CHANGES IN THE BIOCIDE SUSCEPTIBILITY OF STAPHYLOCOCCUS-EPIDERMIDIS AND ESCHERICHIA-COLI-CELLS ASSOCIATED WITH RAPID ATTACHMENT TO PLASTIC SURFACES

Differences in opacity between wells of a microtitre plate containing different volumes of inoculated growth medium reflected planktonic gro wth without any contribution from cells attached at the well surface. Simple algebra and a knowledge of the dependence of optical density up on sample path length (volume) for suspensions of differing cell densi ty enables the generation of growth curves for attached populations (b iofilms). In this manner, minimum inhibitory concentrations (MICs) wer e determined at various stages of growth (0-20 h), both for cells grow ing attached to the bases of the plate wells and, simultaneously, for cells growing in suspension above them. Biocides included cetrimide, p olyhexamethylene biguanide, peracetic acid, phenoxyethanol and chlorox ylenol. Results, expressed as planktonic:biofilm MIC ratios, showed su sceptibility to change, not only as a function of attachment and biofi lm formation, but also with respect to the nature of the chemical agen t. In some instances, changes in susceptibility greater than twofold o ccurred immediately on attachment and could occur in the presence of b iocide concentrations which exceeded the MIC.