Abstract
Abstract: :
Purpose: Favorable clinical outcomes while minimizing antimicrobial resistance is optimal. FQs have been compared by mutant prevention concentration (MPC) – a novel in vitro measurement that represents a drug concentration threshold above which an organism would need 2 simultaneous mutations for growth. For MPC testing, higher bacterial inoculums are used as these may more accurately represent organism numbers at the site of infection & most likely contain resistant subpopulations if present. MPC measures inhibition, not kill – nor does minimal inhibitory concentration (MIC) testing. We determined killing of ocular isolates of PA and KP by GA over a range of bacterial inoculums and on MIC and MPC drug concentrations. Methods: For MIC testing, 105 cfu/ml was inoculated into Mueller–Hinton broth (MHB) containing drug, incubated 18–24 hours (h) at 35°C and examined for growth. For MPC testing, >109 cells were inoculated to agar plates containing drug, incubated at 35°C and screened for growth at 24 and 48 h. The lowest concentration preventing growth was the MIC or MPC respectively. For kill experiments, organism grown in MHB was adjusted to inoculums of 106–109 cfu/ml. Killing was based on MIC and MPC drug concentrations with log10 reduction in viable cells converted to percent kill and recorded at 0, 10, 30 minutes, 1, 2, 3, 4, 6, 12 and 24 h following drug exposure. Results: MIC and MPC values for GA against 2PA strains were 0.5 µg/ml and 8 µg/ml respectively. Exposure of 106–109 cfu/ml of PA to GA at MIC drug concentration resulted in a 60–94% reduction in viable cells 2–4 h after drug exposure, 84–99% by 6 h and >99% by 12 h. At the MPC concentration, more rapid killing was observed with >90% of viable cells killed by 2–4 h. MIC and MPC values for GA against two KP were 0.31–0.63 µg/ml and 0.25–1 µg/ml respectively. For 106–109 cfu/ml of KP exposed to the MIC concentration, 32–99% of cells were killed by 4 h. At the MPC drug concentration, 51–99% of cells were killed by 2 h, 81–99% by 4 h and 91–>99% by 12 h. Conclusion: These data confirm rapid bactericidal activity of GA against gram–negative bacilli. Dosing to achieve MPC concentration results in further reductions in viable cells over a range of bacterial inoculums. While dosing to achieve and maintain MIC drug concentrations contributes to a favorable clinical outcome, achieving and maintaining MPC drug concentrations ensures more rapid bactericidal activity and may help prevent the emergence of resistance.
Keywords: antibiotics/antifungals/antiparasitics • Pseudomonas