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.

Biofilms of Staphylococcus epidermidis were cultured at various specific growth rates, and susceptibilities to tobramycin were compared with those of equivalent plank-tonic populations. In all instances, susceptibility increased significantly with increasing specific growth rate. However, resuspension of the biofilms increased susceptibility, suggesting some involvement of the glycocalyx in reducing antibiotic permeation of the biofilm. Cells that dispersed spontaneously from the biofilms at steady state were particularly susceptible to this agent. Since such cells correspond to newly-divided daughter cells, the relationship between tobramycin susceptibility and the phase in the division cycle was investigated. Susceptibility was enhanced in cultures dividing synchronously shortly before, during and shortly after cell separation. Perfusion of actively-growing S. epidermidis biofilms with tobramycin also demonstrated increased susceptibility with increasing growth rate, but also showed rapid recovery following removal of the agent.

Surface hydrophobicity, surface electrokinetic potential and the ability to adhere to nitric-acid cleansed glass surfaces has been assessed throughout the growth, in batch culture, of Escherichia cola and Staphylococcus epidermidis. In both instances adhesiveness and surface hydrophobicity decreased in early-to mid-exponential phase. Cell surface charge, on the other hand became more electro-negative for E . coli but electro-neutral for Staph. epidermidis as the cells proceeded to divide. Adhesiveness correlated directly with surface electronegativity and hydrophobicity for Staph. epidermidis b u t inversely with surface electro-negativity for E. coli.

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Surface hydrophobicity, surface electrokinetic potential and the ability to adhere to nitric-acid cleansed glass surfaces has been assessed throughout the growth, in batch culture, of Escherichia coli and Staphylococcus epidermidis. In both instances adhesiveness and surface hydrophobicity decreased in early- to mid-exponential phase. Cell surface charge, on the other hand became more electro-negative for E. coli but electro-neutral for Staph. epidermidis as the cells proceeded to divide. Adhesiveness correlated directly with surface electronegativity and hydrophobicity for Staph. epidermidis but inversely with surface electro-negativity for E. coli.