We have recently demonstrated that antibiotic pressure can induce the viable but
non-culturable (VBNC) state in Staphylococcus aureus biofilms. Since dormant bacterial
cells can undermine anti-infective therapy, a greater understanding of the role of antibiotics
of last resort, including daptomycin, is crucial. Methicillin-resistant S. aureus 10850
biofilms were maintained on non-nutrient (NN) agar in the presence or absence of the MIC
of daptomycin until loss of culturability. Viable cells were monitored by epifluorescence
microscopy and flow cytometry for 150 days. All biofilms reached non-culturability at 40
days and showed a similar amount of viable cells; however, in biofilms exposed to
daptomycin, their number remained unchanged throughout the experiment, whereas in
those maintained on NN agar alone, no viable cells were detected after 150 days. Gene
expression assays showed that after achievement of non-culturability, 16S rDNA and mecA
were expressed by all biofilms, whereas glt expression was found only in daptomycin-exposed biofilms. Our findings suggest that low daptomycin concentrations, such as those that are
likely to obtain within biofilms, can influence the viability and gene expression of
non-culturable S. aureus cells. Resuscitation experiments are needed to establish the
VBNC state of daptomycin-exposed biofilms