Vesicle-independent extracellular release of a proinflammatory outer membrane lipoprotein in free-soluble formReport as inadecuate

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BMC Microbiology

, 8:18

First Online: 28 January 2008Received: 02 July 2007Accepted: 28 January 2008


BackgroundAggregatibacter actinomycetemcomitans is an oral bacterium associated with aggressively progressing periodontitis. Extracellular release of bacterial outer membrane proteins has been suggested to mainly occur via outer membrane vesicles. This study investigated the presence and conservation of peptidoglycan-associated lipoprotein AaPAL among A. actinomycetemcomitans strains, the immunostimulatory effect of AaPAL, and whether live cells release this structural outer membrane lipoprotein in free-soluble form independent of vesicles.

ResultsThe pal locus and its gene product were confirmed in clinical A. actinomycetemcomitans strains by PCR-restriction fragment length polymorphism and immunoblotting. Culturing under different growth conditions revealed no apparent requirement for the AaPAL expression. Inactivation of pal in a wild-type strain D7S and in its spontaneous laboratory variant D7SS resulted in pleiotropic cellular effects. In a cell culture insert model filter pore size 0.02 μm, AaPAL was detected from filtrates when strains D7S and D7SS were incubated in serum or broth in the inserts. Electron microscopy showed that A. actinomycetemcomitans vesicles 0.05–0.2 μm were larger than the filter pores and that there were no vesicles in the filtrates. The filtrates were immunoblot negative for a cytoplasmic marker, cyclic AMP cAMP receptor protein. An ex vivo model indicated cytokine production from human whole blood stimulated by AaPAL.

ConclusionFree-soluble AaPAL can be extracellularly released in a process independent of vesicles.

Electronic supplementary materialThe online version of this article doi:10.1186-1471-2180-8-18 contains supplementary material, which is available to authorized users.

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Author: Maribasappa Karched - Riikka Ihalin - Kjell Eneslätt - Deyu Zhong - Jan Oscarsson - Sun N Wai - Casey Chen - Sirkka E A


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