No abstract

The lack of antagonism seen with tigecycline combinations in both chequerboard and time-kill kinetic studies is an encouraging outcome, suggesting that tigecycline may prove to be effective in combination therapy as well as in monotherapy.

Pseudomonas aeruginosa is a Gram-negative bacterium causing chronic infections in cystic fibrosis patients. Such infections are associated with an active type VI secretion system (T6SS), which consists of about 15 conserved components, including the AAA+ ATPase, ClpV. The T6SS secretes two categories of proteins, VgrG and Hcp. Hcp is structurally similar to a phage tail tube component, whereas VgrG proteins show similarity to the puncturing device at the tip of the phage tube. In P. aeruginosa, three T6SSs are known. The expression of H1-T6SS genes is controlled by the RetS sensor. Here, 10 vgrG genes were identified in the PAO1 genome, among which three are co-regulated with H1-T6SS, namely vgrG1a/b/c. Whereas VgrG1a and VgrG1c were secreted in a ClpV1-dependent manner, secretion of VgrG1b was ClpV1-independent. We show that VgrG1a and VgrG1c form multimers, which confirmed the VgrG model predicting trimers similar to the tail spike. We demonstrate that Hcp1 secretion requires either VgrG1a or VgrG1c, which may act independently to puncture the bacterial envelope and give Hcp1 access to the surface. VgrG1b is not required for Hcp1 secretion. Thus, VgrG1b does not require H1-T6SS for secretion nor does H1-T6SS require VgrG1b for its function. Finally, we show that VgrG proteins are required for secretion of a genuine H1-T6SS substrate, Tse3. Our results demonstrate that VgrG proteins are not only secreted components but are essential for secretion of other T6SS substrates. Overall, we emphasize variability in behavior of three P. aeruginosa VgrGs, suggesting that, although very similar, distinct VgrGs achieve specific functions.

Beneficial microorganisms are widely used in agriculture for control of plant pathogens but a lack of efficacy and safety information has limited the exploitation of multiple promising biopesticides. We applied phylogeny-led genome mining, metabolite analyses and biological control assays to define the efficacy of Burkholderia ambifaria, a naturally beneficial bacterium with proven biocontrol properties, but potential pathogenic risk. A panel of 64 B. ambifaria strains demonstrated significant antimicrobial activity against priority plant pathogens. Genome sequencing, specialized metabolite biosynthetic gene cluster mining and metabolite analysis revealed an armoury of known and unknown pathways within B. ambifaria. The biosynthetic gene cluster responsible for the production of the metabolite, cepacin, was identified and directly shown to mediate protection of germinating crops against Pythium damping-off disease. B. ambifaria maintained biopesticidal protection and overall fitness in soil after deletion of its third replicon, a non-essential plasmid associated with virulence in B. cepacia complex bacteria. Removal of the third replicon reduced B. ambifaria persistence in a murine respiratory infection model. Here we show that by using interdisciplinary phylogenomic, metabolomic and functional approaches, the mode of action of natural biological control agents related to pathogens can be systematically established to facilitate their future exploitation. Main Numerous bacterial and fungal species have been recognised for their biological control abilities and plant growth-enhancing properties. Pesticides conventionally used in agriculture are under increasing scrutiny regarding their bioaccumulation and toxicity, which includes their fatal impact on pollinator species. Concern over chemical pesticides has reinvigorated research into biological control agents and their secreted bioactive compounds as viable natural alternatives for agriculture. One feature common to most biopesticidal species is their ability to secrete antimicrobial compounds into the environment and inhibit pathogenic microbes from causing crop disease. Bacteria within the genus Burkholderia are particularly diverse in their specialized metabolism and have a documented ability to produce a range of potent anti-bacterial, anti-nematodal and anti-fungal compounds 1,2. They have demonstrated excellent promise as biological control agents with multiple strains used commercially as biopesticides until 1999. In common with other biological control genera such as Bacillus, Pseudomonas and Stenotrophomonas, certain Burkholderia species may also cause human, animal and plant infections. Therefore, in 1999 the US Environmental Protection Agency (EPA) placed a moratorium on new registrations of Burkholderia biopesticides unless such agents were defined as safe in terms of their risk of opportunistic infection 2. Multiple species within the Burkholderia cepacia complex group were characterised or used as biological control agents 2. They are highly active ...