Bacterial Interactions in Xanthomonas Disease Complexes: From Synergism to Antagonism

Xanthomonas bacteria cause severe economic losses in major crops, threatening global food security. While xanthomonads exhibit host specificity and can act independently, they frequently participate in polymicrobial infections, forming consortia with other bacteria. This review examines Xanthomonas-bacteria interactions within disease complexes, focusing on synergistic and antagonistic relationships. Synergistic relationships commonly involve non-pathogenic microorganisms that help Xanthomonas form robust biofilms, horizontal gene transfer that creates resistant strains, and pathogenic microorganisms that break plant defenses to aid tissue invasion. Antagonistic relationships involve competition for space and nutrients with other pathogens. Additionally, numerous reports describe environmental bacteria that secrete enzymes capable of degrading Xanthomonas diffusible signal factors (DSFs), as well as specific antibacterial metabolites and toxic effectors (such as those secreted via the Type VI secretion system) that target xanthomonads. Some of these antagonistic strains have been screened as biocontrol agents. Many studies merely identify the bacterial genera in infected tissues without mechanistic investigation. Critical knowledge gaps include the absence of transcriptomic analyses during Xanthomonas co-infection and limited understanding of molecular mediators governing interbacterial interactions. To translate research into management approaches, future progress requires systematically mapping environmental and host determinants that shape microbial communities and elucidating the antibacterial mechanisms of antagonistic strains.