Susceptibility of LPS mutants of Actinobacillus pleuropneumoniae to cationic antimicrobial peptides

Actinobacillus pleuropneumoniae is an important pathogen of swine. We previously reported that lipopolysaccharides (LPSs) are involved in the adherence of A. pleuropneumoniae to host respiratory tract cells. Rough LPS and core LPS mutants of A. pleuropneumoniae serotype 1 were generated by using a mini-Tn10 transposon mutagenesis system, and the gene affected by the transposon was identified in each of these mutants. The purpose of the present study was to evaluate the susceptibility of various A. pleuropneumoniae LPS mutants to cationic antimicrobial peptides which are important components of the innate immune response. We determined the minimal inhibitory concentration of the peptides polymyxin B, protamine, cecropin P1, melittin, protegrin-1, and mastoparan. A rough LPS mutant of A. pleuropneumoniae exhibited the same susceptibility to these cationic peptides as that of the wild-type (WT) parent strain 4074 Nal super(r). On the other hand, three core LPS mutants were more susceptible to cationic peptides than the WT strain. Structural analysis of the LPS from all mutants was performed. Our data indicate that an intact outer core is required for optimal protection of A. pleuropneumoniae against the antimicrobial activity of cationic peptides. It would be most interesting to infect pigs experimentally with the core LPS mutants and compare their virulence with that of the WT strain.