The antisense fragment used in this study is identical to the corresponding region of porM1. While it displays a homology of 71.4% to porM2, the antisense fragment and porM2 still exhibit long stretches of identical nucleic acid sequences. Of particular importance is the similarity in the beginning of the antisense fragment covering the Shine-Dalgarno Sequence and the start codon (40 bp, 95% identity). We therefore are convinced that a down-regulation of both, porM1 as well as porM2, may be achieved using the strategy described in this study. Deletion- or insertion mutagenesis of either porM1 or porM2 might result in complementation VX-661 mw of the deleted porin gene by the

remaining one. Such an effect has been observed in M. smegmatis, where the deletion of the mspA gene caused the activation of the transcription of mspB and/or mspD [28]. Mutagenesis of both porin genes in the same derivative, selleck chemicals on the other hand, would

probably restrain the diffusion across the OM to an IWP-2 mw extent compromising cellular functions. The effects of an over-expression of porin in our M. fortuitum strains depended on characteristics of the strains as well as the amount of kanamycin added to the medium. The over-expression of porM1 and porM2 showed the most considerable influence on growth rate in strain 10851/03. Among the tested strains, 10851/03 has the slowest growth rate and produces least porin. Therefore, this strain probably benefits most from a better nutrient supply caused by porin over-production. Otherwise, the adverse effect of kanamycin on the growth rate was most pronounced in strain DSM 46621, which expresses the highest amount of porin among the analysed

strains. Disposing of a relatively high amount of porin, this strain probably takes less advantage of an ameliorated nutrient supply and instead suffers most from more kanamycin diffusion into the cells. When the kanamycin concentration in the plates was reduced to 25 μg ml-1, the over-expressing DSM 46621 derivatives did not show any growth inhibition compared to the control strain and even had a slight growth advantage. It seems that at this kanamycin concentration the beneficial effects of better nutrient influx slightly exceed the adverse effects of better antibiotic influx. The changes in growth behaviour in 10851/03 as well as in DSM C59 46621 were more pronounced upon over-expression of porM2 compared to over-expression of porM1. The down-regulation of the expression of PorM1 together with PorM2 by antisense-technology reduced the growth of both M. fortuitum strains to a similar and very low level suggesting that lack of porins in the knock-down strains strongly impairs the nutrient supply. Our observations point to a passage of kanamycin through the PorM porins. Studies performed with M. smegmatis gave rise to contrarious conclusions [29, 30]. Stephan et al. [29] observed no reduction of kanamycin resistance in a mspA mutant compared to the M.