Growth-rate-independent killing by ciprofloxacin of biofilm-derived Staphylococcus epidermidis; evidence for cell-cycle dependency.

Cell culture methods that allow culture of Staphylococcus epidermidis biofilms at controlled growth rates were used to examine susceptibility to ciprofloxacin. Changes in biofilm susceptibility, dependent upon growth rate, were compared with those for suspended populations grown in chemostat, and also for newly-formed daughter cells shed from the biofilm during its growth and development. Susceptibility increased for intact and resuspended biofilms, and also for planktonic cultures, with increases in growth rate. The dependence of susceptibility upon growth rate was greatest for slow growing cells (mu, 0.01-0.15/h). At any particular growth rate, biofilms appeared more susceptible than their planktonic counterparts. Newly-formed daughter cells were relatively tolerant to ciprofloxacin at all rates of growth. Lack of growth rate dependency for the newly-formed cells suggested a role for the cell-division cycle in determining resistance. This was confirmed by examining the susceptibility of S. epidermidis throughout batch cultures with cell division synchronized. Perfusion of various steady-state biofilms with ciprofloxacin demonstrated killing of the adherent population even at much reduced rates of growth.