Identification of a signalling molecule involved in bacterial intergeneric communication

Abstract

The development of complex multispecies communities such as biofilms is controlled by interbacterial communication systems. We have previously reported an intergeneric communication between two oral bacteria, Streptococcus cristatus and Porphyromonas gingivalis, that results in inhibition of fimA expression. Here, we demonstrate that a surface protein, arginine deiminase (ArcA), of S. cristatus serves as a signal that initiates intergeneric communication. An ArcA-deficient mutant of S. cristatus is unable to communicate with P. gingivalis. Furthermore, arginase activity is not essential for the communication, and ArcA retains the ability to repress expression of fimA in the presence of arginine deiminase inhibitors. These results present a novel mechanism by which intergeneric communication in dental biofilms is accomplished.

Fig. 1.

SDS-PAGE analysis and inhibitory activity of the fractions of S. cristatus surface proteins. Surface extracts of S. cristatus were precipitated with ammonium sulfate at increasing concentrations, separated by SDS-PAGE and stained with Coomassie blue. Lane 1, molecular size standards; lane 2, ammonium sulfate (AS) fraction AS3; lane 3, AS4; lane 4, AS5; lane 5, AS6; lane 6, Blue Sepharose unbound fraction of AS6. Molecular sizes and the ArcA band are denoted by arrows. Inhibitory activity is expressed as % reduction (compared to buffer control) of LacZ activity in P. gingivalis UPF, a strain carrying the transcriptional fusion of a promoterless lacZ and the fimA promoter region.

Fig. 2.

Comparison of the growth curves of S. cristatus strains in TSB medium. The data points are means±sem of four samples (error bars not shown where smaller than symbols). Samples of 1 ml were taken and the OD₆₀₀ was measured over a period of 30 h.

Fig. 3.

Inhibition of fimA expression in P. gingivalis by ArcA. (a) S. cristatus surface proteins were subjected to SDS-PAGE (12 %) and stained with Coomassie blue. Lane 1, molecular mass markers; lane 2, ammonium sulfate fraction AS6 of CC5A; lane 3, ammonium sulfate fraction AS6 of ArcAE (arcA mutant); lane 4, ammonium sulfate fraction AS6 of cArcAE (arcA mutant complemented with the wild-type allele); lane 5, recombinant ArcA purified from E. coli. (b) P. gingivalis UPF carrying a fimA promoter–lacZ fusion and P. gingivalis Mflac carrying an mfa1 promoter–lacZ fusion were tested for LacZ activity in the presence or absence of surface extracts (50 μg) isolated from the S. cristatus strains indicated, or rArcA. The results are means±sem (n=3). Means with different letters are significantly different (P<0.05; Bonferroni test); means with the same letter are not significantly different.