Methods: Clonal relatedness of P. aeruginosa isolates was verified by pulsed-field gel electrophoresis. The antimicrobial susceptibility was determined by an automated VITEK 2 system and/or broth micro-dilution method. Biofilm formation was quantified by the crystal violet method; swarming motility was measured on soft agar, and susceptibility to normal human serum determined by reduction of CFUs.
Results: High prevalence of P. aeruginosa colonization among Mexican children with CF was confirmed; 20% (10/49) of clones identified showed a multidrug-resistant phenotype and 8.2% (4/49) an extensive drug resistance phenotype; 26.5% (13/49) of the isolates were resistant to colistin, 42.9% (21/49) presented a phenotype of adaptation associated with chronic infection, and 79.6% (39/49) showed increased ability to survive in normal human serum.
Conclusions: This cohort of children with cystic fibrosis reveals that colonizing P. aeruginosa strains predominantly display resistance to several first-line antibiotics although most isolates were susceptible to meropenem and tobramycin; 42.9% of isolates showed a phenotype consistent with adaptation to chronic lung infection.