The effect of a bacteriophage on diversification of the opportunistic bacterial pathogen, Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa is an opportunistic human pathogen that colonizes the lungs of cystic fibrosis (CF) patients. CF lungs often contain a diverse range of P. aeruginosa phenotypes, some of which are likely to contribute to the persistence of infection, yet the causes of diversity are unclear. While the ecological heterogeneity of the lung environment and therapeutic regimes are probable factors, a role for parasitic bacteriophage cannot be ruled out. Parasites have been implicated as a key ecological variable driving the evolution of diversity in host populations. PP7 drove cycles of morphological diversification in host populations of P. aeruginosa due to the de novo evolution of small-rough colony variants that coexisted with large diffuse colony morph bacteria. In the absence of phage, bacteria only displayed the large diffuse colony morphology of the wild-type. Further assays revealed there to be two distinct types of resistant bacteria; these had very different ecological phenotypes, yet each carried a cost of resistance.

Figure 1

Mean±s.e.m. sympatric bacterial diversity (closed circles) in the presence (a) and absence (c) of phage. Diversities were calculated as the complement of Simpson's Index, based on the frequencies of colony morphology variants. Mean±s.e.m. densities of bacteria (LD morphology, open squares, SR morphology, closed squares) and phage (open triangles) are shown in the presence (b) and absence (d) of phage. Panel 1 of each figure shows cycle 1, panel 2 shows cycle 2.

Figure 2

Bacterial diversity on day three of cycle one in the presence (a) and absence (b) of phage. Bars show the frequency of colonies with a given swim diameter that grow predominantly as a biofilm at the air–broth interface (black bars) or in the broth (white bars) of a static microcosm. The grey area shows the proportion of bacteria with a given swim diameter that are resistant to ancestral PP7.

Figure 3

The relative fitness ±s.e.m. of MFR-type resistant mutants (black bars), BDR-type resistant mutants (white bars) and sensitive bacteria (light grey bars). All measurements are relative to a marked ancestor and were carried out over 4 and 24 h in the absence of phage, and 24 h in the presence of phage.

Figure 4

Bacterial diversity after 3 days in static microcosms inoculated with MFR-type resistant mutants (a) and BDR-type resistant mutants (b). Bars show the frequency of colonies with a given swim diameter that grow predominantly as a biofilm at the air–broth interface (black bars) or in the broth (white bars) of a static microcosm. The grey area shows the proportion of bacteria with a given swim diameter that are resistant to ancestral PP7.