Anaerobes can be the main cause of infection in the following: The pleural spaces and lungs Intra-abdominal, gynecologic, central nervous system, upper respiratory tract, and skin and soft-tissue diseases Bacteremia Etiology of Mixed Anaerobic Infections The principal anaerobic gram-positive cocci involved in mixed anaerobic infections are Peptococci Peptostreptococci These anaerobes are part of the normal flora of the mouth, upper respiratory tract, and large intestine.