The absence of intervening sequences in 23S rRNA genes of Campylobacter coli isolates from Turkeys is a unique attribute of a cluster of related strains which also lack resistance to erythromycin

Abstract

Certain Campylobacter strains harbor a transcribed intervening sequence (IVS) in their 23S rRNA genes. Following transcription, the IVS is excised, leading to fragmentation of the 23S rRNA. The origin and possible functions of the IVS are unknown. Furthermore, the distribution of IVS-harboring strains within Campylobacter populations is poorly understood. In this study, 104 strains of Campylobacter coli from turkeys, representing 27 different multilocus sequence typing-based sequence types (STs), were characterized in terms of IVS content and erythromycin susceptibility. Sixty-nine strains harbored IVSs in all three 23S rRNA genes, whereas the other 35 strains lacked IVSs from at least one of the genes. The STs of the latter strains belonged to an unusual cluster of C. coli STs (cluster II), earlier found primarily in turkey strains and characterized by the presence of the C. jejuni aspA103 allele. The majority (66/69) of strains harboring IVSs in all three 23S rRNA genes were resistant to erythromycin, whereas none of the 35 strains with at least one IVS-free 23S rRNA gene were resistant. Cluster II strains could be transformed to erythromycin resistance with genomic DNA from C. coli that harbored IVS and the A2075G transition in the 23S rRNA gene, associated with resistance to erythromycin in Campylobacter. Erythromycin-resistant transformants harbored both the A2075 transition and IVS. The findings suggest that the absence of IVS in C. coli from turkeys is characteristic of a unique clonal group of erythromycin-susceptible strains and that IVS can be acquired by these strains via natural transformation to erythromycin resistance.

FIG. 1.

Detection of IVS in C. coli strains from turkeys. Primers 1162 and CG1425 were used, as described in Materials and Methods. (A) Agarose gel showing sizes of PCR products from genomic DNA of C. coli strains 6818 (lane 1; ST-1101), 7080 (lane 2; ST-1175), 6667 (lane 3; ST-1161), 7755 (lane 4; ST-1150), and 6017 (lane 5; ST-1150). Lane M is a 100-bp ladder molecular weight marker (exACTGene cloning DNA ladder; Fisher); the location of the 500-bp marker band is indicated by the arrow. Lane 5 shows an example of a strain yielding both 310-bp and 167-bp amplicons. (B) Agarose gel showing PCR products from genomic DNA of C. coli strain 6979 (lane 1; ST-1150) and from genomic DNA of individual colonies of erythromycin-resistant strain 6979 transformants (lanes 2 to 5). Lane M is a 100-bp ladder molecular weight marker, as described above.

FIG. 2.

23S rRNA gene sequences in the IVS region. The locations and sequences of IVS are shown for C. jejuni NCTC 11168 (11168), C. coli RM2228 (RM2228), C. coli 6818 (6818), C. coli 6979 (6979), C. coli strain 6979 Em^r transformant (6979T), and C. coli strain 6979 Em^r spontaneous mutant (6979mut). Coordinates correspond to nt 1180 to nt 1195 of the 23S rRNA gene of C. jejuni NCTC 11168.

FIG. 3.

Detection of the A2075G transition in the 23S rRNA gene of Campylobacter. Sequences are from C. jejuni NCTC 11168 (11168), C. coli RM2228 (RM2228), C. coli 6818 (6818), C. coli 6979 (6979), C. coli strain 6979 Em^r transformant (6979T), and C. coli strain 6979 Em^r spontaneous mutant (6979mut). The A→G transition is indicated by the arrow.