The Control Of Microbial Growth

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Terminology of microbial growth

1. Sterilization
2. Disinfection
3. Antisepsis
4. Degerming
5. Sanitization
6. Biocide
7. Germicide
8. Bacteriostasis
9. Asepsis

Sterilization

The destruction of all forms of microbial life, including endospores, which are the most resistant form. Hearing is the most common method used for sterilization.

Commercial sterilization

Treatment required to ensure absolute sterility of canned food would degrade the quality of the food. Instead, food is subjected only to enough heat to destroy the endospores of clostridium botulinum, which can produce a deadly toxin.

Disinfection

Is directed at destroying harmful organisms. It usually refers to the destruction of vegetative pathogens, which is not the same as complete sterility. Disinfection might make us of chemicals, ultraviolet radiation, boiling water, or steam. In practice, the term is most commonly applied to the use of a chemical to treat an inert surface.

Antisepsis and antiseptic

When a disinfectant treatment is applied to living tissue, it is called antisepsis, and the chemical is then called an antiseptic. Therefore, in practice the same chemical might be called a disinfectant for one use and an antiseptic for another.

Degermination

The mechanical removal, rather than the killing, of most microbes in a limited area. For example, when someone is about to receive and injection, the skin is swabbed with alcohol.

Sanitization

The process of Sanitization is intended to lower microbial counts to safe public health levels and to minimize the chances of disease transmission from one user to another. Restaurant glassware, china, and tableware are subjected to sanitization.

Suffix --cide

Names of treatments that cause outright deaths of microbes have the suffix -cide, meaning kill. For example, a biocides or germicide, kills Microorganisms (usually with certain exceptions, such as endospores); a fungicide kills fungi; a viracide inactivated viruses.

Suffix -stat or -stasis

Treatments that only inhibit the growth and multiplication of bacteria have the suffix -stat or -stasis, meaning to stop or to steady, as in bacteriostasis. Once a bacteriostatic agent is removed, growth might resume.

Sepsis

From the Greek for decay or putrid, indicates bacterial contamination, as in septic tanks for sewage treatment. Sepsis is also used to describe a disease condition.

Aseptic

An object or area that is free of pathogens. In our laboratory and in hospitals we practice aseptic technique to minimize contamination from instruments, and each other.

The rate of microbial death

When bacterial populations are heated or treated with antimicrobial agents, they usually die at a constant rate. For example, suppose a population of 1 million microbes has been treated for one minute, and 90% of the population has died, we are now left with 100,000 microbes. Thus for every minute the treatment is applied 90% of the remaining population is killed. If the death curve is plotted logarithmically, the death rate is constant.
Factors that influence the effectiveness if antimicrobial agents:
1. The number of microbes
2. Environmental influences
3. Time of exposure
4. Microbial characteristics

The number of microbes

The more microbes there are, the long it will take to eliminate the entire population.

Environmental influences

The presence of organic matter often inhibits the action of chemical antimicrobials. For example, in hospitals, the presence of organic matter in blood, vomit, or feces influence the selection of disinfectants. Temperature, and pH may also influence the efficacy of antimicrobial agents.

Time of exposure

Chemical antimicrobials often require extended exposure for more resistant microbes or endospores to be affected. For example, in heat treatments, a longer exposure can compensate for a lower temperature.

Microbial characteristics

Many biocides tend to be more effective against gram positive bacteria as a group, than against gram negative bacteria due to the lipopolysaccharide layer of gram negative bacteria. The genera pseudomonas are unusually resistant to biocides and will even grow actively in some disinfectants and antiseptics and even antibiotics. This resistance is most notably due to the characteristics of their porins.

Porins

Structural openings in the wall of gram negative bacteria. Porins are highly selective of molecules that they permit to enter the cell.

Actions of microbial agents

1. Alteration of membrane permeability
2. Damage to proteins and nucleus acids

Alteration of membrane permeability

A Microorganism's plasma membrane, located just inside of the cell wall, is the target of many microbial control agents. Damage to the lipids or proteins of the plasma membrane by antimicrobial agents causes cellular contents to leak into the surrounding medium and interferes with the growth of the cell.

