![]() In addition to biochemical tests and morphological differences bacteria can be identified through Sanger sequencing, which sequences genomes from purified 16S RNA PCR. The formation of a string indicates that the bacteria is Gram-negative, while no formation means it is Gram-positive. KOH “string” test shows if a bacterium is Gram-negative or Gram-positive based on if a string is formed after mixing a small sample of the bacteria into a 3% KOH concentration. No change means the bacteria does not have cytochrome c oxidase. The test itself uses oxidase slides that cause color change in the bacteria added correlates to the presence of the enzyme, if the bacteria sample changes to dark blue or purple it means an electron was received indicating that the enzyme is present. ![]() The oxidase test is to see if bacterial species has cytochrome c oxidases, which “functions as the last enzyme of the respiratory chain that transfers electrons to oxygen” in aerobic respiration (Holbrook 2018). A visible reaction indicates that the bacteria has the enzyme, while little or no reaction at all means that there is no enzyme present and the cellular toxin kills or damages the cells. A concentrated hydrogen peroxide solution is added to bacteria to see if bubbling occurs. The catalase test shows if a bacteria strain contains the enzyme catalase which breaks down H2O2 into water and oxygen. The phenylethyl alcohol prevents or reduces the growth of Gram-negative bacteria by preventing proper DNA synthesis. The PEA plate test is also related to whether or not tested bacteria are Gram-positive or negative. The color of bacteria colony growth also indicates if it can actively ferment lactose (Lal 2007). No growth indicates a Gram-positive species, while the opposite points to a Gram-negative species. The EMB-lactose growth is related to whether or not a bacterium is Gram-negative or Gram-positive. Sensitivity primarily occurs in Gram-positive bacteria, while growth is due to Gram-negative bacteria being resistant to the antibiotic (Saleh 2018). Growth on the vancomycin plate shows that the bacteria is not sensitive to vancomycin, while lack of growth indicates that the strain is sensitive to vancomycin. If the plate stays the same reddish pink color that means the bacteria cannot ferment mannitol however, if the plate changes color then it means that it can ferment mannitol. In addition to growth indicating tolerance, the bacteria also causes a color change to the agar. MSA plate growth would indicate if the strain of bacteria is salt tolerant. The ways to determine these differences is through biochemical tests, like growth on mannitol salt agar (MSA), vancomycin, EMB-lactose, and PEA plates, response to hydrogen peroxide in the catalase test, the color change in the oxidase test, and “string” formation in the KOH test. Other than by their morphology, bacteria can be differentiated by their source of energy, thermotolerance, antibiotic resistance, style of movement, salt tolerance, and if they are anaerobic vs. Some identification is based on morphological differences between bacteria, like: cell size, shape, color, colony shape, surface appearance, elevation, and colony edge shape. Bacteria are very diverse organisms, and their diversity is the reason multiple different tests and differences need to be taken account of in order to identify specific species.
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