I recently encountered problems when reading microdilution plates for the determination of MICs of S. aureus for different antibiotics. For vancomycin, gentamicin and TMP-SMX, I see the skip phenomenon where more than one well without growth are observed after resumption of growth. This is random and I can see two to five wells without growth before I see growth again. CLSI recommends repeated the analysis when this phenomenon is observed, but in my case I see this phenomenon only recently and for many different clinical strains of S. aureus.
Have you ever seen the same phenomenon for your strains of S. aureus?
How do you report the MIC?
Do you have a possible solution to resolve this phenomenon?
This is called well jump and may be indication of contamination according to Mahoon Textbook of Diagnostic Microbiology Page 244 Chapter 13 Saunders - Elsivier 2011
Skipped wellsinvolvegrowth at higherconcentrationsand
no growth at one or more of the lower concentrations. This
may occur as a result of contamination, improperly inoculated
wells, improper concentrations of antimicrobial agent in
the wells, the presence of unusual resistance with the...
Also Trailing involves heavy growth at lower concentrations
followed by one or more wells that show greatly reduced
growth in the form of a small button or a light haze. This
commonly occurs with sulfonamides, trimethoprim, and trimethoprim-
sulfamethoxazole;
Good Luck
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