Quality Control Strain Campylobacter jejuni ATCC 33560 Contains a Frameshift Mutation in the CmeR Regulator

LETTER

Campylobacter jejuni ATCC 33560 (CCM 6214) is the quality-control (QC) strain for the antimicrobial susceptibility testing of Campylobacter spp. (1, 6). We have utilized the C. jejuni ATCC 33560 strain in antimicrobial susceptibility testing and in studies to explore antimicrobial resistance mechanisms. In our previous study, C. jejuni ATCC 33560 was a parental strain to induce ciprofloxacin-resistant mutants (4). According to sequence analysis, these mutants did not have point mutations in the quinolone resistance-determining region (QRDR) of the gyrA gene or mutations in the promoter region of cmeABC that encodes a multidrug efflux pump. However, the ciprofloxacin MICs were increased (8 to 16 μg/ml [4]; ATCC 33560, 0.12 to 0.5 μg/ml [6]), indicating the presence of unknown mutations.

As the multidrug efflux pump CmeABC is repressed by CmeR (5), we included it in our further characterization of the mutants. Unexpectedly, the cmeR gene of C. jejuni ATCC 33560 parental strain, as well as its resistant mutants, contained a single A nucleotide deletion in the 8-nucleotide A repeat unit (bases 463 to 470 of cmeR, locus tag Cj0368c in NCTC 11168), causing a shift out-of-frame and yielding a truncated form of CmeR (Fig. 1). Since the C-terminal portion of CmeR is essential for dimer formation, it is likely that the truncated CmeR (168 amino acids [aa]) is unable to form a homodimer and to act on the target genes like the full-length CmeR regulator (210 aa) (5). The C-terminal domain of CmeR consists of 6 α-helices (α4 to α10), and according to the crystal structure, helices 6, 8, 9, and 10 are involved in the dimerization that occurs via pairs of helices (α6 and α9′, α8 and α10′) (2). The truncated form of CmeR in ATCC 33560 contains only helix α6 (aa 106 to 118) and part of helix α8 (aa 152 to 170) (Fig. 1). We excluded the possibility that the C. jejuni ATCC 33560 strain utilized in our laboratory had undergone a single-nucleotide deletion and obtained the genomic DNA from the same strain from another laboratory. The sequence analysis of CmeR from another C. jejuni ATCC 33560 isolate verified the same single-nucleotide deletion in the CmeR regulator.

Fig 1.

Amino acid alignment of C-terminal domain (aa 101 to 210) of CmeR proteins of genome-sequenced C. jejuni NCTC 11168 strain and QC strain ATCC 33560. The four helices (α6, α8, α9, and α10) important in dimerization of CmeR homodimer are underlined. The asterisk indicates the stop codon.

Although the out-of-frame deletion, likely causing the nonfunctional CmeR regulator in C. jejuni ATCC 33560, does not affect its use as a QC strain, we would like to inform Campylobacter researchers employing it for other studies, e.g., antibiotic resistance mechanisms. CmeR has also been shown to be a pleiotropic regulator involved in the control of membrane transporters, C₄-dicarboxylate transport and utilization, periplasmic proteins and lipoproteins, capsule biosynthesis, and proteins with unknown function (3). Therefore, if C. jejuni ATCC 33560 is utilized in other studies concerning the above-mentioned proteins and pathways, this fact needs to be considered in the interpretation of the results. However, the use of C. jejuni ATCC 33560 as a QC strain is still applicable in susceptibility testing of antimicrobial agents.

Nucleotide sequence accession number.

The DNA sequence data determined in this report has been submitted to the GenBank/EMBL/DDBJ databases under accession number HE614280.