Lab 6: Susceptibility Tests

This week in lab we set up several different susceptibility tests. We performed a disk diffusion on a given organisms as well as a control, a microdilution broth, as well as an E-test with the drugs penicillin and ceftriaxone. We also screened a nasal swab specimen for MRSA and determined beta-lactamase resistance of a given organism using a nitrocefin disk.

I performed a disk diffusion susceptibility test with the drugs ampicillin, trimethoprim sulfamethoxazole, cephalothin, ciprofloxacin, gentamicin, ticarcillin, ticarcillin/clavulonic acid, and tobramycin on Enterobacter cloacae. It was susceptible to all but cephalothin. I used the same disks for the control which was Escherichia coli ATCC 25922 and all results were within acceptable limits. Becuase the control results were all acceptable, I could rely on the results for the E. cloacae. Below is an example of a disk diffusion test set up on a Mueller Hinton plate.

The diameter of the zone of inhibition is measured to determine susceptibily. 

Next I performed an E-test with the drugs penicillin and ceftriaxone on Streptococcus pneumonia. The MIC is determined based on where the elliptical intersects the strip.  This MIC value is compared to the susceptibility ranges provided. The interpretation for peniccilin was intermediate and the interpretation for ceftriaxone was susceptible. 

When MRSA is grown on CHROMagar, it produces mauve colonies. My specimen did not grow on the agar at all. In this case, it should be incubated for an additional 24 hours. If mauve colonies are sceen, a coagulase test should be done to confirm the isolate as Staphylococcus aurues. Below I’ve included a picture of what MRSA should look like on CHROMagar.

Mauve colonies on a CHROMagar plate.

In order to determine the resistance of Neisseria gonorrheae against beta-lactam drugs, I performed a nitrocefin disk test. If the test was positive, a pink color should be produced, meaning the organism produces the enzyme beta-lactamase and is capable of breaking down beta-lactam drugs such as penicillin and cephalosporin. In my case, the test was negative meaning the organism does not produce beta-lactamase. However, additional testing should be done to determine if there is any resistance due to other mechanisms.

The disk on the left shows the positive (red) result.  The disk on the right shows no color change indicative of a negative result.