Molecular Tests That Target the RTX Locus Do Not Distinguish between Kingella kingae and the Recently Described...

Kingella kingae is an important invasive pathogen in early childhood. The organism elaborates an RTX toxin presumably restricted to this species. Consequently, real-time quantitative PCR (qPCR) assays targeting the RTX locus have been developed in recent years and are gaining increasing use for the molecular diagnosis of K. kingae infections. However, the present study shows that Kingella negevensis, a Kingella species newly identified in young children, harbors an identical Kingella RTX locus, raising the question of whether K. negevensis can be misidentified as K. kingae by clinical microbiology laboratories. In silico comparison of Kingella sp. RTX and groEL genes and in vitro studies provided evidence that targeting the rtxA and rtxB genes could not differentiate between strains of K. kingae and K. negevensis, whereas targeting the groEL gene could. This prompted the design of a highly specific and sensitive qPCR assay targeting K. negevensis groEL (kngroEL). Ninety-nine culture-negative osteoarticular specimens from 99 children younger than 4 years of age were tested with a conventional 16S rRNA gene-based broad-range PCR assay and Kingella-specific rtxB, K. kingae-specific groEL (kkgroEL), and kngroEL qPCR assays. Forty-two specimens were rtxB positive, including 41 that were also kkgroEL positive and 1 (the remaining one) that was kngroEL positive. Thus, this study discloses an invasive infection caused by K. negevensis in humans and demonstrates that targeting the RTX locus cannot be used for the formal diagnosis of K. kingae infections. These findings stress the need for further studies on the epidemiology of asymptomatic carriage and invasive infections caused by K. negevensis in humans.