Abstract
The Gram-negative bacterium Kingella kingae is part of the commensal oropharyngeal flora of young children. As detection methods have improved, K. kingae has been increasingly recognized as an emerging invasive pathogen that frequently causes skeletal system infections, bacteremia, and severe forms of infective endocarditis. K. kingae secretes an RtxA cytotoxin, which is involved in the development of clinical infection and belongs to an ever-growing family of cytolytic RTX (Repeats in ToXin) toxins secreted by Gram-negative pathogens. All RTX cytolysins share several characteristic structural features: (i) a hydrophobic pore-forming domain in the N-terminal part of the molecule; (ii) an acylated segment where the activation of the inactive protoxin to the toxin occurs by a co-expressed toxin-activating acyltransferase; (iii) a typical calcium-binding RTX domain in the C-terminal portion of the molecule with the characteristic glycine- and aspartate-rich nonapeptide repeats; and (iv) a C-proximal secretion signal recognized by the type I secretion system. RTX toxins, including RtxA from K. kingae, have been shown to act as highly efficient ‘contact weapons’ that penetrate and permeabilize host cell membranes and thus contribute to the pathogenesis of bacterial infections. RtxA was discovered relatively recently and the knowledge of its biological role remains limited. This review describes the structure and function of RtxA in the context of the most studied RTX toxins, the knowledge of which may contribute to a better understanding of the action of RtxA in the pathogenesis of K. kingae infections.
Highlights
The fastidious and facultatively anaerobic Gram-negative coccobacillus Kingella kingae, from the family of Neisseriaceae, was first isolated by Elizabeth King in 1960 [1–3]
Advanced culture techniques and newly developed molecular detection methods revealed that K. kingae is a common cause of septic arthritis and osteomyelitis in children and can cause other invasive diseases such as infective endocarditis, bacteremia, meningitis, ocular infections, pneumonia, pericarditis, or peritonitis
RtxA belongs to a broad family of pore-forming RTX cytotoxins, which are secreted by many Gram-negative pathogens and share several functional domains and characteristic segments
Summary
The fastidious and facultatively anaerobic Gram-negative coccobacillus Kingella kingae, from the family of Neisseriaceae, was first isolated by Elizabeth King in 1960 [1–3]. The K. kingae rtx locus contains five genes, namely, rtxA, rtxB, rtxC, rtxD, and tolC, all of which are involved in the production, activation, and secretion of the RtxA toxin [23]. The first contains the rtxC, rtxA, and tolC genes and the second the rtxB, rtxD, and rtxC genes (Figure 3c) This goes well with the previous study of Opota and colleagues who examined the genomes of K. kingae strains KWG1 and ATCC 23330, referred to as NCTC 10529 [26]. All these data suggest that the rtx locus of K. kingae is plastic, and other rtx loci arrangements m5 oafy be found in additional isolates of K. kingae
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