Extracellular virulence factors of streptococci associated with animal diseases.

A virulence factor denotes a bacterial product or strategy that contributes to virulence or pathogenicity. Streptococci produce a variety of protein toxins and enzymes that are capable of killing host cells and breaking down cell constituents, presumably to provide nutrients for the bacteria or to promote their spread. Some of these secreted products are hemolysins, streptokinases, hyaluronidases, exotoxins and proteases. In some cases, they play an important role in resistance to the host immune system, acting alone or in combination with cell-associated virulence factors (such as the capsule and surface proteins). Thus, the virulence of streptococci is considered as a multifactorial process. In contrast to well known human pathogens, and in spite of their veterinary importance, knowledge of virulence factors of most animal disease-associated streptococci is limited or almost inexistent. In the present article, the available information regarding the extracellular virulence factors of the most important animal disease-related streptococci is reviewed.