H. labeled C after drink and that invasive stomach Hellcobe Comme (a) of Tradi (e) (b) FIGURE 10.4 Biochemical Tests Results, left to right (a) Catalese positive and negative. (b) Sugar fermentation negative, positive (acid and gas positive (acid only), and uninoculated control. (c) Urease positive and uninoculated control. Several tional b a strip F (figure added t ing the tional 16-h What specifically couses the celer to change in the sugar fermentation and urease tests? already present in a bacterial colony. Most, however, require an incubation period of at least 18 hours. One of the easiest and fastest biochemical tests is an assay for the enzyme catalase (figure 10.4a). Recall that catalase is an enzyme that many organisms produce to protect against hydrogen peroxide (see table 4.3), To detect catalase, a small portion of a colony is transferred to a microscope slide or the inside of a Petri dish, and then a drop of hydrogen peroxide (H2O) is added. If catalase is present, it immediately breaks down the hydrogen peroxide to form O, and water; the O, can be observed as bubbles in the reagent. Most bacteria that grow in the presence of O2 are catalase-positive. Important excep- tions are the lactic acid bacteria, which include members of an inverted tube traps any gas produced (figure 10.4b) medium designed to detect urease, an enzyme that degrades urea to produce carbon dioxide and ammonia, contains urea and a pH indicator (figure 10.4c). The basic strategy for identifying bacteria bused os biochemical tests relies on a dichotomous key, a series of alternative choices that lead to the identification of an organ ism (figure 10.5). In the case of bacterial identification, the choices are results of laboratory tests such as those listed in table 10.1. Because each test often requires an incubation period, however, it would be too time-consuming to proceed one step at a time, In addition, relying on a single biochemical test at each step could lead to misidentification. For example if a strain that normally gives a positive result fora certain tes loses the ability to produce a key enzyme, it would instead have a negative result. Therefore, several different biochemi- cal tests are inoculated at the same time in order to identify the organism faster and more conclusively In certain cases, biochemical testing can be done with out culturing the organism. Helicobacter pylori, the cause of most stomach ulcers, can be detected using the breath test, a are det verted puter microt wells wide allow of mi waste the genus Streptococcus. Thus, if B-hemolytic colonies grow from a throat culture but testing reveals they are all catalase- positive, then Streptococcus pyogenes has been ruled out. 2 sys with catalase, p. 101 H Streptococcus pyogenes. p. 535 Most biochemical tests rely on a chemical indicator that changes color when a compound is degraded. To test for the ability of an organism to ferment a given sugar, that organ- ism is added to a broth growth medium containing the sugar and a pH indicator. If the organism ferments the sugar, acid is which for the presence of urease. The patient drinks a assays produced, which lowers the pH, resulting in a color change; solution containing urea labeled with an isotope of carbon. If FIGURE 10.5 Dichotomous Key This shows an example of steps that can be used to distinguish some of the common causes of urinary tract infections. Additional tests may be done to confirm the identity of the pathogen. Gram stain Gram-positive coccus Gram-negative rod Catalase Oxidase test When identifying organisms, why are most biochemical tests usually inoculated at the same time, rather Positive Negative Positive Negative than waiting for one result before starting the next test? Coagulase Pseudomonas Enterococcus sp. Lactose fermentation aeruginosa Positive Negative Positive Negative Staphylococcus Staphylococcus saprophyticus E. coli or other coliform aureus Proteus sp