LAUSR

For most of the 20th century, Xanthomonas euvesicatoria (Xe) was the only known bacterium associated with bacterial spot of tomato (BST) in Florida. X. perforans (Xp) quickly replaced Xe mainly because of production of three bacteriocins (BCNs) against Xe; however, Xp outcompeted Xe even when the three known bacteriocins were deleted. Surprisingly, we observed antimicrobial activity against Xe in the BCN triple mutant when the triple mutant was grown in Petri plates containing multiple spots but not in Petri plates containing only one spot. We determined that changes in the headspace composition (i.e., volatiles) rather than a diffusible signal in the agar were required for induction of the antimicrobial activity. Other Xanthomonas species also produced volatile-induced antimicrobial compounds against Xe and elicited antimicrobial activity by Xp. A wide range of plant pathogenic bacteria, including Clavibacter michiganensis subsp. michiganensis, Pantoea stewartii, and Pseudomonas cichorii, also elicited antimicrobial activity by Xp when multiple spots of the species were present. To identify potential antimicrobial compound(s), we performed liquid chromatography-high resolution mass spectrometry of the agar surrounding the spot in the high cell density Petri plates where the antimicrobial was present, compared to agar surrounding the spot in Petri plates with one spot where antimicrobial activity was not observed. Among the compounds identified in the zone of inhibition were N-butanoyl-L-homoserine lactone and N-(3-hydroxy-butanoyl)-homoserine lactone, which are known quorum sensing metabolites in other bacteria.