In the RT-qPCR experiments, sample to sample variation was corrected by normalizing to the reference gene rpsJ [31] . The rpsJ gene was selected for normalization, since rpsJ was overall the highest ranked reference gene regarding gene expression stability [31] .Reverse transcription was carried out using random hexamers, to prevent gene specific biases [32] . PCR efficiency in the qPCR was determined using a standard curve for each gene, enabling post run correction [33] . To obtain objective data concerning the quantification of the secreted toxins, we used an end point titer assay and an enzyme immunoassay rather than a manual (subjective) cell scoring system [26] .