The Use Of Agars In Microbiology

From its infancy, microbiology has depended on several advances in the field by not only its founders, but by the giants in the field

. The problem of isolating and culturing, or growing, microorganisms for study in the laboratory has seen many resolutions, and some of those are centered around the growth media used to enrich pathogens for further study.

The discovery of agar, the extract from a certain species of red seaweed, was an enormous breakthrough for the microbiological field, as the growth media prior to this discovery was inefficient for the proper culturing of organisms. Meat proteins and gelling materials were easily broken down by the organisms, thus becoming ineffective at successful growth. Agar solved this problem, as it is a firm yet clear gel that can have any chemical or material suspended within it, leading to specialized media we know of today.

Growth media comes in many forms, and each has its place in the microbiology laboratory. Broths are used for the proliferation of organisms, especially those that are fastidious and hard to grow on standard media types. Broths have a high content of nutrients that is not normally found in the organisms' natural environment, giving better nutrition for reproduction of the organism.

Fastidious bacteria are notorious for inhibited growth on standard agar media plates. Broths incorporate special infusions that pathogens thrive in, such as beef or brain extracts. This is highly effective in culturing a large volume of the organisms needed for research or identification of disease from hard to grow pathogens.

Specialized agar media such as blood infused agar plates are effective for determining the hemolytic reaction that the organisms may produce. The breaking down of the blood cells suspended within the agar matrix can be easily seen after 18-24 hour incubation, for example, with Staphylococcus Aureus, on elf the causes of MRSA in humans. The gold-to-white, flat and concentric colonies will have a clear halo where the organisms have broken down and consumed, completely, the blood suspension. This is indicative of the harmful nature of this pathogen.

MacConkey agar is an effective media for the determination of gram-negative organisms such as Enterobacteriaceae like Escherichia coli. This organism has been responsible for contaminated foods and the deaths of human being due to the toxic nature of the by-products the organism secretes. On a MacConkey plate, the organism colonies take on an unmistakable pink cast, and the colony morphology has an added parameter used in identification. MacConkey agar has a dye indicator and sugars specific for gram-negative rods, and inhibitors to prevent any other organisms from growing on the agar.

Chocolate agar is not made from the confection, which gives it its namesake, but from the colour of the agar itself. It also is made from blood suspension within the agar, although the blood cells have been broken down to release nutrients into the agar, causing even the most fastidious organisms to proliferate.

There are other highly specialized agar mixtures, which can give positive identification of organisms that would otherwise require intensive culturing techniques. These culture mediums are prized by researchers and medical microbiologists worldwide for the successful isolation and identification of organisms.

by: Andrew Long

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