Present systems for the classification of anaerobic bacteria have eliminated many of the problems of earlier systems. There are still a number of major problems that require attention, however. Anaerobic bacteria produce many different enzymes that are of importance in providing nutrients to the bacterial cell, as virulence factors, and in permitting organisms to colonize or survive under adverse conditions (including exposure to antimicrobial agents). Some enzymes effect several types of modifications to bile acids, neutral steroids, and corticosteroids. Anaerobes are clearly important in a variety of infections in humans and animals as well as in various other types of pathologic processes.