Although definitive identification of these organisms required additional testing, MacConkey used the previous observation by Theodor Escherich (for whom the genus Escherichia is named) that E. Of particular interest was determining whether a colony represented Escherichia coli (then Bacillus coli communis) or Salmonella enterica serovar Typhi (then B. In addition to enriching for Gram-negative bacteria, MacConkey also wanted to be able to differentiate between types of enteric organisms. MacConkey Agar Differentiates Lactose fermenters and Non-fermenters Therefore, enterics (and a select group of other Gram-negative bacteria, notably Pseudomonas) are selected for on media containing bile. Enteric organisms, however, must withstand a constant assault from bile acids in the gut and have thus evolved mechanisms to resist their action. Bile acids are amphipathic molecules found in the gut that aid in digestion by emulsifying fats and allowing them to be transported in an aqueous environment.Ĭellular membranes also look very much like fats, so bile acids are toxic to many organisms through disruption of this barrier. His strategy for selection of enteric organisms was to add bile acids to standard media. A medium that can perform this function is now known as a selective medium. Therefore, MacConkey needed a way to limit this background of environmental flora and allow only his organisms of interest to grow. MacConkey Agar Is Selective for Non-fastidious Gram-negative Organisms Despite dilution, it proved difficult to identify enterics that may have been present. Many of these are environmental organisms were not predictive of contamination, what MacConkey called “ordinary earth organisms.” Not surprisingly, his samples often grew large numbers of colonies on standard nutrient media. However, MacConkey's efforts were frustrated by the fact that every milliliter of treated water may still contain hundreds or thousands of bacteria. To identify enteric organisms, water samples were plated on solid media and the colonies that formed were enumerated and identified. Although they do not always cause disease themselves, their presence is an indicator of fecal contamination and therefore, the potential presence of other fecally transmitted pathogens. These bacteria are normal inhabitants of the gastrointestinal tract of humans and are also found in other mammals, reptiles, and birds. Part of MacConkey's role on the commission was to survey drinking water sources for the presence of Gram-negative enteric organisms. To evaluate the efficacy of various sewage treatment regimens, the commission's work necessarily involved determining whether treated water remained contaminated by feces. This group was charged with protecting the public from waterborne disease through developing best practices for treatment of sewage. In the late 1890's, Alfred MacConkey was working at the University of Liverpool under the auspices of the Royal Commission on Sewage Disposal.
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