Chapter 7 Controlling Microbial Growth

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
  

Effectiveness of antimicrobial treatment depends on:

• Number of microbes

• Environment (organic matter, temperature, concentration, biofilms)

• Time of exposure

• Microbial characteristics (e.g. glycocalyx,cell wall, resistance)

Autoclaving

• Moist heat denatures proteins

• Autoclave: Steam under pressure

• 15 min at 121oC at 15 psi

Pasteurization

• It reduces spoilage organisms and pathogens

• 60oC for 30 minutes

• 72oC for 15 seconds

• 140oC for 4 seconds

Dry Heat Sterilization

• It kills by oxidation

• Includes

- Flaiming

- Incineration

- Hot-air sterilization

• EX: Autoclave (121oC, 15 min) & Hot-air (170oC, 2 hr)

Physical Methods of Microbial Control

• Filtration

• Low Temperature

• High Pressure

• Desiccation

• Osmotic Pressure

Filtration

• Remove microbes by trapping them in filter

Low Temperature

• Inhibits microbial growth

• Includes

- Refrigeration

- Deep freezing (-20oC or -80oC)

- Lyophilization

High Pressure

• Denatures Proteins

Dessication

• Prevents metabolism

Osmotic Pressure

• Causes plasmolysis (shrinkage of cytoplasm)

Radiation damages to DNA

• Ionizing Radiation - X rays, gamma rays, electron beams

• Nonionizing Radiation (UV) - surface sterilization only

• 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 placed in diluted disinfectant (according to manufacturer’s instructions) for min at 20 C

• Rings transferred to culture media to determine whether bacteria survived treatment 

Evaluating a Disinfectant

• Disk-diffusion method

• Particular species are evaluated in each test

• Zone of inhibition must be at or beyond a certain diamter

Phenolics

• Disruption of the plasma membrane

• EX

- Phenol

- O-phenylphenol (Lysol Spray, Amphyl)

- Thymol (Listerine mouth wash)

- Hexachlorophene (hand soap)

- Pine Sol / Pine oil (turpineol)

- 

Alcohols

• Denatures proteins, dissolve lipids

• EX: ethanol, isopropanol 

Heavy Metals

• Denature proteins by breaking disulfied bonds

• EX

- Merurachrome - antiseptic for wounds

- Coppuer Sulfate (CuSO4) - algicide

- Zince chloride (ZnCl2) - mouthwashes

- Silver nitrate (AgNO3) - antiseptic for eyes, wounds

Suface Active Agents 

• Surfactants (ex: Soap and Detergents

• EX:

- Soap: Degerming

- Acid-anionic Detergents: Sanitizing 

Quaternary Ammonium Compounds (Quats)

• Disruption of plasma membrane, denaturation of proteins

• EX

- Benzalkonium chloride (Zephiran)

- Roccal (lab disinfectant)

- Cationic detergents

- Cetylpyridinium chloride (Cepacol)

Aldehydes

• Inactive proteins by cross-linking with functional groups (-NH2, -OH, -COOH, -SH)

• Glutaraldehyde, formaldehyde

• EX: Disinfection of hospital equipment, especially in respiratory therapy, for kidney dialysis machines, endoscopes

Alkali and Acids

• NaOH (sodium hydroxide)

• NH4OH in ammonia, good at breaking down lipids

• Benzoic acid in mouthwashes ; benzoic acid, sorbic acid, citric acid, and ascorbic acid, in food products

Gaseous Sterilants

• Denature proteins

• Ethylene oxide to Sterilize hospital equipment and disposable lab plasticware

Other Oxidizing Agents

• Steals electrobs from biomolecules causing breakdown

• O3 (swim pools), H2O2 (skin), Cl2 (pools), peracetic acid (dialysis equipment), bleach (sodium hypochlorite), bromine

Microbial Characteristics & Microbial Control

From Most Resistant to Least Resistant

• Prions (most resistant)

• Endospores of bacteria

• Mycobacteria

• Cysts of protozoa

• Vegetative protozoa

• Gram-negative bacteria

• Fungi

• Viruses without envelopes

• Gram-positive bacteria

• Viruses with lipid envelopes (least resistant)

Effectiveness against Endospores & Mycobacteria

• Phenolics - Poor Endospores, Good Mycobacteria

• Quats - None Endospores, None Mycobacteria

• Chlorines - Fair Endospores, Fair Mycobacteria

• Alcohols - Poor Endospores, Good Mycobacteria

• Glutaraldehyde - Fair Endospores, Good Mycobacteria