Chapter 7

The Control of Microbial Growth


The 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

-    Degerming: Removing microbes from a limited area

-    Sanitization: Lowering microbial counts on eating utensils

-    Biocide/germicide: Kills microbes

-    Bacteriostasis: Inhibiting, not killing, microbes


Microbial Death Curve


Effectiveness of Treatment

-    Depends on

-    Number of microbes

-    Environment (organic matter, temperature, biofilms)

-    Time of exposure

-    Microbial characteristics


Actions of Microbial Control Agents

-    Alteration of membrane permeability

-    Damage to proteins

-    Damage to nucleic acids



-    Thermal death point (TDP): Lowest temperature at which all cells in a culture are killed in 10 min

-    Thermal death time (TDT): Time during which all cells in a culture are killed


Decimal Reduction Time (DRT)

-    Minutes to kill 90% of a population at a given temperature


Moist Heat Sterilization

-    Moist heat denatures proteins

-    Autoclave: Steam under pressure


Steam Sterilization

-    Steam must contact items surface



-    Reduces spoilage organisms and pathogens

-    Equivalent treatments

-    63C for 30 min

-    High-temperature short-time: 72C for 15 sec

-    Ultra-high-temperature: 140C for <1 sec

-    Thermoduric organisms survive


Dry Heat Sterilization

-    Kills by oxidation

-    Dry heat

-    Flaming

-    Incineration

-    Hot-air sterilization



-    HEPA removes microbes >0.3 m

-    Membrane filtration removes microbes >0.22 m



Physical Methods of Microbial Control

-    Low temperature inhibits microbial growth

-    Refrigeration

-    Deep-freezing

-    Lyophilization

-    High pressure denatures proteins

-    Desiccation prevents metabolism

-    Osmotic pressure causes plasmolysis



-    Ionizing radiation (X rays, gamma rays, electron beams)

-    Ionizes water to release OH

-    Damages DNA

-    Nonionizing radiation (UV, 260 nm)

-    Damages DNA

-    Microwaves kill by heat; not especially antimicrobial


Principles of Effective Disinfection

-    Concentration of disinfectant

-    Organic matter

-    pH

-    Time


Use-Dilution Test

-    Metal rings dipped in test bacteria are dried

-    Dried cultures are placed in disinfectant for 10 min at 20C

-    Rings are transferred to culture media to determine whether bacteria survived treatment

Disk-Diffusion Method


Phenol & Phenolics

-    Disrupt plasma membranes



-    Hexacholorphene, triclosan

-    Disrupt plasma membranes



-    Chlorhexidine

-    Disrupt plasma membranes



-    Iodine

-    Tinctures: In aqueous alcohol

-    Iodophors: In organic molecules

-    Alter protein synthesis and membranes

-    Chlorine

-    Bleach: Hypochlorous acid (HOCl)

-    Chloramine: Chlorine + ammonia

-    Oxidizing agents



-    Ethanol, isopropanol

-    Denature proteins, dissolve lipids

-    Require water


Heavy Metals

-    Ag, Hg, and Cu

-    Silver nitrate may be used to prevent gonorrheal ophthalmia neonatorum

-    Silver sulfadiazine used as a topical cream on burns

-    Copper sulfate is an algicide

-    Oligodynamic action

-    Denature proteins


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

-    Nitrite prevents endospore germination

-    Antibiotics

-    Nisin and natamycin prevent spoilage of cheese



-    Inactivate proteins by cross-linking with functional groups (–NH2, –OH, –COOH, –SH)

-    Use: Medical equipment

-    Glutaraldehyde, formaldehyde, and ortho-phthalaldehyde


Gaseous Sterilants

-    Denature proteins

-    Use: Heat-sensitive material

-    Ethylene oxide



-    Free radicals destroy microbes

-    Use: Tubular instruments


Supercritical Fluids

-    CO2 with gaseous and liquid properties

-    Use: Medical implants



-    Oxidizing agents

-    Use: Contaminated surfaces

-    O3, H2O2, peracetic acid


Endospores and Mycobacteria