Methods: We used three strains in which we previously generated non-polar deletions of liaR: i) a DAP-R derivative of Efs S613 in which three alleles associated with DAP-R (liaF, cls and gdpD) were introduced, ii) OG1RF, an Efs laboratory strain, and iii) R497 a DAP-R clinical isolate of Efm. The liaR deletion derivatives of the above strains (981, 1029 and 1452, respectively) had DAP MICs of 0.094, 0.064 and 0.094 µg/ml. The strains were passaged daily for 19 days with sub-inhibitory concentrations of DAP in Muller-Hinton II broth supplemented with 50 mMol of calcium chloride. DAP E-test was performed every other day, and the DAP concentration for growth was adjusted to the highest tolerable. Whole genome sequencing was performed in DAP-R derivatives to assess for genetic changes.
Results: DAP-R isolates of Efs 981 (981-R, MIC 24 µg/mL) and Efs 1029 (1029-R, MIC 6 µg/mL) were generated. Conversely, the highest MIC for strain Efm 1452 was 3 µg/mL (CLSI breakpoint 4 µg/mL) after 19 days of passage. Genes associated with DAP-R in Efs included dihydroxyacetone kinase, an enzyme involved in phospholipid metabolism, a network of ABC transporters implicated in bacitracin resistance and two adhesisns, aggregation substance and collagen adhesion protein. Efm 1452 exhibited changes in sortase C, general metabolic enzymes and a variety of transposases, but not in genes involved in known DAP resistance pathways.
Conclusion: Efs lacking liaR generated de novo DAP-R upon passage with the antibiotic whereas a liaR-deletion derivative of Efm remained susceptible to DAP. LiaR may be an intriguing new target for the development of “anti-adaptation” compounds to prolong the usefulness of DAP against Efm, the most recalcitrant of enterococcal species.
R. Rios, None
J. Reyes, None
D. Smith, None
A. Narechania, None
R. Sebra, None
G. Deikus, None
D. Panesso, None
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