CONTROL OF MICROBIAL GROWTH

 

What is media ?

Microbiological media, or bacterial culture media, is a growth medium used to grow bacteria. In other words, it contains everything bacteria need to grow outside the body and under laboratory conditions. An organic media contains organic material that a certain bacterium may need to grow.

What is contamination :-

Cell culture contaminants can be divided into two main categories, chemical contaminants such as impurities in media, sera, and water, endotoxins, plasticizers, and detergents, and biological contaminants such as bacteria, molds, yeasts, viruses, mycoplasma, as well as cross contamination by other cell lines.

 CONTROL OF MICROBIAL GROWTH

Control of microbial growth means to inhibit or prevent growth of microorganisms. This control is affected in two basic ways:

1. By killing microorganisms or

2. By inhibiting the growth of microorganisms.

Control of growth usually involves the use of physical or chemical agents which either kill or prevent the growth of microorganisms.

Agents which kill cells are called cidal agents and agents which inhibit the growth of cells are referred to as static agents.

 Thus, the term bactericidal refers to killing bacteria, and bacteriostatic refers to inhibiting the growth of bacterial cells. Terms commonly used to describe antimicrobial agents and processes:

Antiseptics

A disinfectant that is nontoxic enough to be used on skin. Examples include alcohols (ethanol, 50-70% - denatures proteins and solubilizes lipids), silver nitrate (precipitates proteins), iodine solution (inactivates proteins), detergents (disrupts cell membranes).

Disinfectants

A chemical used to destroy many microorganisms and viruses. Examples include, hypochlorites, chlorine compounds, copper sulfate, formaldehyde (reacts with NH2, SH and COOH groups), phenolic compounds (denature proteins and disrupt cell membranes), and mercuric chloride (Inactivates proteins by reacting with sulfide groups).

Sterilization

Sterilization refers to the complete and total destruction or elimination of all viable organisms, including microbial endospore, in or on a substance being sterilized.

Sterilization procedures involve the use of heat, radiation or chemicals, or physical removal of cells.

Pasteurization

Pasteurization is a brief heat treatment used to reduce the number of spoilage organisms and to kill diseasecausing microbes. It uses precisely controlled heat to reduce the microbial populations in milk and other heat-sensitive liquids.

The process is named after Louis Pasteur, who first used heat for controlling the spoilage of wine. Pasteurization does not kill all organisms and is therefore not synonymous with sterilization.

Milk is usually pasteurized by heating, typically at 63°C for 30 minutes (batch method) or at 71°C for 15 seconds (flash method), to kill bacteria and extend the milk's usable life.

MICROBIAL CONTROL METHODS

Physical agents

Heat, radiation and filtration are most common physical agents normally used to control microbial growth. Heat is the most rapid and efficient physical agent used to control a population of microbes.

Both dry and moist heat is used to kill microbes, Moist heat generally coagulates and causes denaturation in microoganisms. In dry heat, by contrast, the primary effect on microbes is due to oxidation of large molecules.

Similarly there are many different kinds of radiations having antimicrobial effects. Ionizing radiations which include x-rays and gamma rays, form free radicals in cytoplasm and the free radicals damage microbial protein and DNA.

Another important form of radiation is ultraviolet radiations. These radiation damage nucleic acids.

Filtration involves the physical removal of all cells in a liquid or gas. It is especially important for sterilization of solutions which would be denatured by heat (e.g. antibiotics, amino acids, vitamins, etc.).

In this process solutions or gases are passed through a filter of sufficient pore diameter (generally 0.22 micron) to remove the smallest known bacterial cells.

Chemical agents

Large number of chemicals such as alcohol, aldehyde, heavy metals, halogens and dyes are commonly used as antimicrobial agents.

 Alcohols like ethyl alcohol and isopropyl alcohol cause denaturation of proteins and used as an antiseptic.

Two heavy metals, mercury and silver, are used an antimicrobial agents. Mercury is used as mercuric chloride and as a component of Mercurochrome. Silver is commonly used as silver nitrate.

 Two important halogens, chlorine and iodine, act as oxidizing agents that react with the amino acids in proteins and change the nature of the protein. Similarly, gases like ethylene oxide, formaldehyde, and beta propiolactone are used to achieve sterilization.

ANTIBIOTICS

Antibiotics are low molecular-weight antimicrobial agents produced as secondary metabolites by microorganisms that kill or inhibit other microorganisms.

 The term, coined by Selman Waksman, originally described only those formulations derived from living organisms.

A more broadened definition of an antibiotic includes any chemical of natural origin which can kill or inhibit the growth of other types cells. Since most clinically-useful antibiotics are produced by microorganisms and are used to kill or inhibit infectious bacteria,

we will follow the classic definition.

Antibiotics may have a cidal effect or a static effect on a range of microbes. The range of bacteria or other microorganisms that are affected by a certain antibiotic is expressed as its spectrum of action.

Antibiotics effective against a wide range of microoganisms such as both Gram-positive and Gram-negative bacteria are said to be broad spectrum antibiotics. If effective mainly against Gram-positive or Gram-negative bacteria but not both, they are narrow spectrum.

 If effective against a single organism or disease, they are referred to as limited spectrum. Antibiotic molecules produced by a microbe that are subsequently modified by an organic chemist to enhance their antimicrobial properties or to render them unique for a pharmaceutical patent are described as semisynthetic antibiotics.