Antibiotic susceptibility of Kingella kingae isolates from children with skeletal system infections.

To the Editors: Traditionally, the initial drug therapy for suspected skeletal infections in children pending diagnostic bacteriology results targets Gram-positive organisms and usually consists of intravenous administration of a -lactamase–stable drug, such as oxacillin or cloxacillin, or a secondor third-generation cephalosporin. In regions where communityassociated methicillin-resistant Staphylococcus aureus is prevalent, a combination of a -lactam antibiotic and vancomycin or clindamycin is recommended. In recent years, as the result of the widespread use of improved culture methods and sensitive nucleic acid amplification assays, Kingella kingae, a Gram-negative member of the Neisseriaceae family, is being increasingly recognized as a common etiology of septic arthritis and osteomyelitis in young children. The susceptibility of the organism to antimicrobial drugs that are administered to children with confirmed or suspected skeletal system infections has not been adequately investigated. Eighty-five K. kingae strains, isolated between 1990 and 2011 from Israeli children younger than 4 years with skeletal system infections, were studied. Infections included septic arthritis (n 70), osteomyelitis (n 5), osteomyelitis and arthritis of the adjacent joint (n 1), tenosynovitis (n 3), abortive osteoarthritis (n 5), and undetermined skeletal system infection (n 1). The susceptibility of isolates to penicillin G, cefuroxime, oxacillin, clindamycin, cotrimoxazole, and linezolid were determined using the Etest on cation-adjusted MuellerHinton plates with added 5% defibrinated blood. Because all K. kingae isolates are intrinsically resistant to glycopeptide antibiotics, susceptibility to vancomycin was not tested. Isolates exhibiting minimum inhibitory concentrations (MICs) of 4 mcg/mL for penicillin or 80 mcg/mL for cotrimoxazole (ie, 4 mcg/mL for trimethoprim or 76 mcg/mL for sulfamethoxazole) were considered resistant to these drugs according to established criteria for HACEK group organisms. For the other antibiotics tested, no interpretive criteria have been defined; therefore, antibiotic susceptibility results were reported as MIC50 and MIC90 values only (Table 1). Production of -lactamase was determined by the nitrocefin method. All 85 isolates tested negative for -lactamase production and were highly susceptible to penicillin G. A single isolate (1.2%) was resistant to cotrimoxazole. MICs of cefuroxime and linezolid were low, suggesting susceptibility to these drugs. Reduced susceptibility to oxacillin and clindamycin were detected in a substantial fraction of organisms. In the absence of interpretive criteria for antibiotic susceptibility of K. kingae, if cutoff values defined for Staphylococcus aureus would have been employed ( 4 mcg/mL), 32 (37.6%) and 9 (10.6%) isolates would have been considered resistant to these drugs, respectively. The present study results indicate that oxacillin or clindamycin does not provide adequate antimicrobial coverage against K. kingae organisms. Although the consequences of delayed administration of effective antibiotics in K. kingae septic arthritis or osteomyelitis are currently unknown, it seems prudent to recommend the empiric use of cephalosporins, alone or in combination with vancomycin or clindamycin, in children aged 6 to 48 months with suspected skeletal system infections, pending identification of the causative organism and antibiotic susceptibility testing results.