TORTORA • FUNKE
• CASE
Microbiology
AN INTRODUCTION
EIGHTH EDITION
B.E Pruitt & Jane J. Stein
Chapter 7
The Control of Microbial Growth
The Control of Microbial
Growth
• Sepsis refers to microbial
contamination.
• Asepsis is the absence of significant
contamination.
• Aseptic surgery techniques prevent
microbial contamination of wounds.
Terminology
• Sterilization: Removal of all microbial life
– Commercial Sterilization: Killing C. botulinum
endospores
• Disinfection: Removal of pathogens
• Antisepsis: Removal of pathogens from living
tissue
• Degerming: Removal of microbes from a limited
area
• Sanitization: Lower microbial counts on eating
utensils
• Biocide/Germicide: Kills microbes
• Bacteriostasis: Inhibiting, not killing, microbes
• Bacteriocidal: Kills bacteria
• Bacterial populations die at a constant
logarithmic rate.
Figure 7.1a
Effectiveness of antimicrobial
treatment depends on:
• Number of
microbes
• Environment
(organic matter,
temperature,
biofilms)
• Time of
exposure
• Microbial
characteristics
• Condition of
microbes - lag,
log or sport
Figure 7.1b
Actions of Microbial Control
Agents
Mechanisms of action for Antimicrobial control
agents
• Alternation of membrane permeability
• Damage to proteins
• Damage to nucleic acids
• Interfere
with
metabolic
pathways
Methods
may be
Physical
or Chemical
•Physical like heat
•Chemical like disinfectants
Physical Methods of Microbial
Control
Heat is very commonly used, heat may be
moist or dry.
• Heat
– Thermal death point (TDP): Lowest temperature
at which all cells in a culture are killed in 10 min.
– Thermal death time (TDT): Time to kill all cells in
a culture
– Decimal reduction time (DRT): Minutes to kill
90% of a population at a given temperature
Moist Heat
• Moist
heat
denature
s
proteins
• Autoclav
e: Steam
under
pressure
Figure 7.2
Physical Methods of Microbial
Control
• Pasteurization reduces spoilage organisms
and pathogens
• Equivalent treatments
– 63°C for 30 min
– High-temperature short-time 72°C for 15 sec
– Ultra-high-temperature: 140°C for <1 sec
– Thermoduric organisms survive
Physical Methods of Microbial
Control
• Dry Heat Sterilization kills by oxidation
– Flaming
– Incineration
– Hot-air sterilization
Equivalent treatments
Hot-air
Autoclave
170˚C, 2 hr
121˚C, 15 min
Physical Methods of Microbial
Control
• Filtration removes microbes
• Low temperature inhibits microbial growth
– Refrigeration
– Deep freezing
– Lyophilization
• High pressure denatures proteins
• Desiccation prevents metabolism
• Osmotic pressure causes plasmolysis
Physical Methods of Microbial
Control
Electromagnetic spectrum, shorter wavelength
means great energy and can penetrate
further.
• Radiation damages DNA
• Two types used:
– Ionizing radiation (X rays, gamma rays, electron
beams)
– Non-ionizing radiation (UV)
– (Microwaves kill by heat; not especially
Electromagnetic Spectrum
Figure 7.5
Chemical Methods of Microbial
Control
• Principles of effective disinfection
– Concentration of disinfectant
– Organic matter
– pH
– Time
• Often chemical methods are compared to
Phenol. If more effective given a phenol
coefficient of >1. If less effective give a
phenol coefficient of <1.
Chemical Methods of Microbial
Control
• Evaluating a disinfectant
– Use-dilution test
1.
2.
3.
Metal rings dipped in test bacteria are dried
Dried cultures placed in disinfectant for 10
min at 20°C
Rings transferred to culture media to
determine whether bacteria survived
treatment
Chemical Methods of Microbial
Control
Evaluating a disinfectant
Disk-diffusion method
Figure 7.6
Types of Disinfectants
Phenol
Phenolics. Lysol
Bisphenols.
Hexachlorophene, Triclosan
Disrupt plasma
membranes
Figure 7.7
Types of Disinfectants
• Biguanides. Chlorhexidine
– Disrupt plasma membranes
Types of Disinfectants
• Halogens. Iodine, Chlorine
– Oxidizing agents
– Bleach is hypochlorous acid (HOCl)
Types of Disinfectants
Alcohols. Ethanol,
isopropanol
Denature proteins,
dissolve lipids
Table 7.6
Types of Disinfectants
• Heavy Metals. Ag, Hg, Cu
– Oligodynamic action
– Denature proteins
Types of Disinfectants
• Surface-Active Agents or Surfactants
Soap
Degerming
Acid-anionic detergents
Sanitizing
Quarternary ammonium compounds
Cationic detergents
Bactericidal, Denature proteins, disrupt
plasma membrane
Types of Disinfectants
• Chemical Food Preservatives
– Salts
– Sugar
– Dry
– Organic Acids
• Inhibit metabolism
• Sorbic acid, benzoic acid, calcium propionate
• Control molds and bacteria in foods and cosmetics
– Nitrite prevents endospore germination
– Antibiotics. Nisin and natamycin prevent
spoilage of cheese
Types of Disinfectants
• Aldehydes
– Inactivate proteins by cross-linking with
functional groups (–NH2, –OH, –COOH, —
SH)
– Glutaraldehyde, formaldehyde
Types of Disinfectants
• Gaseous Sterilants
– Denature proteins
– Ethylene oxide - not Ethyl oxide
Types of Disinfectants
• Peroxygens
– Oxidizing agents
– O3, H2O2, peracetic acid
Microbial Characteristics and
Microbial Control
Figure 7.11
Microbial Characteristics and
Microbial Control
Chemical agent
Effectiveness against
Endospores
Mycobacteria
Phenolics
Poor
Good
Quats
None
None
Chlorines
Fair
Fair
Alcohols
Poor
Good
Glutaraldehyde
Fair
Good