Background : Klebsiella pneumoniae is an opportunistic pathogen with multiple antibiotic resistance responsible for hospital-acquired infections consecutively to intestinal microbiota colonization. It is considered as an important threat by the World Health Organization (WHO) and Center for Disease Control and Prevention (CDC). Although surface adhesion processes are well described for this pathogen (through biofilm formation), competitive interactions within complex microbial communities remain poorly understood. Type VI secretion systems (T6SS) are known to be involved in bacterial warfare; they act as molecular syringes injecting toxic effectors into adjacent prokaryotic and/or eukaryotic cells through direct contact. Such a system has been recently described in hypervirulent K. pneumoniae (hvKp) strains but is poorly described in classical clinical strains (cKp).Materials: An in silico approach was used to investigate the prevalence of T6SS and their effector among Klebsiella genus in the KL428 database (comprising all Klebsiella complete genomes in NCBI databank -April 2019-). The cKp CH1157 strain was used as reference to investigate the toxic effect of the T6SS-1 phospholipase (Tle1) in vitro using site-directed E. coli toxicity assays. Finally, colonization assays were performed in streptomycin pre-treated mice to assess the role of T6SS-1 in intestinal tract colonization kinetic and its impact on intestinal microbiota through 16S rRNA high-throughput sequencing.Results/Conclusions: The presence of one to three T6SS related clusters was detected in Klebsiella genus, independently of the origin of the strains (environment, hospital-acquired infections, community-acquired infections, carriage). Over-representation of T6SS clusters and of the tle1 effector gene encoding a phospholipase was found in the K. pneumoniae species. A periplasmic-specific toxic effect of Tle1 phospholipase was shown against E. coli DH5α. T6SS-1 isogenic mutants, deficient for different core genes (tssB or clpV), colonized the mice gastrointestinal tract less efficiently than their parental strain. Moreover, preliminary results indicated that K. pneumoniae T6SS-1 globally modulates the mice intestinal microbiota by limiting its resilience (DESeq2, Metacoder, and MetagenomeSeq). K. pneumoniae T6SS antibacterial activity towards microbiota bugs remains to be determined, but complex regulation of this system is likely to promote stable intestinal implantation of the pathogen.< Réduire