Damage to proteins and nucleic acids

Enzymes which are primarily proteins are viral to all cellular activities. Agents that target proteins may result in denaturation resulting in their inactivation. The nucleic acids DNA and RNA are carriers of the cells genetic information. Damage to these nucleic acids by heat, radiation, or chemicals is frequently lethal to the cell; the cell can no longer replicate, nor can it carry out normal metabolic functions such as synthesis of enzymes.

Physical methods of microbial control

Heat
Moist heat: boiling, autoclaving
Pasteurization
Dry heat: direct flaming, hot-air sterilization
Filtration
Low temperatures
Desiccation
Osmotic pressure
Radiation: ionizing and non ionizing radiation

Heat

Kills Microorganisms by denaturing their enzymes, thus inactivating them.

Thermal death point (TDP)

Refers to the lowest temperature at which all the microorganisms in a liquid suspension will be killed in 10 minutes.

Thermal death time (TDT)

Refers to the minimal length of time for all bacteria in a liquid culture to be killed at a given temperature.

Decimal reduction time (DRT)

Refers to the time in minutes, in which 90% of a population of a bacterium at a given temperature will be killed. This is especially useful in the canning industry. (Does not achieve sterilization)

Moist heat

Moist heat kills microorganisms primarily by the coagulation if proteins, which is caused by breakage of the hydrogen bonds that holds the proteins in their three-dimensional structure. Protein coagulation or denaturation occurs more quickly in the presence of water.

Boiling

Boiling is one type of moist heat sterilization. It primarily kills vegetative forms of bacterial pathogens, most viruses, fungi and their spores within 10 minutes. However, endospores and some viruses are not destroyed this quickly. For example, some hepatitis viruses can survive up to 30 minutes of boiling, and some endospores have resisted boiling for more than 20 hours. Boiling is therefore not always a reliable sterilization procedure.

Autoclaving

Reliable sterilization with moist heat requires temperatures above that of boiling water. These high temperatures are most commonly achieved by steam under pressure in an autoclave. Autoclaving is the preferred method is sterilization, unless the material to be sterilized can be damaged by heats or moisture. Sterilization is an autoclave is most effective when the organisms are either contacted by the steam directly or are contained in a small volume of water. Under these conditions, steam at a pressure of about 15 psi (121 degrees Celsius) will kill all organisms and their endospores in about 15 minutes. Autoclaving is used to sterilize culture media, instruments, dressings, intravenous equipment and other medical devices.

Pasteurization

Louis Pasteur used mild heating, sufficient to kill the organisms that caused a particular spoilage problem without seriously damaging the taste of the product. The same principle was applied to milk to produce what we now call pasteurized milk. In classical pasteurization, the milk was exposed to a temperature of about 63 degrees Celsius for 30 minutes. However, most milk pasteurization today uses higher temperatures, at least 72 degree Celsius for only 15 seconds. This heat treatment is know as high temperature short-time pasteurization. Milk can also be sterilized by ultra high temperature treatments so that it can be store without refrigeration.

Dry heat sterilization

Dry heat kills by oxidation effects. One of the simplest method of dry heat sterilization is direct flaming.

Hot-air sterilization

Another form of dry heat sterilization is hot-air sterilization. Items to be sterilized by this procedure are placed in an oven. Generally, a temperature of about 170 degrees calculus for nearly 2 hours ensure sterilization.

Filtration

The passage of a liquid or has through a screenlike material with pores small enough to retain microorganisms. Filtration is used to sterilize heat-sensitive materials. Some operating rooms receive filtered air to lower the numbers of airborne microbes. For example, high-efficiency particulate air filter (HEPA) remove almost all microorganisms larger then about 0.3 um in diameter.

Low temperatures

The effect of low temperatures on microorganisms depends I. The particular microbe and the intensity of the application. For example, at temperatures of ordinary refrigerators (0-7 degrees Celsius), the metabolic rate of most microbes is so reduced that they cannot reproduce or synthesize toxins. In other words, ordinary refrigeration has a bacteriostatic effect. Ye

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