avium [34, 52]. The GPL produced by this serotype is not well characterised, but the presented results indicate that they may be able to produce biofilm despite the apparent lack of some genes involved in production of the most common nsGPL. As stated above, GPL has been associated

with biofilm forming abilities. In the present study, presence of the GPL genes tested was not correlated with biofilm formation, but an association might be due to expression and not presence of the genes. The significant differences in biofilm forming abilities observed between porcine and human TGF-beta family isolates are surprising since these isolates were very similar when tested for other characteristics. BI 2536 purchase Other studies have reported that isolates of human origin may form biofilm [30,

33], so although a significant difference in biofilm formation was observed between human and porcine isolates of M. avium subsp. hominissuis in the present study, this is not a consistent difference. The ability to invade bronchial epithelial cells has been demonstrated to be impaired in biofilm deficient mutants of CB-839 the M. avium strain A5, and the same mutants had an impaired ability to cause infection in mice [53]. It has thus been suggested that the ability of an isolate to form biofilm is linked to virulence. Biofilm forming isolates may also reach their hosts in large numbers if loosening in clusters from a naturally occurring biofilm. The condition of the host may differ between humans and swine. Human hosts are often immunocompromised or have predisposing lung conditions [6, 54], while porcine hosts probably

are not. Swine rarely present with clinical disease caused by M. DNA ligase avium subsp.hominissuis [4]. It could be speculated that swine get infected only when exposed to a large infective dose of the bacterium, for instance originating from naturally occurring biofilms, or that these biofilm related isolates are more virulent. This may lead to a selection for biofilm forming isolates in swine, explaining the differences observed in the present study. Conclusion An optimised method to screen isolates of Mycobacterium avium for biofilm formation was established, and this method was used to examine 97 isolates retrieved from humans, swine and birds. Nine isolates, all of porcine origin, formed biofilm. No correlation was found between the ability of the isolates to form biofilm with the presence of selected GPL genes. The biofilm forming isolates were not related by RFLP or hsp65 sequencing. The differences observed between the porcine and human isolates raises questions regarding their biofilm forming abilities and the importance of biofilm production for their infectious potential. Acknowledgements We would like to thank Prof. Tone Tønjum (Rikshospitalet University Hospital, Norway) and Dr. Ulf R.