Allelic replacement of the streptococcal cysteine protease SpeB in a Δsrv mutant background restores biofilm formation

BACKGROUND: Group A Streptococcus (GAS) is a Gram-positive human pathogen that is capable of causing a wide spectrum of human disease. Thus, the organism has evolved to colonize a number of physiologically distinct host sites. One such mechanism to aid colonization is the formation of a biofilm. We have recently shown that inactivation of the streptococcal regulator of virulence (Srv), results in a mutant strain exhibiting a significant reduction in biofilm formation. Unlike the parental strain (MGAS5005), the streptococcal cysteine protease (SpeB) is constitutively produced by the srv mutant (MGAS5005Δsrv) suggesting Srv contributes to the control of SpeB production. Given that SpeB is a potent protease, we hypothesized that the biofilm deficient phenotype of the srv mutant was due to the constitutive production of SpeB. In support of this hypothesis, we have previously demonstrated that treating cultures with E64, a commercially available chemical inhibitor of cysteine proteases, restored the ability of MGAS5005Δsrv to form biofilms. Still, it was unclear if the loss of biofilm formation by MGAS5005Δsrv was due only to the constitutive production of SpeB or to other changes inherent in the srv mutant strain. To address this question, we constructed a ΔsrvΔspeB double mutant through allelic replacement (MGAS5005ΔsrvΔspeB) and tested its ability to form biofilms in vitro. FINDINGS: Allelic replacement of speB in the srv mutant background restored the ability of this strain to form biofilms under static and continuous flow conditions. Furthermore, addition of purified SpeB to actively growing wild-type cultures significantly inhibited biofilm formation. CONCLUSIONS: The constitutive production of SpeB by the srv mutant strain is responsible for the significant reduction of biofilm formation previously observed. The double mutant supports a model by which Srv contributes to biofilm formation and/or dispersal through regulation of speB/SpeB.