DNA–DNA hybridization (DDH) values have been used by bacterial taxonomists since the 1960s to determine relatedness between strains and are still the most important criterion in the delineation of bacterial species. Since the extent of hybridization between a pair of strains is ultimately governed by their respective genomic sequences, we examined the quantitative relationship between DDH values and genome sequence-derived parameters, such as the average nucleotide identity (ANI) of common genes and the percentage of conserved DNA. A total of 124 DDH values were determined for 28 strains for which genome sequences were available. The strains belong to six important and diverse groups of bacteria for which the intra-group 16S rRNA gene sequence identity was greater than 94 %. The results revealed a close relationship between DDH values and ANI and between DNA–DNA hybridization and the percentage of conserved DNA for each pair of strains. The recommended cut-off point of 70 % DDH for species delineation corresponded to 95 % ANI and 69 % conserved DNA. When the analysis was restricted to the protein-coding portion of the genome, 70 % DDH corresponded to 85 % conserved genes for a pair of strains. These results reveal extensive gene diversity within the current concept of ‘species’. Examination of reciprocal values indicated that the level of experimental error associated with the DDH method is too high to reveal the subtle differences in genome size among the strains sampled. It is concluded that ANI can accurately replace DDH values for strains for which genome sequences are available.

Summary: Members of the genus Burkholderia are versatile organisms that occupy a surprisingly wide range of ecological niches. These bacteria are exploited for biocontrol, bioremediation and plant growth promotion purposes, but safety issues regarding human infections, especially in cystic fibrosis patients, have not been solved. This minireview gives an overview of the taxonomic and ecological diversity of the genus with particular emphasis on strains belonging to the Burkholderia cepacia complex and addresses the important question whether ‘good’ and ‘bad’ strains are actually the same.

Although the exact role of quorum sensing (QS) in various stages of biofilm formation, maturation, and dispersal and in biofilm resistance is not entirely clear, the use of QS inhibitors (QSI) has been proposed as a potential antibiofilm strategy. We have investigated whether QSI enhance the susceptibility of bacterial biofilms to treatment with conventional antimicrobial agents. The QSI used in our study target the acyl-homoserine lactone-based QS system present inPseudomonas aeruginosaandBurkholderia cepaciacomplex organisms (baicalin hydrate, cinnamaldehyde) or the peptide-based system present inStaphylococcus aureus(hamamelitannin). The effect of tobramycin (P. aeruginosa,B. cepaciacomplex) and clindamycin or vancomycin (S. aureus), alone or in combination with QSI, was evaluated in variousin vitroandin vivobiofilm model systems, including two invertebrate models and one mouse pulmonary infection model.In vitrothe combined use of an antibiotic and a QSI generally resulted in increased killing compared to killing by an antibiotic alone, although reductions were strain and model dependent. A significantly higher fraction of infectedGalleria mellonellalarvae andCaenorhabditis eleganssurvived infection following combined treatment, compared to treatment with an antibiotic alone. Finally, the combined use of tobramycin and baicalin hydrate reduced the microbial load in the lungs of BALB/c mice infected withBurkholderia cenocepaciamore than tobramycin treatment alone. Our data suggest that QSI may increase the success of antibiotic treatment by increasing the susceptibility of bacterial biofilms and/or by increasing host survival following infection.