Bacterial Species

Section I- Introduction and Definition of a Bacterial Species

Microscopic examination is essential in a microbiology lab. Scientists are able to differentiate between different organisms with the help of a microscope, because many microbes require a microscope to view very small structures such as flagellum. There are different types of microscopy, which include bright-field, phase-contrast, dark-field, fluorescence and differential interference contrast. The most common type of microscopy used in a microbiology lab is bright-field microscopy with the use of a compound microscope, which provides a contrast between the organism and the background making the organism easier to view. The ocular lens is responsible for viewing the specimen,

The transfer of electrons is important in the electron transport chain, and cytochrome c is responsible for this transfer. If an organism tests positive in the oxidase test, a blue/purple color will be immediately visible on the oxidase swab. The oxidase swab will indicate that electrons have been transferred, and cytochrome c oxidase is present and has made that transfer to electrons. The swab can be directly placed in the organism being tested. The swab contains a chromogenic reducing agent, which will change color, such as in the positive control Alcaligenes faecalis (color change within 10-20 seconds). The negative control for the oxidase test, E. coli, did not show an immediate blue color, proving that the reducing agent was not oxidized. It is important to ensure that there was an immediate color change once the bacterium has come in contact with the swab, because a false positive could occur with a delayed color change. If the color change occurs after the 20 seconds, then it is negative indication for the carrier molecule. Unknown 4 showed an immediate color change after running the test twice, solidifying that it was positive for cytochrome c

This test along with the other IMViC tests is used to differentiate between the gram-negative species, similar to unknown 4. If the organism uses citrate as the primary carbon source, then they also contain ammonium phosphate as the nitrogen source. The medium used for this differential test is the Simmons Citrate Agar, which contains sodium citrate, and bromthymol blue is the pH indicator used to test for the presence of citrate. This test is performed with a light sample of the unknown bacterium, and a proper transfer should occur using aseptic technique between the unknown sample and the agar. If the organism presents growth or color change to blue, then this indicates a positive result for citrate as the carbon source such as in the species Enterobacter aerogenes. The agar will remain a green color such as in E.coli, which is the negative control for the citrate test. No growth has occurred. When unknown 4 was tested, it presented a strong blue color, resembling that of the positive control E.