Regulation of leukotoxin synthesis in Actinobacillus actinomycetemcomitans

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Actinobacillus actinomycetemcomitans, a Gram-negative periodontopathic bacterium, produces a leukotoxin belonging to the RTX family. We have examined the regulation mechanism of leukotoxin production by a toxin-production-variable strain (301-b) during the growth in a chemostat. The following results were obtained :The cellular content of leukotoxin was dependent on the availability of sugar substrate. When the bacterium was grown in a fructose-limited chemostat culture, it produced higher amounts of leukotoxin comparing those in fructose-excess cultures. By the addition of extra fructose (50 mM) to the fructose-limited culture (D=0.15h^<-1>), the leukotoxin level decreased at the constatnt rate identical to the dilution rate. Interestingly, the intracellular level of cyclic AMP also decreased at a high rate to 7 % of the original level during the period of fructose-excess condition. Furthermore, the addition of extra cyclic AMP (3 mM) to a no-leukotoxin-producing, fructose-excess cult … More ure resulted in the production of leukotoxin. These results suggest that the production of leukotoxin is subject to catabolite repression.The apparent leukotoxin production was a result of both growth-rate-dependent production and growth-rate-independent decomposition. The specific rate of leukotoxin production (q) in the presence and absence of 10 mM bicarbonate was estimated to be the following equations :q=9.3 D-0.407 (in the presence of 10 mM bicarbonate)q=2.7 D-0.058 (in the absence of bicarbonate)where D is growth rate. From the slopes of the above relationships, a theoretical maximum leukotoxin yield was estimated as 9.3 and 2.7 mug (mg dry weight)^<-1> in the presence and absence of 10 mM bicarbonate, respectively. The increased leukotoxin yield in the cultures containing bicarbonate indicated that the sddition stimulated the efficiency of leukotoxin synthesis up to about threefold.These results indicate that the production of leukotoxin by A.actinomycetemcomitans is influenced by several environmental factors. Less