Sulfonamide resistance in clinical isolates of Campylobacter jejuni: Mutational changes in the chromosomal dihydropteroate synthase

The characterization of the genetic basis of sulfonamide resistance in Camp ylobacter jejuni was attempted. The resistance determinant from a sulfonami de-resistant strain of C. jejuni was cloned and was found to show 42% ident ity with the folP gene (which codes for dihydropteroate synthase, the targe t of sulfonamides) of the related bacterium Helicobacter pyloni, The sequen ces of the areas surrounding the folP gene in C. jejuni showed similarity t o those of the areas surrounding the corresponding gene in H,pylori. The fo lP gene of C. jejuni, which mediates the resistance, was observed to show p articular features when it was compared to other known folP genes. One of t hese features is the presence of two pairs of direct repeats (15 and 27 bp) within the coding sequence of the gene. Comparison of the C. jejuni folP g enes that mediate susceptibility and resistance revealed the occurrence of mutations that changed four amino acid residues, Resistance of C. jejuni to sulfonamides could be associated with one or several of these four mutatio nal substitutions, which all occurred in the five different resistant isola tes studied. The codon for one of these changed amino acids was found to be located in the second direct repeat within the coding sequence of the gene . The change made the repeat perfect. The transformation of both the resist ance and the susceptibility variants of the gene into an Escherichia coli f olP knockout mutant was found to complement the dihydropteroate synthase de ficiency, confirming that the characterized sulfonamide resistance determin ant codes for the C. jejuni dihydropteroate synthase enzyme. Kinetic measur ements established different affinities of sulfonamide for the dihydroptero ate synthase enzyme isolated from the resistant and susceptible strains. In conclusion, sulfonamide resistance in C. jejuni was shown to be associated with mutational changes in the chromosomally located gene for dihydroptero ate synthase, the target of sulfonamides.