Type VI secretion systems (T6SSs) are a class of macromolecular machines that are
recognized as an important virulence mechanism in several Gram-negative bacteria. The
genome of Pantoea ananatis LMG 2665T , a pathogen of pineapple fruit and onion plants, carries two gene clusters whose predicted products have homology with T6SS-associated gene products from other bacteria. Nothing is known regarding the role of these T6SS-1 and T6SS-3 gene clusters in the biology of P. ananatis. Here, we present evidence that T6SS-1
plays an important role in the pathogenicity of P. ananatis LMG 2665T in onion plants, while
a strain lacking T6SS-3 remains as pathogenic as the wild-type strain. We also investigated the role of the T6SS-1 system in bacterial competition, the results of which indicated that
several bacteria compete less efficiently against wild-type LMG 2665T than a strain lacking
T6SS-1. Additionally, we demonstrated that these phenotypes of strain LMG 2665T were reliant on the core T6SS products TssA and TssD (Hcp), thus indicating that the T6SS-1 gene cluster encodes a functioning T6SS. Collectively, our data provides the first evidence
demonstrating that the T6SS-1 system is a virulence determinant of P. ananatis LMG 2665T and plays a role in bacterial competition.