Final answer:
In a disk-diffusion assay, bacteria resistant to SXT, such as GAS, will grow up to the disk with no zone of inhibition, while susceptible bacteria will not. The presence or absence of inhibition zones helps differentiate bacterial species and assess their antibiotic resistance. Standard overnight incubation at 37°C is used to allow for proper growth and test results interpretation.
Step-by-step explanation:
Organisms that are resistant to SXT (sulfamethoxazole/trimethoprim) would grow up to the edge of the disk during testing. This phenomenon occurs because these organisms are not inhibited by the antibiotic present in the SXT disk. In the disk-diffusion method, if Group A Streptococcus (GAS) is present, it will be susceptible to bacitracin and an inhibition zone will be observed; on the other hand, Group C or G streptococci may grow up to the bacitracin disk indicating resistance. The presence of such zone of inhibition around the disk is critical for evaluating the susceptibility of bacteria to antibiotics.
To conduct this test, overnight TSA plate cultures are used, such as those of Staphylococcus aureus, S. saprophyticus, Escherichia coli, and Pseudomonas aeruginosa. The plates are inoculated and an antibiotic-saturated disk is placed on the bacterial lawn. Post incubation, typically overnight at 37°C, the zone of inhibition is measured. The test is essential in determining the effectiveness of antibiotics against different bacteria and their potential resistance.
It should be noted that extended use of germicides may lead to sterility, and in the context of disk-diffusion assay, a large zone of inhibition is indicative of susceptibility of the test microbe to the chemical.