Porphyromonas gingivalis Outer Membrane Vesicles Mediate Coaggregation and Piggybacking of Treponema denticola and Lachnoanaerobaculum saburreum

Porphyromonas gingivalis sheds outer membrane vesicles that contain several virulence factors, including adhesins. In this study, we investigated the ability of P. gingivalis outer membrane vesicles to mediate the coaggregation and piggybacking of Treponema denticola and Lachnoanaerobaculum saburreum. Marked coaggregation between T. denticola and L. saburreum occurred in the presence of P. gingivalis outer membrane vesicles. Sucrose was an effective chemoattractant for the motile species T. denticola. The addition of outer membrane vesicles to a mixture of T. denticola and L. saburreum significantly increased the number of nonmotile bacteria that migrated into a sucrose-filled capillary tube immersed in the bacterial mixture. Under optimal conditions, the number of nonmotile L. saburreum in the capillary tube increased approximately 5-fold, whereas no increase occurred when boiled vesicles were used. This study showed that P. gingivalis outer membrane vesicles mediate coaggregation between T. denticola and L. saburreum and that nonmotile bacteria can be translocated by piggybacking on spirochetes. 1. Introduction Outer membrane vesicles (10–300？nm in diameter) are naturally shed from the surfaces of Gram-negative bacteria. While they are produced under a variety of growth conditions, shedding appears to occur mainly in response to a stress [1]. The exact mechanism of vesiculation is not fully understood but seems to result from the budding of the bacterial envelope in areas where lipoprotein links between the outer membrane and the peptidoglycan are broken or missing [2]. Porphyromonas gingivalis, a major pathogen in chronic periodontitis, is known to shed large numbers of outer membrane vesicles [3] which have been reported to increase when P. gingivalis is grown in hemin-restricted conditions [4]. Since several determinants of P. gingivalis virulence such as adhesins and proteinases are surface associated, the shedding of outer membrane vesicles may contribute to the pathogenic process of periodontitis. Biologically active substances are present in larger amounts in outer membrane vesicles than in whole bacterial cells, likely due to the higher surface？:？volume ratio of the vesicles [3]. Previous studies have reported that P. gingivalis outer membrane vesicles can adhere to bacteria [5–8], hydroxyapatite surfaces [7], and host cells [6, 8]. Inagaki et al. [8] provided evidence for a role of P. gingivalis outer membrane vesicles in periodontal tissue invasion. More specifically, they showed that outer membrane vesicles can mediate the adherence to and