Novel genes associated with biofilm formation of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae is a gram-negative bacterium and is the causative agent of swine pleuropneumonia, a highly contagious respiratory disease. Biofilm formation is an important ability possessed by numerous bacterial pathogens. The purpose of this study was to identify and characterize biofilm mutants of A. pleuropneumoniae serotype 1 strain S4074 created using a mini Tn-10 transposon. The transposon library was screened to identify mutants with a modified ability to form biofilms in polystyrene microtiter plates. A total of 1200 mutants were screened and the analysis identified 24 mutants that exhibited abnormal biofilm formation, at least 16 unique genes were identified. Most genes identified in the enhanced-biofilm mutants encoded proteins with unknown functions, whereas most genes identified in the biofilm-reduced mutants encoded proteins related to transport, protein synthesis and nucleic acid synthesis. Approximately 50% of genes, including hns, potD2, ptsI, tig and rpmF, identified in our screen have been previously associated with biofilm formation in A. pleuropneumoniae and other bacterial species, and thus validated the screening method. The rest of genes identified, such as APL_0049, APL_0637 and APL_1572, have not been previously associated with biofilm formation. Interestingly, gene APL_0049 was previously seen among the genes differentially expressed during a natural infection of pig lungs. Preliminary characterization of the mutants was also initiated by assessing their hydrophobicity, their biofilm matrix composition and their ability to adhere to a polystyrene surface or NPTr cells. Based on the preliminary characterization, some of the mutants identified appear to have deficiencies during the initial attachment or growth of the biofilm. In conclusion, transposon mutagenesis analysis allowed the identification of new genes associated with biofilm formation in A. pleuropneumoniae.