’ Here, find more we used MFCs to assess several behaviors of wild types and TFP/polar flagellum mutants of A. citrulli. TFP and polar flagella are involved in motility, attachment and biofilm formation in different bacterial species (Josenhans & Suerbaum, 2002; Mattick, 2002; Craig et al., 2004). We have demonstrated previously that TFP and polar flagella are involved in the pathogenicity of A. citrulli (Bahar

et al., 2009; O. Bahar and S. Burdman, unpublished results). We also showed that functional TFP are required for biofilm formation of this bacterium on glass and polystyrene surfaces (Bahar et al., 2009). Acidovorax citrulli has the ability to colonize the xylem vessels of melon seedlings (Bahar et al., 2009). Here, studies with xylem-mimicking MFCs revealed an even more drastic effect of TFP on surface attachment and biofilm formation. Under flow conditions, cells of the TFP-null mutant M6-M were unable to attach to the surface. This result was in contrast to findings from conventional assays, where cell attachment and biofilm formation by this mutant were observed to some extent (Bahar et al., 2009). These

results were corroborated by the use of an additional TFP-null mutant in a different A. citrulli strain, W1-A, which is impaired in pilA (major TFP subunit pilin), and showed a behavior similar to that of M6-M in MFCs. The W1-A mutant, generated in the background of wild-type M6, was used in these assays because numerous attempts to generate a pilA mutant in the background of strain M6 were unsuccessful (Bahar et al., 2009). It is important to mention that strain W1 is not a typical A. citrulli strain as it lacks a polar flagellum and BTK activity possesses reduced virulence in comparison with other group II strains of this bacterium

(Bahar et al., 2009). Nevertheless, in this specific study, utilization of Galeterone the W1-A mutant served as an additional means to assess the role of A. citrulli TFP in the MFC system. An interesting phenotype was seen with the hyperpiliated pilT mutant M6-T. In contrast to M6-M, M6-T cells were able to attach to the surface; however, the strength of attachment was significantly weaker than M6, supporting the fact that functional TFP is crucial for surface attachment under flow. Our findings also demonstrate that under flow, functional TFP play an important role in biofilm growth by A. citrulli. In contrast, under the conditions tested, polar flagella appear to be less important for adhesion and biofilm formation of A. citrulli. This statement is supported by the fact that the flagellin mutant M6-flg and wild-type W1 (both lacking flagella) were able to attach to the surface and form a biofilm in a manner similar to that of M6. TFP are well-established virulence determinants of animal pathogenic bacteria, and were recently shown to contribute to the virulence of several phytopathogenic bacteria, including Ralstonia solanacearum, Xanthomonas oryzae pv.