*2.4. Analysis of Genetic Diversity*

To analyze the genetic diversity, 45 *Campylobacter* isolates from poultry were streaked on Colombia agar (bioMérieux, Marcy-l'Étoile, France), followed by microaerobic incubation at 42 ◦C for 48 h. DNA was extracted from *Campylobacter* isolates using a commercial kit (UltraCleanTM Microbial DNA Isolation Kit, MoBio Laboratories, Solana Beach, CA, USA). The extracted DNA was amplified using DiversiLab *Campylobacter* Kit (bioMérieux, Marcy-l'Étoile, France). The amplified products were separated by electrophoresis on microfluidics chips (Agilent Technologies, Palo Alto, CA, USA) and analyzed with the Agilent 2100 Bioanalyzer (Agilent Technologies, Palo Alto, CA, USA). The peak and band data were analyzed by DiversiLabTM software version 2.1.66 (bioMérieux, Marcy-l'Étoile, France) using Pearson's correlation coefficient and unweighted pair group method with the arithmetic mean, followed by dendrogram generation. The cutoff value was 95% for determining genetic similarity [33,34].

#### *2.5. Analysis of Cytolethal Distending Toxin Genes*

To observe the presence of *cdt* genes (*cdtA*, *cdtB*, and *cdtC*) from isolates, the extracted DNA was amplified using the primers listed in Table 2 [14]. The PCR products were visualized by gel electrophoresis and UV-transillumination.


**Table 2.** PCR primers and amplification conditions used to analysis of *cdt* genes for *Campylobacter* isolates.

(1) Amplification: denaturation-annealing-extension.

#### *2.6. Statistical Analysis*

The data for the prevalence and contamination levels of *Campylobacter* between chicken and duck were statistically analyzed by SAS version 9.3 (SAS Institute Inc., Cary, NC, USA), and Chi-square test and *t*-test were used for prevalence and contamination levels, respectively, to determine significance at α = 0.05.
