In vitro high-throughput screening of the antimicrobial activity of different compounds against Xylella fastidiosa subsp. pauca

The worldwide distribution and the host range of the phytopathogenic bacterium Xylella fastidiosa (Xf) have significantly changed in the last decade with numerous outbreaks reported in the Old Continent. Among the different European isolates, those of the subspecies pauca have been ranked as highly pathogenic, being the causal agents of the olive quick decline decimating olive trees in southern Italy. Significant research investments have been devoted towards finding therapeutic approaches to mitigate the impact of the infections in highly susceptible host species. This study aimed to evaluate in vitro efficacy against Xylella fastidiosa subsp. pauca (Xfp) of different classes of products, including metal ions, micronutrients, antibiotics, and phenolic compounds. The slow and fastidious growth of the bacterium requires optimization of specific protocols to assess antibacterial activities and the effect on biofilm formation. The results showed a dose–response effect against Xf for most products. Notably, among micronutrients and phenolic compounds, CuSO4·5H2O, Dentamet®, pyrocatechol and 4-methylcatechol showed the highest bactericidal and antibiofilm activity. At the same time, antibiotics demonstrated substantial bacteriostatic activity effectively inhibiting biofilm formation. For metal ions, such as CoCl2, K2B4O7·4H2O and MnSO4·H2O, significant effects on bacterial cell viability were recorded but were not able to completely kill the bacterium. Regarding the antibiofilm activity, some of them were able to inhibit biofilm formation, while others increased its formation. Ca(NO3)2·4H2O and Na2MoO4 were found to promote the growth of Xf. The methodologies described proved to be useful for profiling the antimicrobial activity of a large panel of compounds and the data collected provide evidence of their in vitro effectiveness.