Discussion

By examining the results of the various tests performed, it was concluded with a fair amount of certainty that Serratia marcescens had been isolated.  One of the initial indications that Serratia marcescens was isolated was the presence of temperature-sensitive pigmentation.  Since the organism was obtained from molding bread, samples were taken only from those areas that had pigmentation and were red in color. This helped ensure that that the organism was from the Serraria sp.
In the experimental procedure, the organism was successfully grown on the enrichment media, DTC agar.  Since an ingredient for the DTC agar was not immediately available, the organism was initially streaked on MacConkey’s agar plates.  Information gathered from the Bergey’s manual stated that Serratia marcescens is a Gram-negative organism.  As a result, the MacConkey’s agar served as a selective media.  Once there was growth on the MacConkey’s media, and the colonies had similar morphological characteristics to those of Serratia marcescens, different colonies were streaked onto different DTC enrichment agar plates.  As expected, the colonies that grew on the DTC agar plates exhibited bright red pigmentation at room temperature characteristic of Serratia marcescens.
 The first test conducted to confirm the identity of the organism was a Gram stain.  This stain indicated that the organism was a Gram-negative rod.  An oxidase test was also performed and a negative result was obtained.  The catalase test was positive for the sample used.  An oxidation/fermentation test was performed and it was observed that the organism was able to ferment glucose both in the presence and absence of oxygen, indicating that the organism is a facultative anaerobe.  A test for fermentation of mannitol was conducted and it was observed that the organism was able to produce acid but unable to produce gas.  These preliminary tests that were performed helped prove beyond doubt that an organism of the Serratia species had been isolated.  However, all of the tests mentioned above gave results that were similar for two species of Serratia, namely Serratia marcescens and Serratia plymuthica.  Therefore it was necessary to perform more tests to confirm the identity of the organism.
 Research indicated that Serratia plymuthica is able to utilize lactose and has very low motility whereas Serratia marcescens does not ferment lactose and is very motile.  When we performed the motility stab with a sample of the isolated organism, a high level of motility was observed.  The pigmentation of the organism served as an ideal gauge of the motility of the organism.  Since pigmentation was observed in a thin layer at the top of the media used for the motility test, it can be deduced that organisms that were initially inoculated through the entire tube had moved up.  Although these observations show that the organism was fairly motile, it did not provide concrete evidence that the isolated organism was Serratia marcescens.  However, the lactose utilization test for the samplegave a negative result indicating that the isolated organism was in fact Serratia marcescens.
 Although at this point the identity of the organism had been successfully established, abundance of lab time allowed for a final test.  This test involved the use of the Enterotube II which is used as an identification system for Enterobacteriaceae.  The test consisted of inoculating a tube with various compartments containing ingredients for separate tests.  Each test was then scored and the scored were compiled and compared to a preexisting reference table to determine the identity of the organism.  The results we got from this test confirmed that the isolate was Serratia marcescens.
An interesting characteristic of Serratia marcescens is the appearance of the red pigmentation at room temperature and below and the disappearance of this red pigmentation above room temperature. Serratia marcescens has the ability to hydrolyze oil and gelatin.  When this organism is present in the soil, water, plant surfaces, and other environmental sites, its ability to hydrolyze these products can make it a useful decomposer in nature.  Serratia marcescens is also able to reduce nitrates and, therefore, plays a role in the nitrogen cycle.  Furthermore, Serratia marcescens is an effective bacterium for the degradation of chitin.  The four enzymes involved in chitinolysis breaks down chitin, a major component of the exoskeleton of insects and fungi, thereby exhibiting potential as a biocontrol agent (Brurberg et al.).
Serratia marcescens also occurs in human clinical specimens and in digestive tracts of rodents and insects.  It is a prominent opportunistic pathogen that can cause septicemia and urinary tract infections.  It can also cause mastitis in cows and other animal infections (Holt).  In clinical terms, it is useful to isolate a pure sample of the organism as this would help in the determination of its pathogenic qualities.  According to the Bergey’s manual, Serratia marcescens produces extracellular nucleases.  A biotechnology company can harvest this for use in recombinant DNA technology.  A clear step by step protocol is necessary to isolate Serratia marcescens because very often before any major research is conducted on it to assesses its benefits or it’s potential for causing harm, a pure sample must be made available. This is important because it is the basis for further scientific research on the organism.