Microbiology of infected epidermal cysts

Specimens from 231 epidermal cyst abscesses were inoculated on media supportive for growth of aerobic and anaerobic bacteria. Of these, 192 yielded bacterial growth. Aerobic or facultative bacteria only were recovered in 84 specimens (44%), anaerobic bacteria only in 57 specimens (30%), and mixed aerobic and anaerobic bacteria in 51 specimens (26%). A total of 315 isolates (162 anaerobes and 153 aerobes) were recovered. An average of 0.8 aerobic or facultative isolates per infected cyst were recovered, and this number was unrelated to the cysts' anatomic sites. However, the number of anaerobic bacteria varied; they were isolated more frequently in perirectal (1.5 isolates per specimen), vulvovaginal (1.4), and head (1.1) infections, and less frequently in trunk (0.7) and extremities (0.4) infections. The predominant aerobic or facultative bacteria were Staphylococcus aureus (81 isolates), group A streptococcus (9 isolates), and Escherichia coli (7 isolates). The predominant anaerobic organisms were Peptostreptococcus species (85 isolates) and Bacteroides species (55 isolates, including 12 Bacteroides melaninogenicus and 9 Bacteroides fragilis groups). The study highlights the polymicrobial nature and predominance of anaerobes in cyst abscesses in the perirectal, vulvovaginal, and head areas.