<div><p>In the respiratory tract and lung tissue, a balanced physiological response is essential for <em>Actinobacillus pleuropneumoniae</em> to survive various types of challenges. ClpP, the catalytic core of the Clp proteolytic complex, is involved in various stresses response and regulation of biofilm formation in many pathogenic bacteria. To investigate the role of ClpP in the virulence of <em>A. pleuropneumoniae</em>, the <em>clpP</em> gene was deleted by homologous recombination, resulting in the mutant strain S8<em>ΔclpP</em>. The reduced growth of S8<em>ΔclpP</em> mutant at high temperatures and under several other stress conditions suggests that the ClpP protein is required for the stress tolerance of <em>A. pleuropneumoniae</em>. Interestingly, we observed that the S8<em>ΔclpP</em> mutant exhibited an increased ability to take up iron in vitro compared to the wild-type strain. We also found that the cells without ClpP displayed rough and irregular surfaces and increased cell volume relative to the wild-type strain using scanning electron microscopy (SEM). Confocal laser scanning microscopy (CLSM) revealed that the S8<em>ΔclpP</em> mutant showed decreased biofilm formation compared to the wild-type strain. We examined the transcriptional profiles of the wild type S8 and the S8<em>ΔclpP</em> mutant strains of <em>A. pleuropneumoniae</em> using RNA sequencing. Our analysis revealed that the expression of 16 genes was changed by the deletion of the <em>clpP</em> gene. The data presented in this study illustrate the important role of ClpP protease in the stress response, iron acquisition, cell morphology and biofilm formation related to <em>A. pleuropneumoniae</em> and further suggest a putative role of ClpP protease in virulence regulation.</p> </div
