Exceeding the noise level could be detected even in the wild type strain. This was not unexpected, because we had observed previously that also E. coli MG1655 does not produce detectable MliC levels, and because mliC is poorly expressed under normal laboratory order 863405-60-1 growth conditions. All knock-outs caused a considerable reduction of the periplasmic lysozyme inhibitory activity, but the residual activities varied. Knock-out of pliG reduced g-type inhibitory activity in the periplasmic extract to a background level, while the c-type inhibitory activity was only partly reduced in the ivy knock-out. The latter was unexpected, because Ivy is the only known periplasmic lysozyme inhibitor in E. coli, and knock-out of ivy completely eliminated c-type lysozyme inhibitory activity in periplasmic extracts of E. coli MG1655. Finally, double knock-out of ivy and mliC also reduced c-type inhibitory activity in the periplasmic extracts, but somewhat less than the individual ivy knock-out. Complementation of each knock-out strain with the corresponding gene or genes increased the inhibitory activity back to the wild-type level. All the constructed knock-out mutants and genetically complemented mutants showed growth curves in LB broth at 37uC that were undistinguishable from the parental APEC CH2 growth curve. As a first approach to assess the virulence of the different APEC inhibitor knock-outs, a serum sensitivity test was conducted. For many pathogens including APEC, the ability of bacteria to survive and grow in blood serum is a prerequisite for virulence and has proven useful in discriminating virulent and avirulent isolates. Serum resistance depends on the ability of the bacteria to overcome the combined antibacterial effectors that are present in serum. One of the most powerful effectors is the multimolecular attack complex formed by the complement system, but there is also a contribution of several antibacterial peptides and proteins with DCVC (E-isomer) supplier specific modes of action, such as lysozyme. The results of the serum resistance tests are shown in Table 3. All strains including E. coli BL21, which was included as a serum-sensitive control, showed strong growth in serum that had been heat-treated to inactivate the complement system. In untreated serum, growth of BL21 and APEC CH2 was re
