Type III Secretion System and Virulence Markers Highlight Similarities and Differences between Human- and Plant-Associated Pseudomonads Related to Pseudomonas fluorescens and P. putida

Abstract

ABSTRACTPseudomonas fluorescensis commonly considered a saprophytic rhizobacterium devoid of pathogenic potential. Nevertheless, the recurrent isolation of strains from clinical human cases could indicate the emergence of novel strains originating from the rhizosphere reservoir, which could be particularly resistant to the immune system and clinical treatment. The importance of type three secretion systems (T3SSs) in the relatedPseudomonas aeruginosanosocomial species and the occurrence of this secretion system in plant-associatedP. fluorescensraise the question of whether clinical isolates may also harbor T3SSs. In this study, isolates associated with clinical infections and identified in hospitals as belonging toP. fluorescenswere compared with fluorescent pseudomonads harboring T3SSs isolated from plants. Bacterial isolates were tested for (i) their genetic relationships based on their 16S rRNA phylogeny, (ii) the presence of T3SS genes by PCR, and (iii) their infectious potential on animals and plants under environmental or physiological temperature conditions. Two groups of bacteria were delineated among the clinical isolates. The first group encompassed thermotolerant (41°C) isolates from patients suffering from blood infections; these isolates were finally found to not belong toP. fluorescensbut were closely related and harbored highly conserved T3SS genes belonging to the Ysc-T3SS family, like the T3SSs fromP. aeruginosa. The second group encompassed isolates from patients suffering from cystic fibrosis; these isolates belonged toP. fluorescensand harbored T3SS genes belonging to the Hrp1-T3SS family found commonly in plant-associatedP. fluorescens.