Novel selective medium for isolation of bacteroides fragilis from clinical specimens

Abstract

Background Bacteroides fragilis is an obligate anaerobic Gram-negative bacilli that can be found in abdominal wounds and soft tissue infections. It is often present in a mixed culture with other bacteria. B. fragilis isolates resistant to various antimicrobial agents have also been reported.

Objective The objective of this study is to develop and evaluate a novel selective medium (BFS medium) for the isolation of B. fragilis from clinical specimens.

Methodology From February to April 2016, 1209 specimens including wound swabs, bile, pus, peritoneal fluid and tissue were collected in an acute hospital in Hong Kong. The detection of B. fragilis using BFS medium was compared to its detection by routine laboratory method. Identification of colonies was done by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. A duplex polymerase chain reaction was performed to confirm B. fragilis species identification and to detect the presence of cfiA gene.

Results B. fragilis growth was observed as yellow colonies surrounded by a black discoloration on BFS medium. Other Bacteroides and Parabacteroides species included in the evaluation also grew on BFS medium. The growth of other aerobes, facultative anaerobes and anaerobes in the evaluation was inhibited except for Fusobacterium mortiferum. Overall, 19 specimens with B. fragilis were detected by BFS medium only while 41 specimens were detected by both routine practice and BFS medium (P = 0.008, McNemar’s test). B. fragilis strains positive for cfiA gene were isolated from six clinical specimens.

Conclusion B. fragilis showed good growth on BFS medium. BFS medium also resulted in the isolation of significantly more B. fragilis from clinical specimens than routine laboratory method. BFS medium can be a supplementary medium to routine testing to facilitate the isolation of B. fragilis from clinical specimens.