Fluoroquinolone (FQ)-resistant Campylobacter jejuni is a serious problem worldwide that limits effective treatment of infections. The traditional detection method depends on bacterial isolation and MIC testing, or traditional PCR, which is time-consuming and hard to identify the FQ-resistant C. jejuni in a high abundance wild-type background. This study aimed to develop a rapid and accurate ddPCR assay to detect FQ-resistant C. jejuni mutants based on the crucial resistance mutation C257T (Thr-86-Ile) in gyrA. Our ddPCR gyrA assay showed high specificity and accuracy. Sanger sequencing and the qPCR assay could only recognize gyrA mutant sequences when the ratios of wild-type/mutant were 1:1 or 10:1, respectively. Our ddPCR gyrA assay was able to detect gyrA mutant sequences in the mixtures with up to at least 1000:1 wild-type/mutant ratios, which suggested a significant advantage to distinguish the low mutant signal from the wild-type background. We further monitored the occurrence of gyrA mutations under ciprofloxacin pressure using our ddPCR gyrA assay, and clearly showed that the transition of a dominant C. jejuni subpopulation from wild-type to gyrA C257T mutant, resulting in FQ-resistance. We tested 52 samples from live chickens and retail chicken meat and showed that four samples contained wild-type/mutant mixtures comprising 1.7%, 28.6%, 53.3%, and 87.0% gyrA C257T mutants, respectively. These results demonstrated that the ddPCR gyrA assay was a highly sensitive alternative method to distinguish and quantify FQ-resistant C. jejuni infections that could help guide the appropriate use of FQs in clinical practice. IMPORTANCE Campylobacter jejuni is considered to be the leading cause of human bacterial gastroenteritis worldwide, and fluoroquinolones (FQs) are the main choices for the treatment of bacterial gastroenteritis in clinical practice. In theory, antimicrobial susceptibility testing should help us to choose the most appropriate drugs for the treatment. However, to test the susceptibility of C. jejuni to FQs, the standardized method is bacteria isolation and MIC measurement, which will take more than 4 days. In addition, a low abundance of FQ-resistant C. jejuni is also hardly distinguished from a high abundance of wild-type background in the mixed infection. Therefore, the development of rapid and accurate detection technology for FQ-resistant C. jejuni is very important. This study provided a ddPCR gyrA assay, which is a highly sensitive alternative method to distinguish and quantify FQ-resistant C. jejuni infections that may help guide the appropriate use of FQs both in veterinary and human clinical practice.
Keywords: FQ-resistant Campylobacter jejuni; ddPCR; detection; gyrA mutant.