The antimicrobial peptide MPX kills Actinobacillus pleuropneumoniae and reduces its pathogenicity in mice.

Actinobacillus pleuropneumoniae is the causative agent of highly contagious and fatal respiratory infections, causing substantial economic losses to the global pig industry. Due to increased antibiotic resistance, there is an urgent need to find new antibiotic alternatives for treating A. pleuropneumoniae infections. MPX is obtained from wasp venom and has a killing effect on various bacteria. This study found that MPX had a good killing effect on A. pleuropneumoniae and that the minimum inhibitory concentration (MIC) was 16 µg/mL. The bacterial density of A. pleuropneumoniae decreased 1000 times after MPX (1 × MIC) treatment for 1 h, and the antibacterial activity was not affected by pH or temperature. Fluorescence microscopy showed that MPX (1 × MIC) destroyed the bacterial cell membrane after treatment for 0.5 h, increasing membrane permeability and releasing bacterial proteins and Ca2+, Na+ and other cations. In addition, MPX (1 × MIC) treatment significantly reduced the formation of bacterial biofilms. Quantitative RT-PCR results showed that MPX treatment significantly upregulated the expression of the PurC virulence gene and downregulated that of ApxI, ApxII, and Apa1. In addition, the Sap A gene was found to play an important role in the tolerance of A. pleuropneumoniae to antimicrobial peptides. Therapeutic evaluation in a murine model showed that MPX protects mice from a lethal dose of A. pleuropneumoniae and relieves lung inflammation. This study reports the use of MPX to treat A. pleuropneumonia infections, laying the foundation for the development of new drugs for bacterial infections.

Thymol kills bacteria, reduces biofilm formation, and protects mice against a fatal infection of Actinobacillus pleuropneumoniae strain L20.

Actinobacillus pleuropneumoniae is the causative agent of the highly contagious and deadly respiratory infection porcine pleuropneumonia, resulting in serious losses to the pig industry worldwide. Alternative to antibiotics are urgently needed due to the serious increase in antimicrobial resistance. Thymol is a monoterpene phenol and efficiently kills a variety of bacteria. This study found that thymol has strong bactericidal effects on the A. pleuropneumoniae 5b serotype strain, an epidemic strain in China. Sterilization occurred rapidly, and the minimum inhibitory concentration (MIC) is 31.25µg/mL; the A. pleuropneumoniae density was reduced 1000 times within 10min following treatment with 1 MIC. Transmission electron microscopy (TEM) analysis revealed that thymol could rapidly disrupt the cell walls and cell membranes of A. pleuropneumoniae, causing leakage of cell contents and cell death. In addition, treatment with thymol at 0.5 MIC significantly reduced the biofilm formation of A. pleuropneumoniae. Quantitative RT-PCR results indicated that thymol treatment significantly increased the expression of the virulence genes purC, tbpB1 and clpP and down-regulated ApxI, ApxII and Apa1 expression in A. pleuropneumoniae. Therapeutic analysis of a murine model showed that thymol (20mg/kg) protected mice from a lethal dose of A. pleuropneumoniae, attenuated lung pathological lesions. This study is the first to report the use of thymol to treat A. pleuropneumoniae infection, establishing a foundation for the development of new antimicrobials.