Resistance studies with daptomycin

We studied the in vitro emergence of resistance to daptomycin using three m ethods: spontaneous resistance incidence, serial passage in the presence of increasing drug concentrations, and chemical mutagenesis. No spontaneously resistant mutants were obtained for any organism tested (< 10(-10) for Sta phylococcus aureus, <10(-9) for Staphylococcus epidermidis, <less than>10(- 9) for Enterococcus faecalis, <10-9 for Enterococcus faecium, <less than>10 (-8) for Streptococcus pneumoniae), Population analysis demonstrated that b acterial susceptibility to daptomycin is heterogeneous, Assay results were sensitive to calcium concentration and culture density, both of which can a ffect apparent resistance rates. Stable S, aureus mutants were isolated by both serial passage in liquid media and chemical mutagenesis. The daptomyci n MICs for these isolates were 8- to 32-fold higher than for the parental s train. Many mutants with high MICs (> 12.5 mug/ml) had significant growth d efects but did not display phenotypes typical of S, aureus small colony var iants, The voltage component (Delta psi) of the bacterial membrane potentia l was increased in three independent resistant isolates. In vivo data showe d that some daptomycin-resistant mutants had lost significant virulence. Fo r other mutants, the degree of in vitro resistance was greater than the cha nge in in vivo susceptibility, These results suggest that infection,vith so me daptomycin-resistant organisms may still be easily treatable.