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Control Microbial Growth – Microbiology Outline Notes

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Control of Microbial Growth


I. Terminology

A. Sterilization – removal or destruction of all microbial life (including endospores)

B. Disinfection – destruction of pathogens; does not remove all microbial life, but decreases the numbers

1. Disinfectant – chemical used on a surface (kitchen counter)

2. Antiseptic – chemical used on human tissue (wound)

3. Degerming – mechanical removal of microbes

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a. wipe skin with alcohol swab before injection, surgical hand scrub

4. Sanitization – lower microbial count to safe health levels (silverware in a restaurant)


C. Germicide (biocide) – kills microbes, but not endospores

D. Bacteriostasis – inhibits growth of microbes; doesn’t kill

E. Sepsis – bacterial contamination

1. Septicemia – bacteria in blood

2. Aseptic technique – absence of significant contamination

a. minimize contamination during procedure/operation


II. Rate of Microbial Death

A. When treated with heat or antimicrobial agent, bacteria die at a constant rate


Example – For every minute treatment is applied, 90% of the remaining population dies. Microbial death is at a constant rate.



Deaths per Minute

Number of Survivors























B. Factors That Influence the Effectiveness of Antimicrobial Treatments

1. Number of Microbes

a. The more microbes the longer it takes to eliminate the entire population

2. Environment

a. Microbes in organic matter such as feces, blood, and surface biofilms are harder

for germicides to reach

3. Time of Exposure

a. More resistant microbes need to be exposed to antimicrobials longer

4. Type of Microbe

C. How Antimicrobials Work

1. Damage the cell membrane

2. Damage intracellular proteins (enzymes)

3. Damage nucleic acids (DNA and RNA)


D. 2 Ways to Control Microbial Populations

1. Physical

2. Chemical


III. Physical Control

A. Heat

1. Denatures enzymes

2. Moist Heat

a. Boiling – kills most microbes in 10 minutes

1) Some viruses and endospores can survive

2) Therefore, not always reliable sterilization

b. Autoclaving

1) Higher temperature than boiling

2) Steam under pressure

3) Kills everything in 15 minutes

3. Pasteurization

a. Uses mild heat to kill pathogens

b. Also, decreases the number of microbes

c. Prolongs good quality of refrigerated dairy

d. Many thermoduric bacteria (non-pathogenic) survive traditional pasteurization

e. Ultra-high-temperature (UHT) treatment sterilizes milk (coffee creamers)

4. Dry Heat Sterilization

a. Flaming your inoculating loop in the lab


B. Filtration

1. Use a filter with pores smaller than microbes

2. HEPA filters are used to filter air


C. Cold Temperature

1. Refrigeration decreases metabolism of most bacteria; don’t grow

2. Bacteriostatic effect

3. Psychrotrophs do grow, most are not pathogenic

4. Listeria monocytogenes causes listeriosis


D. Desiccation

1. Removal of water

2. Viruses and endospores are resistant to desiccation

3. Infectious microbes can survive in dried mucus, urine, pus, feces


E. Osmotic Pressure

1. Use salt to preserve food

F. Radiation

1. Sterilizing radiation kills microbes

2. Two types: Ionizing and Non-ionizing radiation

3. Ionizing Radiation

a. Example, X-rays and gamma rays

b. Ionize water to form OH• (hydroxyl radicals); interferes with DNA

c. Used to sterilize medical supplies such as gloves, syringes, catheters

4. Non-ionizing radiation

a. Example, Ultraviolet (UV) light

b. Damages DNA by causing Thymine Dimers

c. UV “germicidal” lamps used in hospitals


IV. Chemical Control

A. Chemicals used to control the growth of microbes on living tissue and objects

B. Many Types of Disinfectants / Antiseptics

C. Phenols and Phenolics

1. Phenol

a. Lister used

b. Irritates skin, not used much anymore

c. Antimicrobial action – damages plasma membranes

d. Throat sprays (oral anesthetic, analgesic, and antimicrobial)

2. Phenolics – phenol derivative

a. Stable and lasts a long time after application

b. O-phenyl phenol – main ingredient in Lysol®

c. Triclosan

1) Antibacterial soaps, kitchenware, etc

2) Its overuse is leading to bacterial resistance to it

D. Chlorhexidine

1. Mode of action is to affect cell membranes

2. Surgical hand scrubs and preoperative skin prep


E. Halogens

1. Iodine and Chlorine

2. Iodine (I2)

a. Very effective against most bacteria, many endospores, various fungi/viruses

b. Alters protein synthesis and affects cell membranes

c. Iodophor – Iodine in alcohol (tincture)

1) providone-iodine (Betadine®)


2. Chlorine (Cl2)

a. Germicidal Action is to form hypochlorous acid (HOCl)

b. Hypochlorous acid is strong oxidizing agent

Cl2  +  H2O  —————-à H+  +  Cl  +  HOCl

c. Sodium hypochlorite (NaOCl)

1) Bleach (Clorox®)

F. Alcohols

1. Kill bacteria, but not endospores and non-enveloped viruses

2. Mechanism of action is protein denaturation and disruption of lipid membranes

3. Not good antiseptic when applied to wounds because they cause coagulation of a

layer of protein which bacteria can grow under.

4. Ethanol

a. Purell® hand cleaner

5. Isopropanol

a. Rubbing alcohol

6. Tincture

a. Mixing another chemical with alcohol to enhance effectiveness

1) Iodophore


G. Heavy Metals

1. Silver – used in topical ointments

2. Zinc – mouthwashes


H. Surface Active Agents

1. Soaps and Detergents

a. Not an antiseptic

b. Surfactant – decreases surface tension; emulsification

1) Enhances mechanical removal of microbes during scrubbing

2. Antibacterial soap

a. Contains an antimicrobial, such as Triclosan

b. Fear of resistant microbes


I. Oxidizing Agents

1. Hydrogen Peroxide (H2O2)

a. Not good for antisepsis on open wounds because our cells produce catalase

which breaks it down

b. Good disinfectant, especially for anaerobes


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