The Effect of Inoculum Size on Selection of In Vitro Resistance to Vancomycin, Daptomycin, and Linezolid in Methicillin-Resistant Staphylococcus aureus

Objectives: The inoculum effect (IE) is an increase in the minimum inhibitory concentration (MIC) at high bacterial densities. The effect of three inoculum sizes on the selection of resistance to vancomycin, daptomycin, and linezolid was investigated in methicillin-resistant Staphylococcus aureus (MRSA). Methods: Low (10 4 CFU/ml), medium (10 6 CFU/ml), and high (10 8 CFU/ml) inocula of MRSA were exposed to twofold increasing concentrations of either drug during 15 days of cycling. MICs for low (MIC L ), medium (MIC M ), and high (MIC H ) inocula were determined daily. Conventional MICs were measured at days 1, 5, 10, and 15. Experiments were performed in triplicate. Results: At the beginning of the experiment a small IE was observed for vancomycin (MIC L =1 μg/ml, MIC M =1–2 μg/ml, and MIC H =2 μg/ml) and a significant IE for daptomycin (MIC L =0.25 μg/ml, MIC M =0.25–0.5 μg/ml, and MIC H =2 μg/ml). Linezolid exhibited no IE at low and medium inocula (MIC L =1 μg/ml and MIC M =1–2 μg/ml), but with the high inoculum, concentrations up to 2,048 μg/ml did not fully inhibit visual growth. During cycling, increase of MIC was observed for all antibiotics. At day 15, MIC L , MIC M , and MIC H of vancomycin were 2–4, 4–8, and 4–16 μg/ml and of daptomycin were 0.5–2, 8–128, and 64–256 μg/ml, respectively. MIC L and MIC M of linezolid were 1 and 2–4 μg/ml, respectively. Conventional MICs showed vancomycin and daptomycin selection of resistance since day 5 depending on the inocula. No selection of linezolid resistance was observed. Conclusions: Our results showed the importance of the inoculum size in the development of resistance. Measures aimed at lowering the inoculum at the site of infection should be used whenever possible in parallel to antimicrobial therapy.