The growth and survival of capsulate and non-capsulate Bacteroides fragilis in vivo and in vitro.

The growth of capsulate and non- capsulate Bacteroides fragilis in chambers implanted in the mouse peritoneal cavity was compared. Capsulate and essentially non- capsulate (less than 1% capsulate ) populations of B. fragilis strains NCTC9343 and NCTC10584 consistently grew exponentially to greater than 10(9) cfu/ml within 24 h in vivo, and low numbers of capsulate bacteria were maintained in the essentially non- capsulate population; however, the degree of capsulation of the capsulate population decreased by more than 60%. B. fragilis ATCC23745 differed from strains NCTC9343 and NCTC10584 in that growth was unpredictable and only occurred in some of the implanted chambers. Capsule production by cells of strain ATCC23745 varied from chamber to chamber: sometimes the proportion of capsulate cells increased after prolonged implantation. This could occur with either an increase or decrease in viable numbers in vivo and also after in-vitro incubation of this strain in chambers. The survival of capsulate and non- capsulate B. fragilis strains NCTC9343 and ATCC23745 was compared in aerobic and anaerobic conditions in vitro. In anaerobic conditions, capsulate and non- capsulate strain NCTC9343 survived equally well, whereas capsulate ATCC23745 survived better than its non- capsulate variants. Capsulate populations of both strains survived better than non- capsulate in aerobic conditions.