Impact of rpoS deletion on Escherichia coli biofilms

Abstract

Slow growth has been hypothesized to be an essential aspect of bacterial physiology within biofilms. In order to test this hypothesis, we employed two strains of Escherichia coli, ZK126 (DeltalacZ rpoS(+)) and its isogenic DeltarpoS derivative, ZK1000. These strains were grown at two rates (0.033 and 0.0083 h(-1)) in a glucose-limited chemostat which was coupled either to a modified Robbins device containing plugs of silicone rubber urinary catheter material or to a glass flow cell. The presence or absence of rpoS did not significantly affect planktonic growth of E. coli. In contrast, biofilm cell density in the rpoS mutant strain (ZK1000), as measured by determining the number of CFU per square centimeter, was reduced by 50% (P < 0.05). Deletion of rpoS caused differences in biofilm cell arrangement, as seen by scanning confocal laser microscopy. In reporter gene experiments, similar levels of rpoS expression were seen in chemostat-grown planktonic and biofilm populations at a growth rate of 0.033 h(-1). Overall, these studies suggest that rpoS is important for biofilm physiology.

FIG. 1

Graph showing effects of rpoS deletion and growth rate on planktonic (expressed as log₁₀ CFU per milliliter) and biofilm (expressed as log₁₀ CFU per square centimeter) cultures. Error bars represent standard deviations. Values with the same letter are not significantly different (P = 0.05).

FIG. 2

SCLM micrographs of E. coli biofilms stained with Live/Dead viability stain in the presence (A) and absence (B) of rpoS. The viable (brightly stained) cells are indicated by an arrow. Bars, 2 μm in panel A and 3 μm in panel B.