MIC score, a new tool to compare bacterial susceptibility to antibiotics application to the comparison of susceptibility to different penems of clinical strains of Pseudomonas aeruginosa

This study aimed to compare the susceptibility to carbapenems (imipenem, meropenem and doripenem) of clinical strains of Pseudomonas aeruginosa. It also studied whether susceptibility to imipenem or meropenem could predict, reliably, susceptibility to doripenem. Pseudomonal strains were collected from respiratory specimens, half of them from cystic fibrosis patients. MICs were determined according to European Committee on Antimicrobial Susceptibility Testing recommendations. Carbapenems were compared according to the susceptible, intermediate or resistant categories. A new approach also allowed comparing these carbapenems in a ‘MIC score’ taking into account the differences in breakpoints between drugs. One hundred thirty-nine strains were studied. They were found to be statistically more susceptible to meropenem than to the two other drugs. However, this difference was small: less than one dilution between the agents. This study also highlighted a significant correlation between susceptibility to penems taken in pairs. However, susceptibility to imipenem or meropenem did not reliably predict susceptibility to doripenem. Despite potential differences in resistance mechanisms, the Pseudomonas aeruginosa strains showed close susceptibility to three carbapenems. This was true for both cystic fibrosis patients and others. However, there were variations between strains. That justifies MICs to be determined for each of the three penems. This might be useful in case of elevated MICs and/or for potentially difficult-to-treat infections such as pneumonia in patients with cystic fibrosis patients.