Unknown Sample # 12: Serratia Marcescus

For this experiment, unknown sample #12 was used. The first step in identifying any unknown bacterium is to grow a culture on a nutrient agar plate. This bacterium was also plated on a MAC plate, which is both selective for gram-negative bacteria and differential as well. On the NA plate, bright red colonies were present; on the MAC plate there was bacterial growth as well, but no color change was observed. This lack of color change on the MAC plate indicated that the bacteria did not ferment lactose. A Gram stain was then performed to confirm that bacteria #12 was indeed gram-negative and also showed that it was rod shaped. Following the provided flowchart, the oxidase test was then performed in order to test for the enzyme cytochrome oxidase; unknown bacteria #12 tested negative. Finally, the bacterium was grown on an EMB plate, which resulted in no color change; this indicated again that the bacteria did not ferment lactose. After these tests were completed, the results were compared to the provided identification flowchart. After review it was determined that unknown bacteria sample #12 was Serratia marcescens. S. marcescens is a bacterium that exhibits all of the characteristics shown by the performed tests, as it …show more content…
Different types of bacteria have different enzymes, by-products, etc. that can be identified by tests such as the catalase or oxidase test. These help narrow down the possible types an unknown bacteria could be, as the biochemical tests are usually quite specific. For example, if a catalase test is negative, one can almost be sure that the bacteria sample in question is S. faecalis. Biochemical tests are also some of the quickest, which also makes them very useful in a medical setting when one is trying to treat an infection. Their simplicity, ease, and specificity makes biochemical tests critical to the process of identifying bacteria