Dissemination of K. kingae in a susceptible pediatric population may result in an outbreak of invasive disease. Our experience suggests the need for increased alertness for clusters of K. kingae infections among children in daycare.

Fifty Kingella kingae organisms, isolated from tonsillar cultures of day care center attendees during an 11-month period, and 60 isolates derived from epidemiologically unrelated individuals, including 19 isolates from respiratory carriers and 41 isolates from patients with invasive infections, were typed by immunoblotting, pulsed-field gel electrophoresis, and ribotyping. One strain, defined by unique immunoblotting, pulsed-field gel electrophoresis, and ribotyping patterns, represented 14 day care isolates (28%) and was frequently isolated during the first half of the follow-up period; a second strain represented 23 (46%) isolates and prevailed during the last 5 months. Children frequently carried the same strain continuously or intermittently for weeks or months, when it was replaced by a new strain. Epidemiologically unrelated organisms showed greater variability, and no strain represented >5% of isolates. The present results support person-to-person transmission of K. kingae among young children in the day care setting.

The immune response to Kingella kingae was determined by enzyme-linked immunosorbent assay, using outer-membrane proteins as coating antigen, in 19 children with invasive infection. The age-related incidence of K. kingae disease in southern Israel during 1988-2002 was calculated and correlated with serum antibody levels in healthy children. Significant increases in immunoglobulin G (IgG) levels were found in children convalescing after invasive infections. The incidence was 1.3, 40.3, 23.9, 5.7, and 1.9 cases/100,000 children among those aged 0-5, 6-11, 12-23, 24-35, and 36-47 months, respectively. A low attack rate and undetectable serum IgA and high IgG levels were found during the first 6 months of life, which indicates that protection was conferred by maternally derived immunity. The high attack rate found among 6-24-month-old children coincides with the age at which antibody levels were lowest. Low incidence of disease and increasing antibody levels were found among older children, which probably represents cumulative experience with K. kingae antigens via colonization or infection.

The purpose of this study was to investigate the prevalence of β-lactamase and the genomic clonality of a large collection of Kingella kingae isolates from Israeli patients with a variety of invasive infections and asymptomatic pharyngeal carriers. β-lactamase production was studied by the nitrocefin method and the minimum inhibitory concentrations (MICs) of penicillin and amoxicillin-clavulanate were determined by the epsilon (Etest) method. The genotypic clonality of isolates was investigated by pulsed-field electrophoresis (PFGE). β-lactamase was found in 2 of 190 (1.1 %) invasive isolates and in 66 of 429 (15.4 %) randomly chosen carriage organisms (p < 0.001). Overall, 73 distinct PFGE clones were identified (33 among invasive organisms and 56 among carriage isolates). β-lactamase production was found to be limited to four distinct PFGE clones, which were common among carriage strains but rare among invasive strains, and all organisms in the collection belonging to these four clones expressed β-lactamase. The penicillin MIC of β-lactamase-producing isolates ranged between 0.094 and 2 mcg/mL (MIC50: 0.25 mcg/mL; MIC90: 1.5 mcg/mL) and that of amoxicillin-clavulanate between 0.064 and 0.47 mcg/mL (MIC50: 0.125 mcg/mL; MIC90: 0.125 mcg/mL). The penicillin MIC of β-lactamase non-producing isolates ranged between <0.002 and 0.064 mcg/mL (MIC50: 0.023 mcg/mL; MIC90: 0.047 mcg/mL). Although β-lactamase production is prevalent among K. kingae organisms carried by healthy carriers, the low invasive potential of most colonizing clones results in infrequent detection of the enzyme in isolates from patients with clinical infections. The exceptional presence of β-lactamase among invasive organisms correlates with the favorable response of K. kingae infections to the administration of β-lactamase-susceptible antibiotics.