Antimicrobial activities of black-pigmented gram-negative anaerobes.

The antimicrobial activities of Prevotella intermedia and Porphyromonas gingivalis isolates were tested against other species of Gram-positive and Gram-negative anaerobes as well as against each other. Generally, Pr. intermedia possessed significantly higher antimicrobial activity than P. gingivalis. The strongest activity of P. gingivalis towards Gram-negative anaerobes was directed against Pr. intermedia. Cross-sensitivity between both species was observed with strains from different lesions. Antimicrobial activity towards strains of the same species was detected only with Pr. intermedia. No correlations were found between plasmid content and antimicrobial activity. It was concluded that the inhibitory potency of Pr. intermedia could be one reason for the high proportion of black-pigmented Gram-negative anaerobes in the subgingival flora of periodontitis lesions.