Antibacterial Activity of Polyhexamethylene Guanidine Against Xanthomonas citri pv. citri

The objective of this study is to clarify the in vitro antibacterial activity of polyhexamethylene guanidine (PHMG) against Xanthomonas citri pv. citri (Xcc), a major agricultural pathogenic bacterium that causes citrus canker, and to evaluate the field control effect of PHMG on citrus canker.

When determining the in vitro antibacterial activity, the turbidity method was used to monitor the changes in the turbidity of the bacterial solution, so as to quantify the inhibitory degree of PHMG on the growth of Xcc. By inoculating the leaves to simulate the infection of citrus leaves by Xcc, whether PHMG could block the invasion of Xcc was observed and whether it had the effect of preventing and treating citrus canker was explored. The field experiment was carried out when the autumn new shoots were about to sprout, in an environment that was closest to the actual production problems, to test the actual effect of PHMG in preventing and treating citrus canker. Microscopic observation instruments such as scanning electron microscopy and fluorescence microscopy were used to show the surface and morphological changes of Xcc after the action of PHMG; PI staining was used to visualize the process of cell apoptosis; the crystal violet staining experiment was used to reveal the formation of biofilms of Xcc. Through these experiments, the effects of PHMG on the cells of Xcc were explored.

PHMG showed broad-spectrum antibacterial properties and had good in vitro inhibitory effects on five plant pathogenic bacteria, including Xcc. The EC₅₀ values were approximately 0.5 μg·mL^-1, indicating that a low concentration of PHMG could inhibit the growth of pathogenic bacteria. The EC₅₀ values of PHMG against 51 strains of Xcc ranged from 0.19 to 0.69 μg·mL^-1, and the in vitro antibacterial activity was relatively stable and reliable. The results of leaf inoculation showed that PHMG could protect citrus leaves from Xcc and had a therapeutic effect. The results of field trials showed that spraying PHMG (0.5 mg·mL^-1) had a control effect of about 60% on citrus canker, which was slightly lower than that of the control fungicides, kasugamycin-thiodiazole-copper and kasugamycin-oxine-copper. Furthermore, PHMG not only caused depressions on the surface of Xcc and abnormal cell size, but also promoted the apoptosis of Xcc, inhibited the formation of cell biofilm, and reduced the motility of Xcc.

PHMG has excellent in vitro antibacterial activity against Xcc, and can effectively reduce the occurrence of citrus canker in the field in practical applications. In addition, PHMG affected the cell morphology, cell apoptosis, biofilm formation and motility of Xcc. The results of this study will provide important theoretical guidance for the subsequent application of PHMG for citrus canker and its expansion to other bacterial diseases.