Comprehensive genomic analysis of type VI secretion system diversity and associated proteins in Serratia

The type VI secretion system (T6SS) is a contact-dependent contractile nanomachine widely distributed among Gram-negative bacteria, essential for interbacterial competition, host interactions and environmental adaptation. The T6SS encoded by Serratia is known to play roles in bacterial competition and contribute to niche adaptation. Yet, the diversity and distribution of T6SS loci and relevant proteins across different Serratia species remain underexplored. In this study, we conducted bioinformatic and comparative genomic analyses of the T6SS to expand our understanding of T6SS in Serratia and their distribution within the genus. A dataset comprising 2,337 Serratia genomes was analysed, identifying 4 distinct T6SS loci classified into 3 main types based on TssB sequence variation, with varying distributions across specific Serratia haplogroups. T6SS^serratia-1b subtype was found to be the most prevalent. By integrating homology searches and positional information, we expanded the known database of T6SS proteins in Serratia, identifying a range of effectors likely involved in interbacterial interactions and host adaptation. Additionally, statistical analysis reveals a strong correlation between the presence of T6SS and the diversity of associated genes, including not only known T6SS effectors and immunity proteins but also other co-occurring gene families, enabling the identification of multiple candidate loci potentially involved in bacterial competition and pathogenicity. These findings shed light on the complex distribution, evolutionary dynamics and functional diversity of T6SS and associated proteins in Serratia, advancing our understanding of their role in bacterial competition and environmental adaptation.