Microbiology will be incomplete without the discovery and development of the solid culture media, as well as the Petri dish plates and pure culture systems that allows for the specific isolation and identification of microorganisms (bacteria) from various environments – so that they can be studied further, and their importance and metabolites exploited for the sustainable development of man, plants, animals and the environment especially in the areas of health and food production. In addition to this development was the discovery and development of the microscope – which helped in revolutionizing the field of microbiology and even provided a platform for viewing and understanding the useable or infinitesimally nature/world of microorganisms that are too small to be seen by the naked eyes.
Robert Koch was the first to grow bacteria on solid culture media. Koch’s early use of potato slices as solid media was fraught with problems. Besides being rather selective in terms of which bacteria would grow on the slices, the slices were frequently overgrown with molds. Koch thus needed a more reliable and reproducible means of growing bacteria on solid media, and he found the answer in agar – the solidifying component of culture media.
Koch initially employed gelatin as a solidifying agent for the various nutrient fluids he used to culture bacteria and developed a method for preparing horizontal slabs of solid media that were kept free of contamination by covering them with a bell jar or glass box. Nutrient gelatin was a good culture medium for the isolation and study of various bacteria, but it had several drawbacks, the most important being that it did not remain solid at 37oC, the optimum temperature for the growth of most human pathogens. This necessitated the need for an alternative and different solidifying agent that would exclude the inconsistency applicable to the gelatin.
Agar is a polysaccharide derived from red algae. It was used widely in the nineteenth century as a gelling agent. Walter Hesse, an associate of Robert Koch, first used agar as a solidifying agent for bacteriological culture media. The actual suggestion that agar be used instead of gelatin was made by Hesse’s wife, Fannie. She had used agar to solidify fruit jellies. When agar was tried as a solidifying agent in microbial nutrient media, its superior gelling qualities were immediately evident. Hesse wrote to Koch about this new discovery of another gelling agent (agar) that had more superior gelling qualities to gelatin (which possessed lesser gelling powers). Robert Koch quickly adapted agar to his own studies, including his classic studies on the isolation of the bacterium Mycobacterium tuberculosis, the cause of the disease tuberculosis (TB).
Agar has many other properties that make it desirable as a gelling agent for microbial culture media. In particular, agar remains solid at 37oC (human body temperature) and, after melting during the sterilization process, remains liquid to about 45oC, at which time it can be poured into sterile vessels or Petri dish plates. In addition, unlike gelatin, which many bacteria can degrade, causing the medium to liquefy, agar is not degraded by most bacteria. Agar also renders most solid culture media transparent, making it easier to different bacterial colonies from inanimate particulate matter suspended in the medium. Hence, agar found its place early in the annals of microbiology and is still used today all over the world for obtaining and maintaining pure cultures of bacteria.
In 1887 Richard Petri, a German bacteriologist, published a brief paper describing the modification of Koch’s flat plate technique for the cultivation of bacteria. Petri’s enhancement, which turned out to be amazingly useful, was the development of the transparent double-sided dishes that bear his name. The advantages of Petri dishes were immediately apparent. They could easily be stacked and sterilized separately from the medium, and following the addition of molten culture medium to the smaller of the two dishes, the larger dish could be used as a cover to prevent further contamination of the culture medium in the plate.
Bacterial colonies that formed on the surface of the agar in the Petri dish remained fully exposed to air and could easily be manipulated for further study. The original idea of Petri has not been improved on to this day, and the Petri dish, made either of reusable glass and sterilized by heat or of disposable plastic and sterilized by ethylene oxide (a gaseous sterilant), is a mainstay of the microbiology laboratory. Koch was keenly aware of the implications of his pure culture methods had for the study of microbial systematic. He observed that different colonies (differing in colour, morphology, size, and the like) developed on solid media exposed to a contaminated object and that they bred true and could be distinguished from one another by their colony characteristics.
Cells from different colonies also differed microscopically and often in their temperature or nutrient requirements as well. Koch realized that these differences among microorganisms met all the requirements that taxonomists had established for the classification of larger organisms, such as plant and animal species. in Koch’s own words (translated from the German language): “All bacteria which maintain the characteristics which differentiate one from another when they are cultured on the same medium and under the same conditions, should be designated as species, varieties, forms, or other suitable designation.” Koch also realized from the study of pure cultures that one could show that specific microorganisms have specific effects, not only in causing disease, but in other capacities as well.
Such insightful thinking was significant in the relatively rapid acceptance of microbiology as an independent biological science in the early twentieth century. Koch’s discovery of solid culture media and his emphasis on pure culture microbiology reached far beyond the realm of medical bacteriology. His discoveries supplied critically needed tools for development of the fields of bacterial taxonomy, genetics and several other subdisciplines. Indeed, the entire field of microbiology owes much to Robert Koch and his associates for the intuition they played in grasping the significance of pure cultures and developing some of the most basic methods in microbiology.
Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.