Effect on Pseudomonas aeruginosa alginate expression of direct plating and culture of fresh cystic fibrosis sputum on to pseudomonas isolation agar containing subinhibitory concentrations of roxithromycin and rifampicin.

The effects of subinhibitory concentrations of roxithromycin (16 mg/L) or rifampicin (16 mg/L) on alginate production by Pseudomanas aeruginosa were investigated. The weight of purified alginate from antibiotic-free cultures was significantly greater (52.5 +/- 24.0 mg, range 22.4-109.5), compared with alginate from cultures bacteria exposed to sub-MIC of roxithromycin (21.9 +/- 17.0, 0.0-42.1 (P < or = 0.037)) and to sub-MIC of rifampicin (28.6 +/- 15.0, 2.9-47.5 (P < or = 0.038)). Chromatographic analysis of hydrolysed and chemically transformed sub-units of alginate revealed that the presence and the molar ratio of D-mannuronic acid and L-guluronic acid were not affected in the remnant alginate exposed to sub-MIC of roxithromycin in contrast to that in the remnant alginate exposed to sub-MIC of rifampicin.