Detection and quantification of Staphylococcus aureus heterogeneity to identify antibiotic-induced persistence

Persister cells are characterised as being viable but non-culturable, a state that preserves their metabolic energy to survive the environmental stress, which allows for recurrent infections. Detection of persisters is, therefore, not possible with standard culture-dependent methods. Furthermore, the effect of antibiotics on the induction of persisters has not been assessed. This study aimed to identify antibiotic-induced persistence and determine the percentage of heterogeneity. Vancomycin, daptomycin and dalbavancin were assessed by standard MIC methods against selected Staphylococcus aureus strains. Replicates of MIC assays were stained with propidium iodide to quantify live/dead and a reactive oxygen species (ROS) dye to detect and quantify persisters using culture-independent single-cell sorting, independently. A comparative analysis was then performed. Dalbavancin showed the lowest MIC values against tested S. aureus strains followed by daptomycin and vancomycin. Cell sorting of vancomycin-, daptomycin- and dalbavancin-treated S. aureus strains showed a range of 1.9–10.2 %, 17.7–62.9 % and 7.5–77.6 % live cells based on the strain, respectively, in which daptomycin, in particular, was a strong inducer of a persister population. Persisters represented 3.7–16 % of the bacterial population. The culture-independent identification of antibiotic-induced persistence through studying at the single-cell level showed different efficacy of antibiotics than standard MIC. Vancomycin was the most effective antibiotic against tested strains followed by dalbavancin then daptomycin as assessed by cell sorting. Therefore, re-evaluation of standard MIC methods may be required to assess the efficacy of antibiotics. Additionally, the detection of daptomycin-associated persisters may provide an elucidation to the reported rapid resistance development in vivo.