Daptomycin inoculum effects and mutant prevention concentration with Staphylococcus aureus.
BACKGROUND
Antimutant activity of antimicrobials can be estimated by comparing drug pharmacokinetics with mutant prevention concentration (MPC). Large bacterial inocula known to reduce susceptibility have not been studied for effects on MPC determination.


METHODS
Staphylococcus aureus inoculum size was varied with solid and liquid media containing daptomycin and Ca(2+), a cation expected to lower inoculum effects, to assess effects on MIC and MPC.


RESULTS
With drug-containing agar, individual colonies were obtained over a narrow range of inoculum size that shifted to higher inoculum size as daptomycin concentration increased. Increasing Ca(2+) supplementation from 1 to 50 mM lowered MIC by 2-fold and MPC from 20 to 3 mg/L, the latter determined by extrapolation of population analysis profiles to an inoculum size of 10(10) cfu. Cells of colonies recovered from daptomycin-containing agar had wild-type MIC. With liquid medium, supplemented with 1 mM Ca(2+)and containing 10(10) cfu, MPC was between 2.5 and 5 mg/L at an inoculum density of 10(7) cfu/mL. Bacteria recovered from liquid assays exhibited a 4- to 8-fold increase in MIC and contained point mutations in mprF.


CONCLUSIONS
Inoculum effects on MPC can be reduced by measurement with low-density (large volume) liquid bacterial cultures. Retesting putative mutants for susceptibility can be important: stable mutants having genetic variations in the mprF gene were recovered from liquid medium, but not from agar. Daptomycin MPC with S. aureus was below minimal plasma drug concentration with approved doses, which is consistent with resistance to daptomycin arising rarely.
Quinn
Brian
B
Public Health Research Institute and Department of Microbiology and Molecular Genetics, UMDNJ-New Jersey Medical School, 225 Warren Street, Newark, NJ 07103, USA.
Hussain
Syed
S
Malik
Muhammad
M
Drlica
Karl
K
Zhao
Xilin
X
D000362:Agar; D000483:Alleles; D000900:Anti-Bacterial Agents / Q000493:pharmacokinetics / Q000494:pharmacology; D002118:Calcium; D015169:Colony Count, Microbial; D003470:Culture Media / Q000737:chemistry; D017576:Daptomycin / Q000493:pharmacokinetics / Q000494:pharmacology; D024881:Drug Resistance, Bacterial / Q000187:drug effects* / Q000235:genetics; D009154:Mutation / Q000187:drug effects*; D012641:Selection, Genetic; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development
D000900:Anti-Bacterial Agents; D003470:Culture Media; D000362:Agar; D017576:Daptomycin; D002118:Calcium