Inhibition of Actinobacillus actinomycetemcomitans leukotoxicity by bacteria from the subgingival flora

Actinobacillus actinomycetemcomitans produces a pore-forming leukotoxin tha t lyses human polymorphonuclear leukocytes and monocytes. Certain proteolyt ic bacteria may coexist with A. actinomycetemcomitans in periodontal pocket s. We aimed therefore to examine whether oral bacteria can modify the leuko toxicity of A. actinomycetemcomitans. A total of 55 strains representing 45 bacterial species of the subgingival flora were tested. Each strain was in cubated with the highly toxic strain of A. actinomycetemcomitans HK 1519 an d the leukotoxic activity of the suspension against human polymorphonuclear leukocytes was determined from the activity of the lactate dehydrogenase r eleased upon lysis of the leukocytes. Porphyromonas gingivals. Prevotella i ntermedia, Prevotella nigrescens, Prevotella melaninogenica and Prevstella loeschii inhibited the leukotoxicity of A. actinomycetemcomitans cells as w ell as the activity of leukotoxin purified from the same strain. The bacter ial strains without the ability to block leukotoxic activity also failed to destroy pure leukotoxin even after 5 h of incubation. The proteolytic degr adation of leukotoxin by P. gingivalis was mainly dependent on the activity of the enzymes R- and K-gingipains. P. intermedia and P. nigrescens also d egraded the leukotoxin by enzymes. The results imply a role of the periodon tal microflora in modifying the virulence of A. actinomycetemcomitans by de stroying its leukotoxin.