Abstract
The genetic basis of macrolide resistance in Campylobacter coli (n = 17) and C. jejuni (n = 35) isolates previously subjected to in vivo selective pressure was investigated to determine if the number of copies of 23S rRNA genes with macrolide-associated mutations affects the minimum inhibitory concentration (MIC) of macrolides. Sequence data for domain V of the 23S rRNA gene revealed that two macrolide-resistant C. coli isolates had adenine→guanine transitions at position 2059 (A2059G, Escherichia coli numbering). One of the two isolates had the A2059G transition in only two of the three gene copies. Among the macrolide-resistant C. jejuni isolates (n = 9), two different point mutations within domain V were observed. Three macrolide-resistant C. jejuni isolates had A2059G transitions. One of these three C. jejuni isolates had the A2059G transition in only two of the three gene copies. Six macrolide-resistant C. jejuni isolates had an adenine→cytosine transversion at position 2058 (A2058C, E. coli numbering) in all three copies of the 23S rRNA gene. Campylobacter jejuni isolates with the A2058C transversion had higher erythromycin MICs (>256 μg/mL) compared to C. jejuni isolates with A2059G transitions (64–128 μg/mL). In addition, the C. jejuni and C. coli isolates with only two copies of the 23S rRNA gene having A2059G substitutions had lower macrolide MICs compared to isolates with all three copies of the gene mutated. No isolates were observed having only one copy of the 23S rRNA gene with a mutation. Sequence analysis of ribosomal proteins L4 (rplD) and L22 (rplV) indicated that ribosomal protein modifications did not contribute to macrolide resistance among the collection of Campylobacter examined.