Figure 2.

Acid stress responses in Streptococcus agalactiae (Group B streptococci, GBS) under low pH. GBS possess different defense mechanisms to cope with low pH. They include the Arginine deiminase system (ADI), an F-ATPase transporter, transporter of the BCCT family, chaperones and proteases, SodA, and a NRAMP-type transporter. The Arginine deiminase system (ADI) comprises three units: an arginine deiminase (AD), ornithine carbamoyltransferase (OTC) and carbamate kinase (CK). Arginine is taken up from the extracellular environment and cleaved by AD into citrulline and ammonia. Citrulline is further cleaved to yield ornithine and carbamoyl phosphate by the action of OTC. Finally, CK cleaves carbamoyl phosphate into carbon dioxide and ammonia, thereby generating an alkaline microenvironment. Proton pumps like the F-ATPase represent the most direct approach to counteract acid stress by transporting protons outside the cell to keep a proper level of intracellular pH. Under low pH, the F-ATPase system is induced to pump out protons extracellularly in order to maintain the alkalinity of the intracellular cytoplasm. Additional mechanisms include repair or prevention of acid damage in macromolecules by chaperones and proteases, such as Dnak, GroES, and CLp. Metal ion homeostasis also takes part in the acid response as the NRAMP metal ion symporter takes up Mn²⁺ and expels protons out of the cell. The osmotic stress is tightly controlled as well through the up-regulation of the glycine betaine osmoregulation system where choline and glycine betaine (a powerful osmoprotectant) are taken up by transporters of the BCCT (Betaine/Carnitine/Choline Transporter) family. Defense mechanisms to avoid the damaging effects of superoxide species generated during acid stress are mainly exerted through the activity of the streptococcal superoxide dismutase (SOD).