The cytoplasmic protein SspC of Staphylococcus aureus, referred to as staphostatin B, is a very specific, tightly binding inhibitor of the secreted protease staphopain B (SspB). SspC is hypothesized to protect intracellular proteins against proteolytic damage by prematurely folded and activated staphopain B (M. Rzychon, A. Sabat, K. Kosowska, J. Potempa, and A. Dubin, Mol. Microbiol. 49:1051-1066, 2003). Here we provide evidence that elimination of intracellular staphopain B activity is indeed the function of SspC. An isogenic sspC mutant of S. aureus 8325-4 exhibits a wide range of striking pleiotropic alterations in phenotype, which distinguish it from the parent. These changes include a defect in growth, a less structured peptidoglycan layer within the cell envelope, severely decreased autolytic activity, resistance to lysis by S. aureus phages, extensively diminished sensitivity to lysis by lysostaphin, the ability to form a biofilm, and a total lack of extracellular proteins secreted into the growth media. The same phenotype was also engineered by introduction of sspB into an 8325-4 sspBC mutant. In contrast, sspC inactivation in the SH1000 strain did not yield any significant changes in the mutant phenotype, apparently due to strongly reduced expression of sspB in the sigma B-positive background. The exact pathway by which these diverse aberrations are exerted in 8325-4 is unknown, but it is apparent that a very small amount of staphopain B (less than 20 ng per 200 microg of cell proteins) is sufficient to bring about these widespread changes. It is proposed that the effects observed are modulated through the proteolytic degradation of several cytoplasmic proteins within cells lacking the inhibitor. Seemingly, some of these proteins may play a role in protein secretion; hence, their proteolytic inactivation by SspB has pleiotropic effects on the SspC-deficient mutant.