseptic, sterilization, disinfection/disinfectants, antisepsis/antiseptic, degerming) 2.Describe the patterns of microbial death caused by treatments with microbial control agents. 3.Describe the effects of microbial control agents on cellular structures. Learning Objectives

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Terminology of Microbial Control  Sepsis refers to microbial contamination  Asepsis is the absence of significant contamination  Aseptic surgery techniques prevent microbial contamination of wounds  Sterilization: removing all microbial life  Commercial sterilization: killing C. botulinum endospores  Disinfection: removing pathogens  Antisepsis: removing pathogens from living tissue

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Terminology of Microbial Control  Degerming: removing microbes from a limited area  Sanitization: lowering microbial counts on eating utensils  Biocide/germicide: killing microbes  Bacteriostasis: inhibiting, not killing, microbes

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Checking your understanding.  The usual definition of sterilization is the removal or destruction of all forms of microbial life; how could there be practical exceptions to this simple definition?

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.1a Understanding the Microbial Death Curve. Plotting the typical microbial death curve logarithmically (red line) results in a straight line. log 10 of number of surviving cells Arithmetic number of surviving cells Time (min) (a) Plotting the typical microbial death curve arithmetically (blue line) is impractical: at 3 minutes the population of 1000 cells would only be a hundredth of the graphed distance between 100,000 and the baseline. One log decrease = 90% of population killed

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Microbial Death Rates Figure 9.1

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Effectiveness of Treatment  Depends on:  Number of microbes  Environment (organic matter, temperature, biofilms)  Time of exposure  Microbial characteristics

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.1b Understanding the Microbial Death Curve. log 10 of number of surviving cells sterile surgical equipment High population load Low population load Time (min) (b) Logarithmic plotting (red) reveals that if the rate of killing is the same, it will take longer to kill all members of a larger population than a smaller one, whether using heat or chemical treatments.

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Checking your understanding  How is it possible that a solution containing a million bacteria would take longer to sterilize than one containing a half-million bacteria?

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Action of Antimicrobial Agents  Many types of chemical and physical microbial controls  Modes of action fall into two basic categories  Alteration of cell walls or cytoplasmic membranes  Interference with protein and nucleic acid structure

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings 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 conditions

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings 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

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Selection of Microbial Control Methods  Ideally, agents should be:  Inexpensive  Fast-acting  Stable during storage  Control all microbial growth while being harmless to humans, animals, and objects

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Factors Affecting the Efficacy of Antimicrobial methods  Nature of site to be treated  Degree of susceptibility of microbes involved  Environmental conditions that pertain

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings 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

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Relative Susceptibility of Microorganisms Figure 9.2

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings 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  Low-level germicides kill vegetative bacteria, fungi, protozoa, and some viruses

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Environmental Conditions Figure 9.3

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Methods for Evaluating Disinfectants and Antiseptics  Phenol coefficient  Use-dilution test  In-use test

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Phenol Coefficient  Evaluating the efficacy of disinfectants and antiseptics by determining the ratio of agent’s ability to control microbes to that of phenol  Greater than 1.0 indicates that agent is more effective than phenol  Has been replaced by newer methods

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings Use-Dilution Test  Metal cylinders dipped into broth cultures of bacteria and dried  Contaminated cylinder immersed into dilution of disinfectant for 10 minutes  Cylinders removed, washed, and placed into tube of medium for 48 h  Most effective agent entirely prevents growth at highest dilution  New standard procedure being developed

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings In-Use Test  Swabs taken from objects before and after application of disinfectant or antiseptic  Swabs inoculated into growth medium and incubated  Medium monitored for growth  Accurate determination of proper strength and application procedure for each specific situation

Copyright © 2007 Pearson Education, Inc. publishing as Benjamin Cummings FOR YOUR ATTENTION THANK YOU