Biochemical tests are often used to determine the identity of bacteria and differentiate between its species. There are numerous types of tests meant to detect key characteristics such as morphology, stain, motility, fermentation pathways used, oxygen requirements, enzymes present, and redox tests used. By conducting these tests with aseptic techniques, one can usually narrow down an unknown bacterium to its family; with the help of dichotomous keys and Bergey’s Manual of Determinative Bacteriology, particular genus and species can be identified.
The first tests commonly conducted on a bacterium also give the broadest results. Initial tests employ differential staining and normally include gram, capsule, and endospore stains. All three indicate …show more content…
These tests are done using various sugar broths containing inverted Durham tubes and a pH indicator, typically phenol red. If a bacterium is capable of fermenting a distinct sugar, acidic byproducts are formed, thereby lowering the pH of the test medium. This results in the pH indicator in the medium changing color from red to yellow. If carbon dioxide is produced by the end product of glycolysis, pyruvate, a small bubble of gas will appear at the top of the Durham tube. A test tube that turns yellow but lacks a bubble in the Durham tube indicates a fermenter that goes straight from pyruvate to lactic acid without the production of carbon dioxide. If the liquid medium in the test tube does not change color and there is no gas present in the Durham tube, the bacteria did not ferment the sugar. Numerous sugars can be used to help identify an unknown; dextrose, sucrose, maltose, fructose, galactose, lactose, adonitol, sorbitol, and mannitol are common. The test for dextrose (glucose) is important in identifying enteric bacteria; all of them can ferment it, but only a handful produce gas. All of my sugar tests on my unknown turned yellow, positively indicating fermentation. Dextrose maltose, lactose, adonitol, sorbitol, and mannitol also produced gas and had a bubble in the top of the Durham tube. These results show that my bacterium can definitely …show more content…
It measures motility, hydrogen sulfide production, and the presence of the enzyme tryptophanase. Test tubes containing this media are inoculated with a single stab straight through to the bottom of the tube. An organism’s motility can be observed by the distance that it travels away from the inoculation line. If the entire tube appears turbid, the organism is motile; if the growth stays relatively close to the stab line, it is non-motile. My unknown is very motile, as the whole test tube was opaque. In some bacteria, sulfur can be reduced to hydrogen sulfide by two processes: degradation of the amino acid cysteine or through reduction of sulfur-containing compounds such as thiosulfate. When hydrogen sulfide is present in SIM media, a black precipitate forms; a lack of black color in the test tube is a negative result. My bacterium turned the test tube black, so it is positive for producing hydrogen sulfide. The last characteristic the SIM test looks for is tryptophanase, an enzyme in many bacteria that can convert the amino acid tryptophan to indole. SIM agar contains tryptophan, and with the addition of Kovac’s reagent, indole is indicated by a red color in the media. The red color points to the presence of tryptophanase in the microorganism tested. Bacteria are indole negative if there is no color change. My media turned red; this shows that my unknown is indole