Phenylethyl Alcohol Agar

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Lastly, Enterococcus gallinarum showed a black precipitate after incubation, and therefore this bacteria is definitely bile esculin positive since the formation of a black precipitate shows that the bacteria can tolerate bile and hydrolyze esculin and therefore is a member of enterococci as its’ name suggests. Also, the website http://urbancaliber.hubpages.com/hub/Unknown-Determination states that Enterococcus gallinarum shows a black precipitate on the Bile agar, signifying that it is positive for this test.

Conclusion: In conclusion, I found that Lactococcus lactis and Providencia alcalifaciens were both supposed to be Bile Esculin negative and there should be no dark precipitate formed around either micro-organism after incubation. As the lab manual suggests, the formation of a dark precipitate indicates that the bacteria is in fact bile esculin positive and because these two showed no formation of precipitate, I concluded that they were negative for this test. On the other hand, Enterococcus gallinarum showed a black precipitate and therefore I concluded that Enterococcus gallinarum is actually positive for this test because if a black precipitate forms the organism is able to tolerate bile and hydrolyze esculin and therefore is Bile esculin positive.

References

http://www.studymode.com/essays/Hydrolysis-Report-429546.html

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC275140/

http://urbancaliber.hubpages.com/hub/Unknown-Determination

Exercise 4-4: Mannitol Salts Agar

Introduction: Mannitol Salt Agar (MSA) is both a selective and differential medium and therefore inhibits growth of some organisms and encourages growth of others, and also exposes differences between organisms. Sodium Chloride in the agar makes the medium selective because its’ concentration is so high that it is enough to dehydrate and kill most bacteria. In fact, because Staphylococci are adapted to living in salty habitats, they thrive in this medium. Mannitol which is also present in the agar makes the medium differential by providing the substrate for fermentation. Most importantly, the indicator is phenol red which is yellow below pH 6.8, red at pH 7.4 to 8.4, and pink at pH of 8.4 or higher. Phenol red is an important indicator because depending on its color change as the pH changes, one can interpret whether or not fermentation has taken place. If fermentation takes place on the mannitol, acid is produced and therefore the pH of the medium is lowered and colonies appear bright yellow usually surrounded by a yellow halo. If fermentation does not occur on the mannitol, the growth appears pink or red and the medium does not change. MSA is important because Staphylococcus aureus ferments the mannitol and therefore MSA can be used to isolate and differentiate Staphylococcus aureus from other bactiera. In the end, a nutrient agar plate will be used to compare how well the bacteria grew on the MSA plate compared to the NA plate.

Results: Refer to Data Sheet

Discussion: From my results, I found that Escherichia coli had very poor growth on the MSA plate compared to the NA plate. Looking at the lab manual, because Escherichia coli had very poor growth, I interpreted that the organism was inhibited by NaCl and that no fermentation of mannitol took place. Using the website http://www.wisc-online.com/objects/ViewObject.aspx?ID=MBY4207 I found that E.coli is a gram-negative bacteria that cannot tolerate a high salt concentration and therefore will not grow on the plate, which is exactly what my own results showed.

Next, I saw that Enterococcus gallinarum had poor growth on the MSA plate and good growth on the NA. The website http://www.tgw1916.net/Streptococcus/gallinarum.html shows that Enterococcus gallinarum produces acid from mannitol fermentation, thereby showing that my result was not correct.

Furthermore, for Staphyloccous saprophyticus I saw that the bacteria grew very well on the MSA plate and was yellow and opaque, which can also be seen from the picture in the results section above. Because it was yellow and opaque, I concluded that the organism was not inhibited by NaCl because only poor growth suggests that the organism is inhibited by NaCl. I also concluded that Staphyloccous saprophyticus produces acid from mannitol fermentation since yellow growth was observed and only yellow growth suggests that the organism can produce acid from fermentation. The website http://faculty.ccbcmd.edu/courses/bio141/labmanua/lab15/msass.html shows that Staphyloccous saprophyticus does indeed produce acid from mannitol fermentation because mannitol salt agar is selective for staphylococci because of the high salt concentration. The color change from red to yellow is due to acid fermentation since the pH indicator phenol red turns from red (alkaline) to yellow (acid). Last, Staphyloccous epidermidis produced good growth on the MSA plate and appeared a light pink color suggesting that the organism does not ferment mannitol which I found from table 4-4 in the lab manual. Because the organism did grow on the MSA plate the organism is not inhibited by NaCl since only organisms that can stand the high salt concentration can grow on the plate. The website http://www.wisc-online.com/objects/ViewObject.aspx?ID=MBY4207 shows that my results were correct.

Conclusion: In conclusion, I confirmed that Escherichia coliI exhibited poor growth on the MSA plate indicating that it was inhibited by NaCl and did not ferment mannitol. I than found that Enterococcus gallinarum should have had good growth. Staphyloccous saprophyticus had good growth on the MSA plate and produced a yellow color showing that the bacteria is not inhibited by NaCl since they grew on the MSA plate and produced acid from mannitol fermentation since the color changed from red to yellow. Lastly, Staphyloccous epidermidis grew on the MSA plate showing that it too is not inhibited by NaCl but it produced a pink color representing the fact that it does not ferment mannitol.

References

http://www.wisc-online.com/objects/ViewObject.aspx?ID=MBY4207

http://www.tgw1916.net/Streptococcus/gallinarum.html

http://faculty.ccbcmd.edu/courses/bio141/labmanua/lab15/msass.html

http://www.wisc-online.com/objects/ViewObject.aspx?ID=MBY4207

Bergey, D. H., and John G. Holt. Bergey's Manual of Determinative Bacteriology. Baltimore: