The aim of the present study was to investigate the structure,
phenotypic diversification and competitiveness of Pseudomonas
aeruginosa mutator biofilms. For this purpose, the environmental
strain Hex1T and its respective mutS mutant, Hex1TMS, were
grown in flow-cell biofilms and inspected by confocal laser
scanning microscopy. Structural analyses revealed that biofilms
formed by Hex1TMS strain were similar to those formed by Hex1T
strain, displaying the characteristic mushroom-shaped structures of
P. aeruginosa biofilms. However, mutator biofilms showed
significantly increased phenotypic diversity in comparison to wild
type biofilms, with a Shannon-Weaver Index (H) of 0.31 and 0.03,
respectively (P<0.0001). Normal, giant, small, wrinkly, transparent
and brown pigmented were among the different morphotypes
found. Competition experiments between Hex1T and Hex1TMS
strains with initial ratios 1:1 and 1:100 showed that the
hypermutable strain had increased fitness after 6 days, indicating a
potential adaptive advantage of mutators during biofilm mode of
growth. The fact that this increased competitiveness was afforded
by phenotypic diversification was confirmed by competing the
principal evolved morphotypes with the parental mutS strain. These
results give further evidences of how mutator bacteria can
accelerate the speed of evolution of a bacterial population.
