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Bacteria can invade the body and cause infection. With the ability to identify bacteria according to various properties, we can discern the type and then the sensitivity of the bacteria. Via culture and sensitivity, we can attempt to cure the infection by targeting the bacteria with specific drugs. The process of culture and sensitivity is used in labs to determine the best medical treatment for patients with bacterial infections.
I received unknown bacteria #30 and was to determine which of 12 different bacteria it was. Over the course of four days, I cultured and analyzed bacteria #30. My methods of analysis included gram staining, application of the Mannitol Salt Agar test, blood agar testing and catalase testing.
On day one, I aseptically innoculated the bacteria to the trypticase soy agar (TSA) plate and put it in the incubator at 37oC for 48 hours. This provided time for the bacteria to grow so that I would be able to observe the bacterial morphology.
Bacteria are most often rod-shaped (bacilli), spherical (cocci) or curved (spirilla). The culture of bacteria #30 grew out a yellow colony of bacteria, which were spherical, convex, and with smooth margins. This result was indicative of a coccus bacterial species rather than a baccilus or spirilla species.
As Escherichia coli, Serratia marcesesns, Enterbacter aerogenes, Pseudomonas aervginosa, Citrobacter freundii and Proteus vulgaris are rod-shaped bacteria (bacilli), it was clear that bacteria #30 was none of these.
Instead, I was left with the possibilities of bacteria #30 being Staphylococcus aureus, Staphylococcus epidermidis, Micrococcus luteus, Enterococcus faecalis, Neisseria gonorrhoeae or Streptococcus pyogenes – all cocci bacteria.
On day two, I applied gram stain to the sample to determine if the bacteria had gram-negative or gram-positive properties. The bacteria formed clumps which stained purple (gram positive). The bacteria were cocci shaped and 0.9um in diameter. This result eliminated the possibility of unknown bacteria #30 being Neisseria gonorrhoeae as N. gonorrhoeae is gram negative.
On day three, I used the Mannitol Salt Agar (MSA) test to evaluate whether or not the bacteria was salt tolerant and whether or not it would ferment mannitol. As there was no growth, I determined that the bacteria was not salt tolerant. This exclued Staphylococcus aureus and Staphylococcus epidermidis as unknown bacteria #30 as Staphylococcus species are able to grow in a high concentration of salt. This was confirmed as the specimen did not ferment mannitol (the phenol did not change color from red to yellow).
Enterococcus faecalis ferments mannitol; bacteria #30 was not Enterococcus faecalis.
On day four, I applied the blood agar and catalase tests to differentiate between the remaining bacterial species (Streptococcus pyogenes and Micrococcus luteus). There was growth of yellow colonies. The bacteria was not beta hemolytic, but was catalase positive. The results of the blood agar and catalase tests confirmed that unknown bacteria #30 was not Streptococcus pyogenes as Streptococcus pyogenes is catalase negative.
Micrococcus luteus, on the other hand, is a catalase positive bacteria. I concluded that unknown bacteria #30 was Micrococcus luteus.
The following properties led me to the conculsion that unknown bacteria #30 is Micrococcus luteus:
- Gram positivity
- Salt intolerance
- Beta hemolytic negative
By applying various methodologies, I was able to use processes of elimination and inclusion to identify this bacteria.