Unknown Bacteria

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In the laboratory, identification of an unknown bacterium is often necessary. In the lab, a random sample consisting of three different bacteria was selected. The sample contained one gram-positive, one gram-negative paracolon, and one gram-negative coliform. The purpose of the experiment is to identify each of the three species that the mixture contained. After receiving an unknown mixture, the sample was streaked for isolation onto TSA, blood agar, and MacConkey plates. Each plate serves as a first step to identify the unknowns. The TSA (tryptic soy agar) can be used to do a gram stain, which differentiates gram-negatives from gram-positives, based on the structural make up of the cell wall (Carson, 2015). The blood agar plate is used to test for hemolytic activity, which is useful for distinguishing gram-positives. A MacConkey plate is selective by inhibiting the growth of gram-positives and differential due to the fermentation of lactose by certain gram-negative species. In the…show more content…
The gram negative species are apart of the Enterobacteriaceae family. These are gram-negative rods, which are facultatively aerobic - either respiring or fermenting. Most are motile via a flagella (Carson, 2015). Salmonella enterica and Yersinia enterocolitica are both paracolons, which lack the ability to ferment lactose. Conversely, Klebsiella pneumoniae and Escherichia coli are both coliforms, which are able to ferment lactose. Of the Enterobacteriaceae family, there are genera that are in the normal human flora. Some species such as K. pneumoniae and E. coli are opportunistic pathogens which can capitalize on weakened host defenses and cause food poisoning (Baron, 1996). S. enterica secrete proteins that help aid in intracellular invasion and proliferation (Hensel, 2009). K. pneumoniae is a part of the normal human mouth, skin, and intestine flora, but can wreak havoc if inhaled (Ryan,
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