Single and combination antibiotic susceptibilities of planktonic, adherent, and biofilm-grown Pseudomonas aeruginosa isolates cultured from sputa of adults with cystic fibrosis

Abstract

Evidence suggests that Pseudomonas aeruginosa bacteria form biofilms within the airways of adults with cystic fibrosis (CF). The objective of this study was to determine whether clinical isolates of P. aeruginosa recovered from adults with CF have similar susceptibilities to individual antibiotics and to antibiotic combinations when grown as adherent monolayers or as biofilms compared to when they are grown using planktonic methods. Twelve multiresistant P. aeruginosa isolates, one mucoid and one nonmucoid from each of six CF patients, were grown conventionally under planktonic conditions, as adherent bacterial monolayers, and as biofilms. Each bacterial isolate remained genotypically identical despite being cultured under planktonic, adherent, or biofilm growth conditions. Isolates grown as adherent monolayers and as biofilms were less susceptible to bactericidal killing by individual antibiotics compared to those grown planktonically. More importantly, biofilm-grown bacteria, but not adherent monolayer-grown bacteria, were significantly less susceptible to two- and three-drug combinations of antibiotics than were planktonically grown bacteria (P = 0.005). We conclude that biofilm-grown bacteria derived from patients with CF show decreased susceptibility to the bactericidal effects of antibiotic combinations than do adherent and planktonically grown bacteria.

FIG. 1.

Scanning electron micrograph of P. aeruginosa (nonmucoid isolate from patient 3) grown as a biofilm attached to the plastic peg.

FIG. 2.

Pulsed-field gel electrophoresis of P. aeruginosa isolates. Nonmucoid isolate 1 has an identical banding pattern when grown as a biofilm (lane 2) and when grown planktonically (lane 3). Similar results are pictured for mucoid isolate 5 (lanes 4 and 6). Lanes 1 and 7 are control lanes, lane 5 represents results from another planktonic isolate; its biofilm equivalent had an identical banding pattern but is not shown in the figure.