Cells of mucoid and non-mucoid in colonies were at least one-thousandfold less sensitive to the antibiotics tobramycin or cefsulodin than were cells of the same bacteria in dispersed suspension. We did not detect any difference between the mucoid form and the non-mucoid form in the antibiotic sensitivity of colonies, from which we infer that the exopolysaccharide of the mucoid form does not contribute to colony-resistance by forming a barrier to antibiotic diffusion. Mathematical models were constructed in order to estimate time-courses of penetration of tobramycin and cefsolodin into biofilms and microcolonies of mucoid and non-mucoid . For tobramyen penetration, adsorption of antibiotic to the exopolysaccharide of the glycocalyx and antibiotic uptake by cells were taken into account in the calculations. The longest time-period for the concentration of tobramycin at the base of a biofilm 100 µm deep to rise to 90% of the concentration outside the biofilm was predicted to be 2·4 h. For cefsulodin penetration, irreversible hydrolysis catalysed by β-lactamase was taken into account, using β-lactamase levels taken from the literature. The calculations predicted that the cefsulodin concentration at the base of antifilm 100 µm deep would rise to 90% of the external concentration in 29 s when the β-lactamase was synthesized at the basal level. For a similar biofilm of bacteria synthesizing enhanced levels of β-lactamase (‘derepressed’), the concentration of cefsulodin at the base was calculated to rise to 41% of the external concentration in about 50s and then remain at that level. This was despite the fact that cefsulodin is a poor substrate for this β-lactamase.


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