Microbiological growth media
microbiological-growth-media
Abstract
Microbiology has been largely developed thanks to the discovery and optimization of culture media. The first liquid artificial culture medium was created by Louis Pasteur in 1860. Previously, bacterial growth on daily materials such as some foods had been observed. These observations highlighted the importance of the bacteria's natural environment and their nutritional needs in the development of culture media for their isolation. A culture medium is essentially composed of basic elements (water, nutrients), to which must be added different growth factors that will be specific to each bacterium and necessary for their growth.
The evolution of bacterial culture through the media used for their culture began with the development of the first solid culture medium by Koch, allowing not only the production of bacterial colonies, but also the possibility of purifying a bacterial clone. The main gelling agent used in solid culture media is agar. However, some limits have been observed in the use of agar because of some extremely oxygen-sensitive bacteria that do not grow on agar media, and other alternatives were proposed and tested. Then, the discovery of antimicrobial agents and their specific targets prompted the emergence of selective media. These inhibiting agents make it possible to eliminate undesirable bacteria from the microbiota and select the bacteria desired. Thanks to a better knowledge of the bacterial environment, it will be possible to develop new culture media and new culture conditions, better adapted to certain fastidious bacteria that are difficult to isolate.
Introduction
The discovery of culture media allowed the development of microbiology in the nineteenth century [1]. Bacterial culture was the first method developed to study the human microbiota [2], using an artificial medium that allows growth and isolation of bacteria. The first to have cultured a bacterium in a reproducible way was Louis Pasteur in 1860 thanks to the development of the first so-called artificial culture medium [3]. Recently, after the emergence of molecular techniques in the 1970s, such as PCR, sequencing and more particularly metagenomics, microbiologists have favoured these innovative techniques to the detriment of culture. Nevertheless, metagenomics presents certain disadvantages and in particular a depth bias, due to the lack of sensitivity of the primers used, because it does not detect bacteria present at concentrations <105 bacteria per gram of stool [2]. Moreover, these techniques only detect DNA: it is impossible with these techniques to differentiate DNA belonging to living bacteria from that of the transient bacteria of the microbiota studied, or from that of dead bacteria.
A few years ago, culturomics, a new culture technique that uses a very large number of culture media and culture conditions to extend the repertoire of bacteria, was developed in our laboratory [2]. This technique demonstrates the complementarity between metagenomics and culturomics. Therefore, the metagenomic identification of bacterial species existing in a given microbiota can be exploited by culturomics through the optimization of new specific culture media for the isolation of these species. This complementarity allows culturomics to become a targeted technique.
New culture media today mimic the natural environment of bacteria by adding different elements in culture medium to cultivate bacteria that were previously uncultivated.
We propose here a bibliographical review of culture media and the evolution of techniques through the development of microbiology over time.
Empirical approach of microbiology
Observational microbiology
Microbiology is defined not only by the organisms it studies, but also by the tools used to study them. The first observation of a bacterium was made around 1673 by the Dutch microscopist Anton van Leeuwenhoek thanks to the microscopes he had developed. Those enlarged from 50 to 300 times what he observed [1,3].
In the course of his research, he highlighted small structures that he called ‘animalcules’ [4], because he thought he was observing small animals [5]. Throughout his observations, he described and drew yeast cells, filiform fungi, microscopic algae and protozoa [4]. Leeuwenhoek was also the first to observe a parasite, Giardia lamblia [3]. However, microscopy alone cannot address all the questions about the microorganisms studied. For about 200 years microbiology was stagnating until the development of microbial isolation techniques in pure cultures. This is another important milestone in the history of microbiology.
Cultural microbiology
The birth of culture broth
In the thirteenth century, 400 years before Leeuwenhoek, a blood-like substance appeared on the communion bread. In line with Christian beliefs, this red substance was assumed to be the blood of Christ. Bartholomeo Bizio, an Italian pharmacist, solved this mystery in 1817 thanks to advances in microbiology and showed that it was not blood, but a microorganism that he named Serratia marcescens. This bacterium appeared as red colonies on bread when stored in a warm and humid atmosphere [6]. It is one of the first natural cultures of a bacterium. The origins of culture media date back to the nineteenth century. Many bacteriologists have tried, with varying degrees of success, to grow bacteria on the food or material on which the microorganism was first developed.