Spiral Plating Technique Applied to the Detection of Antagonism between Two Oral Species of Black-Pigmented Bacteroides
Various strains of black-pigmented Bacteroides were screened for antagonistic interactions by using a spiral plating technique. The initial screening procedure included cross combinations of Bacteroides gingivalis , Bacteroides melaninogenicus ssp. intermedius and Bacteroides melaninogenicus ssp. melaninogenicus. Pure cultures grown on an enriched trypticase soy agar (ETSA) medium were transferred to reduced transport fluid. Samples were dispersed, serially diluted and plated on ETSA under anaerobic conditions. Each plate was inoculated with a test strain deposited at a variable amount and an indicator strain deposited at a constant amount. After anaerobic incubation, plates were examined microscopically for inhibition. Antagonististic interactions were found only between strains of B. gingivalis and B. mel. ssp. intermedius. A second antagonism screening included 36 combinations of B. gingivalis and B. mel. ssp. intermedius. Each test consisted of four plates - two control and two double-inoculated plates. After anaerobic incubation, colonies of the control and indicator strains were counted and compared. Inhibition of B. mel. ssp. intermedius by B. gingivalis was found in 16 of the 36 combinations. The 16 cases of positive inhibition were duplicated on two different media-buffered ETSA and MM10 without sucrose. Selected antagonistic combinations were examined by three additional techniques including cross streaking, filtering of end products and diffusion. Positive inhibition of B. mel. ssp. intermedius by B. gingivalis was clearly detected in the first two of these tests.