Determination of an Unknown Bacteria
...d red negative results for the acid/gas mannitol fermentation, whereas Streptococcus reported yellow positive results. Mannitol Salt Agar goes a few steps further in the elimination process by determining both mannitol fermentation and selecting for salt tolerance. Unknown B showed negative results for mannitol fermentation since it did not change the red agar to yellow, and it showed positive results for salt tolerance since colony growth resulted. Micrococcus would be eliminated since it is unable to tolerate the salt concentration of this agar and would show no growth on the plate. To narrow down the possible microbes even further, the computer program BACTI.BAS was used. The top three matches were Staphylococcus aureus, Staphylococcus epidermis, and Streptococcus faecalis. It was already determined that S. faecalis differed by mannitol fermentation in the broth and agar, and the program further differentiated the two through differences in nitrate reduction, and citrate utilization. Nitrate reduction is the conversion of nitrate to nitrite, and many facultative bacteria are able to use the oxygen in nitrite as a hydrogen acceptor in anaerobic respiration. Unknown B, a facultative anaerobe, showed positive results for nitrate reduction, whereas S. faecalis is reported to have negative results. Citrate utilization is the ability of some organisms to utilize citrate as a sole source of carbon, and Unknown B demonstrated negative results. S. faecalis is reported to test positive for citrate utilization. The computer program, along with Bergey’s Manuel, also provided further proof for differentiation between Staphylococcus aureus and Staphylococcus epidermis. The most important of these is the differences in mannitol fermentation test results. Unknown B, as mentioned previously, showed negative results for both mannitol broth fermentation and for the telling color change of the phenyl red indicator in Mannitol Salt Agar. S. aureus is reported in Bergey’s manuel to show the telltale positive yellow color change in both the mannitol broth and in the agar. Unknown B remained red in both tests. On the other hand, S. epidermis is reported to also show negative results in both of these tests, identical to Unknown B. Glucose fermentation (acid/gas production), methyl red (mixed acid fermentation), nitrate reduction, oxidase (presence of cytochrome oxidase), starch hydrolysis, tryptophan hydrolysis (indole), citrate utilization, Voges-Proskauer test, motility, litmus milk, thioglycollate (oxygen requirement), and catalase tests were all identical in Unknown B, S. aureus, and S. epidermis, and therefore only used as confirmatory factors between all three organisms instead of tests that show divergence. Lacto...