Chapter 7: The Control of Microbial Growth

Sterilization

Destruction or removal of all forms of microbial life, including endospores but with the possible exception of prions.

-Usually done by steam under pressure or a sterilizing gas, such as ethylene oxide.

Commercial Sterilization

Sufficient heat treatment to kill endospores of Clostridium botulinum in canned food.

-More-resistant endospores of thermophilic bacteria may survive, but they will not germinate and grow under normal stage conditions.

Disinfection

Destruction of vegetative pathogens.

-May make use of physical or chemical methods.

Antisepsis

Destruction of vegetative pathogens on living tissue.

-Treatment is almost always by chemical antimicrobials.

Degerming

Removal of microbes from a limited area, such as the skin around an injection site.

-Mostly mechanical removal by an alcohol- soaked swab.

Sanitization

Treatment intended to lower microbial counts on eating and drinking utensils to safe public health levels.

-May be done with high-temperature washing or by dipping into a chemical disinfectant.

Germicide

"cide"= kill (aka biocide)

-Rapidly kills m/o, but not necessarily endospores

Fungicide

Kills fungi

Virucide

"Kills" or inactivates viruses

Bacteriostasis

"Stasis"= halt

-Inhabits bacterial growth and multiplication

Sepsis

Greek word for decay/putrid. Indicator of bacterial contamination

Asepsis

The absence of pathogens from an object or area

-Critical in surgical procedures

Microbial death rate

Populations of m/o subjected to a treatment die a constant rate

-This "death rate" can be plotted as a straight line

Conditions influencing microbial death

#1. Types and numbers of microbes

-Pseudomonads, TB bacillus, endospores, and viruses are relatively resistant.

-The more microbes there are to begin with, the longer it will take to kill them.

#2. Temperature

-Disinfectants tend to work better in warm solns

#3. Physiological state of the m/o

-Actively growing organisms are more susceptible to antimicrobials.

#4. Environment

-The presence of organic matter (blood, fecal matter, vomit) may inhibit disinfectants.

-M/O in boils are able to "hide" from disinfectants

#5. Time of exposure...

Where do antimicrobial agents work?

The plasma membrane

-Cell membrane permeability may be affected by altering lipid and protein content-> "leaky membranes"

Nucleic acids (DNA and RNA) and proteins

-Some antimicrobials break bonds that hold these molecules together, others interfere with synthesis

Physical methods of microbial control

-Heat: moist vs. dry

-Filtration

-Temperature

-Dessication

-Osmotic Pressure

-Radiation: Ionizing vs. non-ionizing

1. Heat

A. Dry Heat: Incineration, hot air sterilization requires 170 C for 2 hours.

B. Moist heat: Boiling (10 min) kills most m/o; but spores and viruses may survive.

Autoclaving: Steam under pressure, moisture must touch every surface to sterilize.

15 psi, 121 C, 15 min.

C. Pasteurization: Mild heating that is sufficient to kill spoilage or disease organisms without damaging the taste of the product (72 C, 15s) HTST vs. UHT.

2. Filtration

Used for heat-sensitive liquids passed through a membrane filter with pores too small for m/o to pass through (0.22-0.45) HEPA (high efficiency particulate air) filters=0.33

3. Low temperatures (depends on the bacteria)

At refrigerator temperatures (4C)

-Metabolic rates of most m/o are reduced

-Bacteriostatic

-Most m/o are dormant in the freezer (-20 C)

4. Desiccation (Removal of water)

-Water is required for growth; dried foods won't support m/o growth

-But, restoring water restores growth

-Lyophilization (freeze drying) involves fast freezing (-70 C) and removal of water in a vacuum.

Osmotic pressure: High salt or sugar concentration

-Hypertonic environment

-Generally, molds and yeasts resist osmotic pressure better than bacteria.

6. Radiation

A. Ionizing radiation: X-rays, gamma rays

-Short wavelength

-"Unzip" the strands of DNA

-Used to sterilize pharmaceuticals, disposables, and mail...

B. Non-ionizing radiation: UV light

-Less energy, less penetrating than ionizing

-Causes DNA damage (mutagenesis), thymine dimers

-"Germicidal" lamps

C. Microwaves kill indirectly by heating food, but... -> In general, few chemical agents sterilize, most reduce m/o numbers to safe levels or remove m/o from objects (fomites)

-Antiseptics are used for living tissue

-Disinfectants are used for inanimate objects -> The problem...

->No single disinfectant is an "all purpose" agent. It is essential to find the proper agent/purpose!

Types of disinfectants

1. Phenol (carbolic acid)

-Used by Lister in the 1860s for aseptic surgery

-Seldom used today... smelly and irritating.

-Injures plasma membranes and inactivates enzymes

-Found in some cough drops (anesthetic affect) and Chloraseptic

Types of disinfectants

2. Phenolics: derivatives of phenol

-Not as irritating as phenol

-Used in combination with detergents

-Injures plasma membranes and inactivate enzymes

-Stable in the presence of organic compounds

-Notable phenolics-> o phenylphenol (Lysol): Effective surface disinfectant

-Triclosan: Found in "antibacterial" soaps and toothpaste

-Widespread resistance has been documented in several species of bacteria!

Types Continued