The image shows bacteria, isolated from a stool specimen of a patient suffering from diarrhea, growing on MacConkey agar. MacConkey agar is used for the isolation of gram-negative enteric bacteria and the differentiation of lactose fermenting from lactose nonfermenting gram-negative bacteria. Lactose fermentors will alter the pH of the medium, changing the pH indicator in the medium to a pink color. Colonies of lactose nonfermentors appear colorless on the medium, and the agar surrounding the colonies remains relatively transparent. The various size colonies in the image represent a mixture of Escherichia coli, Proteus spp., and Yersinia enterocolitica (see arrow in labeled image). Yersinia enterocolitica grows poorly on MacConkey agar generating very tiny lactose negative colonies (0.5 to 0.7 mm); therefore, the use of a stereoscopic microscope is mandatory in order to find this pathogen.
The use of the stereoscope is necessary so that the growth of this pathogen is not overlooked within a mixed culture such as the one shown. Also, the use of the stereoscope facilitates colony differentiation of diverse enterobacteria that are nonlactose fermenters.
This colony represents the first isolation of this microorganism in Central America (2). The Y. enterocolitica colonies are 24 hours old. The isolation agar is MacConkey. Infections with Y. enterocolitica in Central America are very rare, compared with colder countries, the tropical weather and temperature may be a contributing factor in this observation.
Yersinia enterocolitica is the causative agent for relatively infrequent diarrhea and abdominal pain, also this is an emerging agent of foodborne illness. This image will provide a useful resource when instructors are teaching the value of differential and selective media, as well as when discussing mixed cultures, the careful analysis required in disease diagnosis, and the characteristics of the pathogen Yersina enterocolitica.
- Allen, M. MacConkey agar plates protocol. MicrobeLibrary Atlas Protocol Project [accessed October 6, 2005] [Online.] http://www.microbelibrary.org/edzine/details.asp?id=1977.
- Gini, G., and M. Torres. 1986. Primeros aislamientos de Yersinia enterocolitica en Centro America y revision de la literatura. Rev. Lat-Am. Microbiol. 28:197-200.