Modification of the CpsA protein reveals a role in alteration of the Streptococcus agalactiae cell envelope.

The bacterial cell envelope is a crucial first line of defense for a systemic pathogen, with production of capsular polysaccharides and maintenance of the peptidoglycan cell wall serving essential roles in survival in the host environment. The LytR-CpsA-Psr proteins are important for cell envelope maintenance in many Gram-positive species. In this study we examined the role of the extracellular domain of the CpsA protein of the zoonotic pathogen Group B Streptococcus in capsule production and cell wall integrity. CpsA has multiple functional domains including a DNA-binding/transcriptional activation domain and a large extracellular domain. We demonstrated that episomal expression of extracellularly truncated CpsA causes a dominant negative effect on capsule production when expressed in the wild type strain. Regions of the extracellular domain essential to this phenotype were identified. The dominant negative effect could be recapitulated by addition of purified CpsA protein or a short CpsA peptide to cultures of wild type bacteria. Changes in cell wall morphology were also observed when the dominant negative peptide was added to wild type cultures. Fluorescently labeled CpsA peptide could be visualized bound at the mid-cell region near the division septae, suggesting a novel role for CpsA in cell division. Finally, expression of truncated CpsA also led to attenuation of virulence in zebrafish models of infection, to levels below that of a cpsA deletion strain, demonstrating the key role of the extracellular domain in virulence of GBS.