A rapid method to identify <i>Salmonella enterica</i> serovar Gallinarum biovar Pullorum using a specific target gene <i>ipaJ</i>

<i>Salmonella enterica</i> serovar Gallinarum biovar Pullorum (<i>S.</i> Pullorum) is the pathogen of pullorum disease, which leads to severe economic losses in many developing countries. Traditional methods to identify <i>S. enterica</i> have relied on biochemical reactions and serotyping, which are time-consuming with accurate identification if properly carried out. In this study, we developed a rapid polymerase chain reaction (PCR) method targeting the specific gene <i>ipaJ</i> to detect <i>S.</i> Pullorum. Among the 650 <i>S.</i> Pullorum strains isolated from 1962 to 2016 all over China, 644 strains were identified to harbour <i>ipaJ</i> gene in the plasmid pSPI12, accounting for a detection rate of 99.08%. Six strains were <i>ipaJ</i> negative because pSPI12 was not found in these strains according to whole genome sequencing results. There was no cross-reaction with other <i>Salmonella</i> serotypes, including <i>Salmonella enterica</i> serovar Gallinarum biovar Gallinarum (<i>S</i>. Gallinarum), which show a close genetic relationship with <i>S.</i> Pullorum. This shows that the PCR method could distinguish <i>S.</i> Gallinarum from <i>S.</i> Pullorum in one-step PCR without complicated biochemical identification. The limit of detection of this PCR method was as low as 90 fg/μl or 10² CFU, which shows a high sensitivity. Moreover, this method was applied to identify <i>Salmonella</i> isolated from the chicken farm and the results were consistent with what we obtained from biochemical reactions and serotyping. Together, all the results demonstrated that this one-step PCR method is simple and feasible to efficiently identify <i>S.</i> Pullorum.