Identification of new IS711 insertion sites in Brucella abortus field isolates

Abstract Background Brucellosis is a zoonosis caused by Brucella spp., a group of highly homogeneous bacteria. The insertion sequence IS 711 is characteristic of these bacteria, and occurs in variable numbers and positions, but always constant within a given species. This species-associated polymorphism is used in molecular typing and identification. Field isolates of B. abortus , the most common species infecting cattle, typically carry seven IS 711 copies (one truncated). Thus far, IS 711 transposition has only been shown in vitro and only for B. ovis and B. pinnipedialis , two species carrying a high number of IS 711 copies, but never in other Brucella species, neither in vitro nor in field strains. Results We found several B. abortus strains isolated from milk and aborted fetuses that carried additional IS 711 copies in two hitherto undescribed insertion sites: one in an intergenic region near to the 3' end of a putative lactate permease gene and the other interrupting the sequence of a marR transcriptional regulator gene. Interestingly, the second type of insertion was identified in isolates obtained repeatedly from the same herd after successive brucellosis outbreaks, an observation that proves the stability and virulence of the new genotype under natural conditions. Sequence analyses revealed that the new copies probably resulted from the transposition of a single IS 711 copy common to all Brucella species sequenced so far. Conclusions Our results show that the replicative transposition of IS 711 can occur under field conditions. Therefore, it represents an active mechanism for the emergence of genetic diversity in B. abortus thus contributing to intra-species genetic polymorphism.