Chapter 9
Controlling Microbial Growth in the Environment
Terminology of Microbial Control
Sterilization
The removal or destruction of all
microbes in or on an object
Includes viruses and bacterial
endospores, but we cant sterilize prion contaminated material without
incinerating it.
Example: autoclaving nutrient agar
to make plates.
Aseptic
Free of microbes, especially
pathogens
Disinfection(disinfectants)
inhibition or killing of
microorganisms on inanimate objects by chemicals
Example: Viruses on glass slides
are killed by soaking in 10% bleach .
Antisepsis(antiseptic)
inhibition or killing of
microorganisms on skin or tissue by chemicals
Example- a phlebotomist uses
alcohol to clean the skin prior to drawing blood.
Degerming
removal by mechanical means, like
scrubbing.
Example: a surgeon washes her hands
prior to performance of a procedure.
Sanitization
an approved procedure known to
produce acceptable levels of microbe reduction or removal
Example: restaurants are required
to have written sanitization procedures that explain how food preparation
surfaces are disinfected
Pasteurization
use of heat to kill pathogens and
reduce numbers of spoilage microbes
Suffix
stasis/-static-suspend the growth without killing
Suffix
cide/-cidal destroys or inactivates a microbe
Types of controls:
Physical Methods of Microbial
Control
Chemical Methods of Microbial Control
Action of Antimicrobial Agents
Modes
of action fall into two basic categories
Alteration
of cell walls or cytoplasmic membranes
Interference
with protein and nucleic acid structure
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
Damage to Proteins and Nucleic Acids
Protein
function depends on 3-D shape
Extreme
heat or certain chemicals denature proteins
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
Factors Affecting the Efficacy of Antimicrobial methods
Nature
of site to be treated
Harsh
chemicals and extreme heat cannot be used on humans, animals, and fragile
objects
Relative Susceptibility of Microorganisms
Environmental Conditions
Physical Methods of Microbial Control
Exposure
to extremes of heat
Exposure
to extremes of cold
Desiccation
Filtration
Osmotic
pressure
Radiation
Moist Heat
Used
to disinfect, sanitize, and sterilize
Kills
by denaturing proteins and destroying cytoplasmic membranes
More
effective than dry heat; water better conductor of heat than air
Methods
of microbial control using moist heat
Boiling
Autoclaving
Pasteurization
Ultrahigh-Temperature
Sterilization
Dry Heat
Used
for materials that cannot be sterilized with or are damaged by moist heat
Denatures
proteins and oxidizes metabolic and structural chemicals
Requires
higher temperatures for longer time than moist heat
Incineration
ultimate means of sterilization
Refrigeration and Freezing
Decrease
microbial metabolism, growth, and reproduction
Chemical
reactions occur slower at low temperatures
Liquid
water not available
Desiccation and Lyophilization
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
Filtration
Used for sensitive materials
like tissue culture medium.
Osmotic Pressure
High
concentrations of salt or sugar in foods to inhibit growth
Radiation
Radiation
described as ionizing or nonionizing
Ionizing Radiation
Electron
beams effective at killing but do not penetrate well
Used
to sterilize some sliced meats, microbiological plastic ware, and medical and
dental supplies
Gamma
rays penetrate well but require hours to kill microbes
Used
to sterilize meats, spices, and fresh fruits and vegetables
X-rays
require too much time to be practical for growth control
Nonionizing Radiation
Wavelengths
greater than 1 nm
Excites
electrons and causes them to make new covalent bonds
Affects
3-D structure of proteins and nucleic acids
Suitable
for disinfecting air, transparent fluids, and surfaces of objects
Chemical Methods of Microbial Control
Affect
microbes cell walls, cytoplasmic membranes, proteins, or DNA
Effect
varies with temperature, length of exposure, and amount of organic matter
Also
varies with pH, concentration, and age of chemical
Tend
to be more effective against enveloped viruses and vegetative cells of
bacteria, fungi, and protozoa
Chemical Methods of Microbial Control
Major
Categories
Phenols
Alcohols
Halogens
Oxidizing
agents
Surfactants
Heavy
Metals
Aldehydes
Gaseous
Agents
Antimicrobics
Development of Resistant Microbes
Major emphasis by CDC on
preventing the development and spread of resistant organisms
Chapter 10
Controlling Microbial
Growth in the Body:
Antimicrobial Drugs
Drugs
Chemicals that affect physiology in
any manner
Chemotherapeutic agents drugs
that act against diseases
Antimicrobial agents drugs that
treat infections
True or False- all antimicrobials are chemotherapeutic
agents, but not all chemotherapeutic agents are antimicrobials
True!
Ideal Antimicrobial Agent
· ·
Readily available
· ·
Inexpensive
· ·
Chemically stable
· ·
Easily administered
· ·
Nontoxic and nonallergenic
· ·
Selectively toxic against wide range of pathogens
Paul Ehrlich
Magic Bullets
Arsenic compound that killed
trypanosomes and another that worked against treponemes
Alexander Fleming
Penicillin released from Penicillium
Antibiotics antimicrobial agents
produced naturally by organisms
Mechanisms of Antimicrobial Action
Key is selective toxicity
Inhibition of Cell Wall Synthesis
Examples
Vancomycin and cycloserine
interfere with particular alanine-alanine bridges that link NAM subunits in
many Gram-positives
Bacitracin blocks secretion of NAG
and NAM from cytoplasm
Isoniazid and ethambutol disrupt
formation of arabinogalactan-mycolic acid in mycobacterial species
Inhibition of Cell Wall Synthesis
Prevent bacteria from increasing
amount of peptidoglycan
Have no effect on existing
peptidoglycan layer
Effective only for growing cells
No effect on plant or animal cells;
no peptidoglycan
Inhibition of Protein Synthesis
True or false? Because
all cells synthesize protein, few drugs selectively inhibit this process.
False- prokaryotic ribosomes are 70S, eukaryotic are 80S
Disruption of Cytoplasmic Membranes
True or false- many antimicrobials that affect the cell
membrane are only used externally because they are toxic to humans.
True!
Inhibition of Metabolic Pathways
Inhibition of Nucleic Acid Synthesis
Prevention of Virus Attachment
Evaluation of Antimicrobial
Spectrum of action
Efficacy
Dosages required to be effective
Routes of administration
True or false- spinal cord
infections are difficult to treat because most drugs cannot diffuse out of the
blood into the spinal column.
True!
Overall safety
Side effects
Disruption of Normal Microbiota
May result in secondary infections
Overgrowth of normal flora
superinfections
The Development of Resistant Organisms in Populations
Some are naturally partially or
completely resistant
Resistance by bacteria acquired in
2 ways
New mutations of chromosomal genes
Acquisition of R-plasmids via
transformation, transduction, and conjugation
Multiple Resistance and Cross Resistance
Retarding Resistance
High concentrations of drug
maintained in patient for long enough time to kill all sensitive cells and
inhibit others long enough for immune system to destroy
Use antimicrobial agents in
combination; synergism vs. antagonism
Limit use of antimicrobials to
necessary cases
Development of new variations of
existing drugs (novel side chains added to original molecule)
Intentionally modifying genomes of
organisms, by natural and artificial processes, for practical purposes