Characterization of paired mucoid/non-mucoid Pseudomonas aeruginosa isolates from Danish cystic fibrosis patients: antibiotic resistance, beta-lactamase activity and RiboPrinting

The purpose of this study was to characterize 42 paired mucoid and non-muco id Danish cystic fibrosis (CF) Pseudomonas aeruginosa isolates collected in 1997, by RiboPrinting, antibiotic susceptibility and beta -lactamase activ ity. Eight P, aeruginosa isolates collected before 1991 were included for c omparison. Eighteen of the 42 paired mucoid and non-mucoid isolates showed the same ribotype; the remaining 24 belonged to different ribogroups. Mucoi d isolates showed higher susceptibility to antibiotics and lower beta -lact amase activity compared with non-mucoid isolates. Significant differences ( P less than or equal to 0.01) between mucoid and non-mucoid isolates were f ound for the meropenem and colistin MICs for the isolates with the same rib otype, and for the MICs of ceftazidime, piperacillin, aztreonam, meropenem, tobramycin, ciprofloxacin and in the basal levels of beta -lactamase for t he paired isolates belonging to different ribogroups. A dominant ribotype 7 3-S2 with hyperinducible beta -lactamase production and significantly highe r MICs of piperacillin, meropenem and tobramycin compared with the other ma jor ribotypes (73-S1, 207-S3 and 227-S8) was present among the 84 CF isolat es. The isolates collected before 1991 had an antibiotic susceptibility pat tern similar to the 1997 isolates. Despite prolonged and intensive antibiot ic treatment, susceptible mucoid isolates were isolated from the CF sputum, possibly because these bacteria are protected from the selective pressure of antibiotics by the resistant non-mucoid isolates co-existing in the biof ilm in the lungs of CF patients.