Biological features of Pseudomonas aeruginosa biofilms: time to rethink antimicrobial practices in clinical settings

Abstract

Bacterial infections in cystic fibrosis (CF) lungs are generally associated with mucoid P. aeruginosa and small colonies variants (SCV). Previous works showed that several other colony variants were isolated from CF lungs showing as well resistance to antibiotics. Those variants until now devaluated may play a relevant role in CF population persistence to antibiotic therapies and immune defences. Phenotypic switching and hypermutability have been pointed out as possible mechanisms responsible for phenotypic heterogeneity. Aim: To seek future target sites of new drugs to control bacterial infections, it is of clinical importance to determine the mechanisms underlying phenotypic heterogeneity in CF lungs. Method: P. aeruginosa strains and LB medium were used to simulate CF infections. Clonal diversification was checked after 24, 48, 72 and 120 h after media inoculation (or initial infection) by colony morphology observation. The different colonies were further characterized in terms of resistance to in-use antibiotics. Results: The colony variants isolated along CF development were susceptible to all antibiotics tested. Despite this, certain colony variants (non-mucoid and non-SCV) encompassed in their populations resistant mutants (RM). Additional susceptibility tests confirmed that those RM were resistant to the selected antibiotics, discarding the possibility of an adaptive or responsive resistance. Those colony variants with RM in their populations were consistently observed along CF infection development. This event may be indicative that these hypermutable variants act as diversity-drivers, generating phenotypes that only persist if advantageous for the entirely population. However, for some P. aeruginosa strains no hypermutable bacteria were detected in CF infection development, which may means that colony variation was caused by phenotypic switching. Conclusions: From these results it can be stated that phenotypic switching and hypermutability caused high phenotypic diversity of P.aeruginosa, highlighting that these events may occur in CF lungs. However, phenotypic switching represents an evolutional advantage since it generates variants without the fitness cost of irreversible mutations. New therapies design and drugs must definitely consider both phenotypic switching and hypermutability mechanisms.