Recovery of anaerobic bacteria from vented blood-culture bottles

From January 1979, to July 1981, blood from each patient suspected of having bacteremia was collected in two bottles of commercially prepared tryptic soy broth; one bottle was vented for 24 hr to the atmosphere. Bottles were incubated at 37 degrees C for a maximum of seven days and were examined daily for signs of growth. Blind subcultures onto chocolate-agar plates were performed 18-24 hr after collection and again at 48 hr. For 63,106 bottles inoculated, a total of 4,788 strains were isolated, for a rate of 6.4% (excluding contaminants). Two hundred and thirty (4.8%) isolates were anaerobes, of which 75 (33%) were identified as Bacteroides fragilis. It is significant that 20 of these anaerobic isolates (approximately 9%) were recovered only from the vented bottles. These included eight strains of B. fragilis; three strains of Clostridium perfringens; and one strain each of Bacteroides distasonis, Fusobacterium nucleatum, Peptococcus saccharolyticus, and Eubacterium species. These results emphasize the need to check vented bottles for anaerobes, since significant numbers of these bacteria could be missed by failing to do so. These data further show the desirability of holding anaerobic blood cultures for a minimum of one week.