The Burkholderia cepacia complex (BCC) comprises at least nine phenotypically similar but genotypically distinct species (genomovars). Previously recognised as phytopathogens and soil saprophytes, these highly adaptable bacteria are also important human pathogens causing life-threatening pulmonary infections in individuals with cystic fibrosis or chronic granulomatous disease. Despite progress in understanding the epidemiology, taxonomy and genomic plasticity of the B. cepacia complex, the factors responsible for virulence in various hosts, including those encoded or acquired via bacteriophages, remain unclear.
We showed previously that the host range of individual B. cepacia complex phages includes multiple species of the complex, and in the case of phage NS2 extends to Burkholderia pseudomallei and Pseudomonas aeruginosa. This unusually broad host range suggests that the potential for horizontal gene transfer is not confined to the transducing phages NS1 and NS2. Evidence of a prophage element was also noted and its expression confirmed by the isolation of a temperate phage from Burkholderia cenocepacia J2315, which we named DK4. Recently, Summer et al.  described the characterisation, sequencing and genomic organisation of a temperate phage, from B. cenocepacia strain J2315, which they named BcepMu. BcepMu is a Mu-like phage with a genome of 36,748 bp. When DNA from nine isolates, representing each of the known B. cepacia complex species and 33 strains of B. cenocepacia, was screened by PCR using putative BcepMu-specific primers, only B. cenocepacia isolates belonging to the ET12 lineage reacted positively.
In this otherwise excellent study, Summer and colleagues unfortunately did not take account of any previous published data on B. cepacia complex bacteriophages. When DK4 phage DNA was used as the template in PCR reactions with pairs of primers designed by Summer et al. to amplify sequences reported to be specific for BcepMu13, BcepMu17, the PCR product was 100% identical to the BcepMu genome sequence. Amplification of three other genes contained within the BcepMu-specific lysis cassette namely, Holin (BcepMu21), Slt (Soluble lytic transglycosylase, or Endolysin, BcepMu 22) and Rz1 (BcepMu 23) yielded PCR products that showed 100% identity with the BcepMu genome genes. Restriction endonuclease digest analysis of DK4 genomic DNA showed it was cut into fragments of similar size to those predicted for the BcepMu genome sequence (GenBank Accession No. AY539836). Analysis of DK4 phage particles by electron microscopy reveals that, like BcepMu, DK4 has an icosahedral head and contractile tail, which are typical features of myophage. Taken together, these data provide compelling evidence that the temperate phages DK4 and BcepMu isolated from B. cenocepacia J2315 are identical.