Multiple antibiotic resistance index, fitness and virulence potential in respiratory Pseudomonas aeruginosa from Jamaica

Respiratory infections are common causes of morbidity and mortality worldwide. We sought to assess the multiple antibiotic resistance (MAR) index, fitness and virulence potential in Pseudomonas aeruginosa from patients with lower respiratory tract infections. Isolates were assessed for antimicrobial susceptibility, in vitro competitive fitness, and pigment, elastase and rhamnolipid production. Oxidative stress tolerance was determined on both planktonic and biofilm cells, and virulence potential was tested in a plant model. Mean MAR index for isolates was 0.34 (range 0.17-0.50). Whilst isolates exhibited good biofilm formation in the presence of ciprofloxacin, there was no significant difference in biofilm production over the concentration range assessed. Several drug-resistant strains were out-competed by a susceptible strain even in the presence of antibiotic. H2O2 exerted a greater oxidative stress than tert-butylhydroperoxide and, as expected, biofilms were more resistant than planktonic cells. Whilst most (81 %) isolates were pigmented there was no significant difference between pigmented and non-pigmented isolates when elastolytic activity was compared (P>0.05). More than half of the isolates produced the quorum sensing mediator rhamnolipid and infection of the plant model by bacteria occurred whether elastase or rhamnolipid was present or absent. These data suggest that non-pigmented strains of P. aeruginosa might pose an equally significant microbiological threat as pigmented strains even though pigment production appeared to be strongly associated with elastase expression. Whilst dual expression of elastase and rhamnolipid by these bacteria would cause severe tissue damage (as seen in the plant model), non-production of either does not prevent bacteria from causing serious infection.