Limitations of antimicrobial agent choice based on MIC value against Pseudomonas aeruginosa.

Introduction. In antibiotic chemotherapy, an antimicrobial agent is selected based on MIC, which is determined by a test method using the principle of the broth microdilution. However, it does not provide a viable count in the mutant selection window. In this study, we used turbidimetry and visual judgement to examine the validity of the selection of various antibiotics to treat Pseudomonas aeruginosa based on MIC determination after 24 h of incubation.Methods. The antibiotics used in this study were piperacillin (PIPC), imipenem (IPM), meropenem, ciprofloxacin and amikacin (AMK). The strains used were 30 P. aeruginosa strains clinically isolated that were susceptible to all the antibiotics used, and the standard PAO1 strain. The viable count was measured after exposure for 3 and 24 h to therapeutic concentrations of various antibiotics, at which time turbidity was examined visually or by transmittance. In addition, the MPCs of IPM and PIPC were measured.Results. In this study, 10²-10⁸ c.f.u. ml-1 of P. aeruginosa survived exposure to PIPC, IPM and AMK at concentrations 2.5-80 times the MIC despite high drug concentrations. No turbidity was observed in the culture medium. Furthermore, both IPM and PIPC showed high mutant prevention concentration (MPC), with 64.5% of strains in IPM and 10% of strains in PIPC showing intermediate or resistance after 24 h.Conclusions. Choosing an appropriate antibiotic based on exceeding the MIC may be insufficient. While the PK/PD theory focuses on MIC, measuring MPC alongside MIC is urgent in clinical practice for optimal antibiotic selection.