Pseudomonas aeruginosa cystic fibrosis isolates of similar RAPD genotype exhibit diversity in biofilm forming ability in vitro

Abstract

Background: Pseudomonas aeruginosa is considered to grow in a biofilm in cystic fibrosis (CF) chronic lung infections. Bacterial cell motility is one of the main factors that have been connected with P. aeruginosa adherence to both biotic and abiotic surfaces. In this investigation, we employed molecular and microscopic methods to determine the presence or absence of motility in P. aeruginosa CF isolates, and statistically correlated this with their biofilm forming ability in vitro.

Results: Our investigations revealed a wide diversity in the production, architecture and control of biofilm formation. Of 96 isolates, 49% possessed swimming motility, 27% twitching and 52% swarming motility, while 47% were non-motile. Microtitre plate assays for biofilm formation showed a range of biofilm formation ability from biofilm deficient phenotypes to those that formed very thick biofilms. A comparison of the motility and adherence properties of individual strains demonstrated that the presence of swimming and twitching motility positively affected biofilm biomass. Crucially, however, motility was not an absolute requirement for biofilm formation, as 30 non-motile isolates actually formed thick biofilms, and three motile isolates that had both flagella and type IV pili attached only weakly. In addition, CLSM analysis showed that biofilm-forming strains of P. aeruginosa were in fact capable of entrapping non-biofilm forming strains, such that these 'non-biofilm forming' cells could be observed as part of the mature biofilm architecture.

Conclusions: Clinical isolates that do not produce biofilms in the laboratory must have the ability to survive in the patient lung. We propose that a synergy exists between isolates in vivo, which allows "non biofilm-forming" isolates to be incorporated into the biofilm. Therefore, there is the potential for strains that are apparently non-biofilm forming in vitro to participate in biofilm-mediated pathogenesis in the CF lung.

Figure 1

Scanning electron microscopy images of Pseudomonas aeruginosa isolates attaching to glass surfaces. Weakly adherent P. aeruginosa isolates formed a monolayer (B and D; isolate 80) while the moderate and strongly adherent isolates formed clumps of cells (A and C; isolate 17) when biofilms were grown on glass cover slips. Microbial attachment first presented as clumps of cells (A and B; 7 and 14 h respectively after inoculation) and as the biofilm matured the spaces between the clumps were covered with a cell lawn (C and D; 20 and 40 h respectively after inoculation).

Figure 2

Box-and-whiskers plots showing the impact of flagella/TFP on the biofilm. P. aeruginosa isolates placed in four groups based on their motility properties. Based on the presence of flagella/TFP the groups were named as C1 (+/+), C2 (-/-), C3 (+/-), C4 (-/+). One-way ANOVA reveals a significant difference (P < 0.001) between the groups with respect to CV absorbance. (This difference can also be observed when the three outliers, marked by stars, in group C2 and the two outliers in group C3 are discarded from the analysis.) Tukey's post-hoc test revealed that the presence of both flagella/pili (group C1) contributes to a significantly higher biofilm biomass (as compared to groups C2-C4).

Figure 3

CSLM images of GFP-tagged Pseudomonas aeruginosa biofilms in a glass capillary flow reactor 72 h post-inoculation, showing variation in biofilm structure. (A) control strain P. aeruginosa ATCC15442; (B) P. aeruginosa CF isolate 17; (C) P. aeruginosa CF isolate 40 (D) P. aeruginosa CF isolate 41.

Figure 4

CSLM images of mixed biofilm produced by Pseudomonas aeruginosa isolates gfp-17 (green) and isolate 80 (red) in a glass capillary flow reactor. Isolate gfp-17 was allowed to establish a biofilm for 48 h and then isolate 80 was inoculated into the flow reactor. After 24 h incubation the mixed biofilm was stained and GFP and rhodamine B were excited at 488 nm and 567 nm respectively.

Figure 5

Cross section of the mixed Pseudomonas aeruginosa biofilm. Isolate gfp-17 was allowed to establish a biofilm for 48 h and then isolate 80 was inoculated into the flow reactor. After 24 h incubation the mixed biofilm was stained and GFP and rhodamine B were excited at 488 nm and 567 nm respectively. As can be seen from the cross section, isolate 80 became incorporated into the biofilm body and was not simply attached to the surface of the isolate gfp-17 biofilm.