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Discussion Section | Microbiology Unknown Report, Pseudomonas aeruginosa

Microbiology Discussion and Conclusion section of a microbiology lab report. In microbiology class students are asked at the end of the semester to identify unknown bacteria using all the lab techniques they have learned and practiced throughout the semester. Here is a sample of a section of the final lab report that must be handed in.



How did the test results lead to the identification?

            Different bacteria can live and flourish in certain environments, while others will die or never grow in the very same environment due to their dissimilar characteristics.  These tests help determine which bacteria can grow in certain situations.  For both bacteria, the Gram stain was a necessity in order to determine if the microbe was Gram Positive or Gram Negative.  Once the Gram Positive bacteria was successfully visible under the microscope, it was clear that it was cocci.  Knowing this information helped to cross two bacteria (both rods) off the unknown list.  From there, the Mannitol test helped narrow down the choices even more.  This particular Gram Positive microbe was able to ferment Mannitol and produce acid giving it a positive result.  This positive result narrowed the choices down from three to two (S. aureus and E. faecalis).  The only test that would allow for differentiation between these two bacteria was the Nitrate reduction test.  This test came back negative, which meant this microbe could not reduce nitrate to nitrite or any other nitrogenous compounds.   Only one Gram Positive bacteria given cannot reduce nitrate, which lead to its identification: E. faecalis.

The other Gram stained slide was viewed under the microscopes and pink/reddish rods were clearly visible making it the Gram negative bacteria.  This Gram stain, however, did not eliminate any of the Gram Negative bacteria because they were all rods.  The first test was the Simmons Citrate test.  This test determines whether or not a microbe can use citrate as its sole carbon source.  It was a positive result, meaning it could in fact use citrate as its only carbon source.  This test narrowed down the choices from five to three bacteria (K. pneumoniae, E. aerogenes, and P. aeruginosa). The Urea test was then conducted.  This test is used to identify bacteria capable of hydrolyzing urea using the enzyme urease.  The test came back negative, which helped to eliminate K. pneumoniae, since that particular bacteria produces urease. Casein was the last test used in order to identify the Gram Negative bacteria. After incubation, there was a clearing in the milk agar, which proved that this bacteria could hydrolyze the milk protein casein using the enzyme casease.  The only bacteria that was able to do that was P. aeruginosa.


Was it the correct identification?

            After showing the professor the results that lead to the identification of these bacteria (E. faecalis and P. aeruginosa), it was confirmed the correct microbes were found.

What problems were encountered?

            The only problem that was encountered was during the original streak plate.  After two days of incubation, it was hard to isolate two different bacteria into two separate nutrient agar plates due to overgrowth.  In order to avoid contamination or future problems, another streak plate was done and more carefully.  After the second try, two separate bacteria were growing and it was possible to successfully isolate them both.



Pseudomonas aeruginosa

Pseudomonas aeruginosa wasn’t successfully isolated until 1882.  In a publication entitled when “On the Blue and Green Coloration of Bandages”, Carle Gessard reported the growth of the organism from cutaneous wounds of two patients with bluish-green pus (3).  P. aeruginosa is a gram negative, rod shaped bacteria.  It is a strictly aerobic microbe, which means it needs oxygen to grow and survive.  It is noted for its ability to live in versatile environments, cause nosocomial infections in susceptible individuals and it’s resistance against antibiotics.

P. aeruginosa is an important microbe to be aware of because it has an ability to survive with minimal nutrients and can tolerate a variety of physical conditions. This allows P. aeruginosa to be prevalent in both hospital settings, as well as, in our overall community. While in the hospital, P. aeruginosa can be found on a variety of equipment.  Such equipment includes: respiratory machines, antiseptics, soaps, mops, etc. (3).  While out in the community, P. aeruginosa can be found in swimming pools, hot tubs, humidifiers, and even vegetables. While P. aeruginosa is still prevalent in our community, it is in hospitals that this bacteria can cause major infection.  It is considered an opportunistic pathogen, which means it usually does not pose a problem, however, it can become pathogenic.  When a person enters a hospital, it is normal for their immune system to be compromised.  When an individual becomes immunocompromised their ability to fight off an opportunistic pathogen like P. aeruginosa becomes more difficult.

While P. aeruginosa normally does not cause illness in a healthy person, it can cause major problems for an individual in the hospital with a compromised immune system.  It is the most common pathogen isolated from patients who have been hospitalized longer than 1 week (4). Some common nosocomial infections caused by P. aeruginosa include: pneumonia, urinary tract infections, blood stream infections, as well as, infection of surgical sites.  Nosocomial P. aeruginosa infections can be dangerous because the individual is already under either physical or mental stress (or both), but another frightening fact is that P. aeruginosa is resistant to many antibiotics.

P. aeruginosa can develop resistance to antibiotics, either through the acquisition of resistance genes on mobile genetic elements (i.e., plasmids) or through mutational processes that alter the expression and/or function of chromosomally encoded mechanisms. (3) While these drug resistant abilities are positive aspects for P. aeruginosa, it is very dangerous for individuals fighting infection. The drug resistance makes treating this pathogen even more difficult and vastly limits avenues in which doctors can take to help treat patients with this infection.  The question then becomes, if there is an overuse of the antibiotics that do work, will P. aeruginosa become resistant to those too?





(1) McDonald, Virginia (et al.). Lab Manuel for General Microbiology. Saint Louis Community College

at Meramec. 2011

(2) Lab Review for Practical Microbiology.

(3) Lister, Philip D. (et al). “Antibacterial-Resistant Pseudomonas aeruginosa: Clinical Impact and

Complex Regulation of Chromosomally Encoded Resistance Mechanisms.”

www.ncbi.nlm.gov/pmc/articles. American Society of Microbiology. 2009. April 28, 2015.

(4) Lessnau, Dieter-Klaus, MD. “Pseudomonas aeruginosa Infections.”

www.emedicine.medscape.com/article/226748. WebMD LLC. 2014. April 28, 2015.




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