A genus of bacteria containing at least 28 species that are collectively referred to as staphylococci. Their usual habitat is animal skin and mucosal surfaces. Although the genus is known for the ability of some species to cause infectious diseases, many species rarely cause infections. Pathogenic staphylococci are usually opportunists and cause illness only in compromised hosts. Staphylococcus aureus, the most pathogenic species, is usually identified by its ability to produce coagulase (proteins that affect fibrinogen of the blood-clotting cascade). Since most other species of staphylococci do not produce coagulase, it is useful to divide staphylococci into coagulase-positive and coagulase-negative species. Coagulase-negative staphylococci are not highly virulent but are an important cause of infections in certain high-risk groups. Although Staphylococcus infections were once readily treatable with antibiotics, some strains have acquired genes making them resistant to multiple antimicrobial agents.