The Control of Microbial Growth

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

PowerPoint® Lecture Slide Presentation prepared by Christine L. Case

M I C R O B I O L O G Ya n i n t r o d u c t i o n

ninth edition TORTORA FUNKE CASE

7The 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 Clostridium

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


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

Figure 7.1b


Actions of Microbial Control Agents

Alternation of membrane permeability

Damage to proteins

Damage to nucleic acids


Physical Methods of Microbial Control

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


Heat

Moist heat

denatures

proteins

Autoclave:

Steam under

pressure

Figure 7.2

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 7.3

Steam Sterilization

Steam must contact

item’s surface.



Pasteurization reduces spoilage organisms and

pathogens

Equivalent treatments

63°C (145.4°F) for 30 min

High temperature, short time: 72°C for 15 sec

Ultra high temperature: 140°C (284°F) for <1 sec

Thermoduric organisms survive


Hot-air Autoclave

Equivalent treatments 170˚C, 2 hr 121˚C, 15 min


Dry heat sterilization kills by oxidation Flaming

Incineration

Hot-air sterilization



Low temperature inhibits microbial growth Refrigeration

Deep freezing

Lyophilization

Desiccation prevents metabolism



Radiation damages DNA Ionizing radiation (X rays, gamma rays, electron

beams) – damages many molecules

Nonionizing radiation (UV) – damages DNA

(Microwaves kill by heat; not especially antimicrobial)



Chemical Methods of Microbial Control

Evaluating a disinfectant Use-dilution test

1. Metal rings dipped in test bacteria are dried.

2. Dried cultures are placed in disinfectant for 10

min at 20°C.

3. Rings are 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:

Hexacholorphene,

Triclosan

Disrupt plasma

membranes

Figure 7.7



Halogens: Iodine, chlorine

Oxidizing agents

Bleach is hypochlorous acid (HOCl)



Alcohols: Ethanol,

isopropanol

Denature proteins,

dissolve lipids

Table 7.6



Heavy metals: Ag, Hg, and Cu

Oligodynamic action

Denature proteins


Soap Degerming

Quarternary ammonium compounds “Quats”Cationic detergents

Bactericidal, Denature proteins, disrupt plasma membrane


Surface-active agents or surfactants




Chemical food preservatives Organic acids

Inhibit metabolism

Sorbic acid, benzoic acid, and calcium propionate

Control molds and bacteria in foods and

cosmetics

Antibiotics. Nisin and natamycin prevent spoilage

of cheese



Aldehydes Inactivate proteins by cross-linking with functional

groups (–NH2, –OH, –COOH, –SH)

Glutaraldehyde and formaldehyde



Peroxygens Oxidizing agents

O3, H2O2, peracetic acid


Microbial Characteristics and Microbial Control

Figure 7.11


Microbial Characteristics and Microbial Control

Table 7.7

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Table 7.8 (1 of 4)



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The Control of Microbial Growth