Kingella kingae

Kingella kingae is a species of Gram-negative facultative anaerobic β-hemolytic coccobacilli. First isolated in 1960 by Elizabeth O. King, it was not recognized as a significant cause of infection in young children until the 1990s, when culture techniques had improved enough for it to be recognized. It is best known as a cause of septic arthritis, osteomyelitis, spondylodiscitis, bacteraemia, and endocarditis, and less frequently lower respiratory tract infections and meningitis. There are four species of Kingella: K. kingae, the most common, is part of the bacterial flora of the throat in young children and is transmitted from child to child. When it causes disease, the clinical presentation is often subtle and preceded by a recent history of stomatitis or upper respiratory infection. Other species are K. indologenes, K. denitrificans (both causing endocarditis) and K. oralis found in dental plaque. One notable exception is in cases of endocarditis (heart valve infection), which can be more refractory to treatment. K. kingae is the fifth member of the HACEK group of fastidious Gram-negative bacteria that cause endocarditis. Routine laboratory tests may be normal because the organism is difficult to culture. Inoculating the fluid from infected joints directly into blood culture vials can enhance the chances of an accurate culture, but extended culture times are not helpful. The organism has also been known as Moraxella kingae. K. kingae is oxidase-positive, catalase-negative, and beta-hemolytic. Kingella kingae is thought to begin infection by colonizing the pharynx, crossing the epithelium by using an RTX toxin, and entering the circulation and reaching deeper tissues, such as bones and joints. K. kingae expresses type IV pili, which allow for enhanced adhesion to respiratory epithelial and synovial cells and thus increased likelihood of colonization. These pili have also been shown to be reduced in number as pathogenesis progresses. σ54 regulates the transcription of pilA1, a major pilus subunit. PilS and PilR, regulatory transcription factors best known from the Pseudomonas aeruginosa pilus system, also may regulate pilA expression. High levels of type IV pili on K.kingae are associated with spreading/corroding colony types, while low levels of type IV pili are associated with nonspreading/noncorroding colony types of K. kingae. The three different types of populations are: spreading/corroding (with high-density pilation), nonspreading/noncorroding colonies (low density pilation), and domed colonies (no pilation, and thus no adherence to epithelium). Generally, respiratory and nonendocarditis infections tend to be highly piliated, while joint fluid, bone, and endocarditis blood isolates are less piliated, if at all. Children under three years of age may become infected with K.kingae and develop spondylodiscitis. Typical symptoms include back pain, abdominal pain, and damage to the bones and joints. It generally targets the lumbar region of the spinal cord, and the only true way of diagnosing it is through biopsy or needle aspiration, as blood plate growth gives many false negatives.