LAUSR

Pseudomonas aeruginosa is an important etiological agent of opportunistic infections. Injectable colistin is available as a last-line treatment option for multidrug-resistant P. aeruginosa infections. When cells were inoculated at a high number, colistin-susceptible P. aeruginosa grew on agar medium containing colistin at a concentration 10-fold higher than the minimum inhibitory concentration without acquiring colistin resistance. This study examined the responsible mechanism for growth in the presence of a high concentration of colistin. Cell wash fluid derived from P. aeruginosa efficiently reduced colistin antimicrobial activity. This reduction was mediated by lipopolysaccharide (LPS) in the wash fluid. Extracellular LPS inhibited colistin activity more effectively than cell-bound LPS in fixed cells. Cell wash fluids from Escherichia coli and Acinetobacter baumannii also reduced colistin activity; however, they were less potent than those from P. aeruginosa. The amount of LPS in cell wash fluid from P. aeruginosa was approximately 10-fold higher than that in fluid from E. coli or A. baumannii. In conclusion, cell-free LPS derived from bacterial cells inhibited the antimicrobial activity of colistin, and this effect was greatest for P. aeruginosa. Thus, large amounts of broken and dead cells of P. aeruginosa at infection foci will reduce the effectiveness of colistin, even against cells that have not yet acquired resistance.