Bacitracins
are a mixture of structurally related cyclic polypeptides with clinically
useful antibiotic properties. They act by indirectly interfering with the
biosynthesis of the bacterial cell wall. In this study, we analysed an avian
necrotic enteritis strain of Clostridium perfringens that was resistant
to bacitracin. We identified a putative bacitracin resistance locus that
resembles a bacitracin resistance determinant from Enterococcus faecalis.
It contained the structural genes bcrABD and a putative regulatory gene,
bcrR. Mutagenesis studies showed that both bcrA and bcrB
were essential for bacitracin resistance. The introduction of shuttle plasmids
carrying bcrA, bcrB, bcrAB or bcrRAB into a bacitracin
susceptible C. perfringens strain confirmed that both bcrA and bcrB
were required for bacitracin resistance. Analysis of the plasmid DNA content of
the parent strain showed that it contained at least three large conjugative
plasmids and that the bcrRABD locus was located on an 89 kb conjugative
plasmid. This plasmid was sequenced and shown to be closely related to members
of the conjugative antibiotic resistance and toxin plasmid family from C. perfringens.
Our analysis revealed that the bcr genes were located on a potential
mobile element (Tn4460) that was related to the Tn916 family of
conjugative transposons and encoded homologues of the Tn916 gene
products that are responsible for recombination, regulation and conjugation.
Subsequent conjugation experiments suggested that the Tn4460 bacitracin
resistance element had the ability to transpose onto both conjugative and
non-conjugative plasmids or onto the chromosome. In summary, we have identified
and characterised a novel mobile bacitracin resistance determinant in a
toxin-producing C. perfringens strain, the first such element to be
identified in this bacterium.