Burkholderia Type VI Secretion Systems Have Distinct Roles in Eukaryotic and Bacterial Cell Interactions

Bacteria that live in the environment have evolved pathways specialized to defend against eukaryotic organisms or other bacteria. In this manuscript, we systematically examined the role of the five type VI secretion systems (T6SSs) of Burkholderia thailandensis (B. thai) in eukaryotic and bacterial cell interactions. Consistent with phylogenetic analyses comparing the distribution of the B. thai T6SSs with well-characterized bacterial and eukaryotic cell-targeting T6SSs, we found that T6SS-5 plays a critical role in the virulence of the organism in a murine melioidosis model, while a strain lacking the other four T6SSs remained as virulent as the wild-type. The function of T6SS-5 appeared to be specialized to the host and not related to an in vivo growth defect, as ΔT6SS-5 was fully virulent in mice lacking MyD88. Next we probed the role of the five systems in interbacterial interactions. From a group of 31 diverse bacteria, we identified several organisms that competed less effectively against wild-type B. thai than a strain lacking T6SS-1 function. Inactivation of T6SS-1 renders B. thai greatly more susceptible to cell contact-induced stasis by Pseudomonas putida, Pseudomonas fluorescens and Serratia proteamaculans—leaving it 100- to 1000-fold less fit than the wild-type in competition experiments with these organisms. Flow cell biofilm assays showed that T6S-dependent interbacterial interactions are likely relevant in the environment. B. thai cells lacking T6SS-1 were rapidly displaced in mixed biofilms with P. putida, whereas wild-type cells persisted and overran the competitor. Our data show that T6SSs within a single organism can have distinct functions in eukaryotic versus bacterial cell interactions. These systems are likely to be a decisive factor in the survival of bacterial cells of one species in intimate association with those of another, such as in polymicrobial communities present both in the environment and in many infections

Antibacterial effects of nitric oxide on uropathogenic Escherichia coli during bladder epithelial cell colonization—a comparison with nitrofurantoin

Uropathogenic Escherichia coli (UPEC) is the predominant causative organism of urinary tract infections (UTI) with a high recurrence rate.1 Recurrence of UTI may involve intracellular localization of bacterial colonies within the bladder mucosa, a process that could benefit the bacteria in terms of protection against antibiotics and host immune cells.2, 3 Once internalized, UPEC may multiply and form intracellular bacterial communities with biofilm-like properties4 and/or enter a non-replicating stable and quiescent state that may serve as a source for recurrent UTI.2, 5, 6 A wide range of antimicrobial agents is used for the treatment of UTI but many antibiotics are unable to penetrate biofilm matrix or inhibit bacteria in a metabolically quiescent state. A recent study demonstrated that of seven different functional classes of antibiotics only a few, including nitrofurantoin and some fluoroquinolones, were able to eliminate internalized UPEC within bladder epithelial cells. Nitric oxide (NO) is a small hydrophobic molecule with antibacterial properties that readily diffuses through lipid bilayer membranes. During infection various host cells produce NO enzymatically from inducible nitric oxide synthase and NO has a key role in the innate immune response. It has been shown previously that NO has antibacterial activity against UPEC isolates, including multidrug-resistant extended spectrum beta-lactamase-producing isolates. Although NO can interact directly with bacteria, it can also be oxidized to reactive nitrogen species.</p