Association of Actinobacillus actinomycetemcomitans leukotoxin with nucleic acids on the bacterial cell surface

Actinobacillus actinomycetemcomitans, a periodontopathic gram-negative bacterium, produces a leukotoxin that is a member of the RTX cytotoxin family. Although genes may function in toxin secretion, the leukotoxin is not secreted extracellularly but remains associated with the bacterial cell surface. We report here that this toxin-cell surface association is mediated by nucleic acids and directly demonstrate that the extracellular secretion of toxin occurs in growing cultures with increased ionic strength of medium. All examinations were performed with freshly harvested A. actinomycetemcomitans 301-b from anaerobic fructose-limited chemostat cultures. The occurrence of cell surface-localized DNA was shown by directly digesting whole cells with the restriction endonuclease EcoRI or HindIII, which yielded many DNA fragments. The cell surface DNA constituted about 20% of the total cellular DNA. The leukotoxin was released from the whole cells by digestion with DNase I as well as restriction endonucleases. Because the leukotoxin binds ionically to DNA, it is dependent on the ionic strength of buffers or media. Accordingly, the toxin was released from cells suspended in saline at pH 7.5 in the presence of increasing amounts of MgCl2 (0 to 10 mM) or NaCl (0 to 50 mM). Moreover, a considerable quantity of leukotoxin was detected in the culture supernatant of fructose-limited chemostat cultures when sodium succinate solution was pumped into the steady state as an additional salt (30 and then 50 mM). This toxin-DNA association was also found in well-characterized strains including not only the leukotoxin-producing ATCC 29522 but also the toxin production-variable ATCC 29523 and the non-leukotoxin-producing ATCC 33384 when these strains were grown in the chemostat culture.