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ABSTRACTKingella kingaeis 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. kingaeinfections. However, the present study shows that Kingella negevensis, a Kingellaspecies newly identified in young children, harbors an identical KingellaRTX locus, raising the question of whether K. negevensiscan be misidentified as K. kingaeby clinical microbiology laboratories. In silicocomparison of Kingellasp. RTX and groELgenes and in vitrostudies provided evidence that targeting the rtxAand rtxBgenes could not differentiate between strains of K. kingaeand K. negevensis, whereas targeting the groELgene 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 kngroELqPCR assays. Forty-two specimens were rtxBpositive, including 41 that were also kkgroELpositive and 1 (the remaining one) that was kngroELpositive. Thus, this study discloses an invasive infection caused by K. negevensisin humans and demonstrates that targeting the RTX locus cannot be used for the formal diagnosis of K. kingaeinfections. These findings stress the need for further studies on the epidemiology of asymptomatic carriage and invasive infections caused by K. negevensisin humans.

We report the investigation methods for the diagnosis of an epidemic and culture-negative Kingella kingaeendocarditis complicating a severe outbreak of hand, foot and mouth disease in a childcare center. The diagnosis was confirmed by polymerase chain reaction testing performed from cardiac tissue. Our findings argue for the systematic investigation of K. kingaeoutbreaks by using molecular tools in such context.

Kingella kingaeis a significant pediatric pathogen responsible for bone and joint infections, occult bacteremia, and endocarditis in early childhood. Past efforts to detect this bacterium using culture and broad-range 16S rRNA gene PCR assays from clinical specimens have proven unsatisfactory; therefore, by the late 2000s, these were gradually phased out to explore the benefits of specific real-time PCR tests targeting the groELgene and the RTX locus of K. kingae.