Fastidious or rarely isolated gram-negative rods with a particular focus on <i>Kingella kingae</i>

<b>SUMMARY</b>The bacterial species covered in this review are taxonomically diverse. <i>Kingella kingae</i>, a well-known infectious agent of pediatric bacteremia and osteomyelitis as well as septic arthritis in children aged 6-36 months, is the focus of this review. Recent advances in molecular diagnostic tools have allowed new aspects of pathogenesis and transmission to be investigated since the last review of <i>K. kingae</i> in this journal. Asymptomatic oropharyngeal colonization with noninvasive strains can be distinguished from invasive clones, so that a positive detection in the oropharynx can be assumed to be compatible with a clinical entity caused by <i>K. kingae</i>. Furthermore, novel and uncommon infections could be attributed to <i>K. kingae</i>. Clinical and microbiological aspects of the other <i>Kingella</i> spp. and of the genera <i>Actinobacillus</i>, <i>Aggregatibacter</i>, <i>Capnocytophaga</i>, <i>Cardiobacterium</i>, <i>Chromobacterium</i>, <i>Dysgonomonas</i>, <i>Eikenella</i>, <i>Pasteurella</i>, <i>Simonsiella</i>, <i>Streptobacillus</i>, and <i>Suttonella</i> are discussed separately. The identification of most genera by phenotypic methods is difficult. Direct microscopy and phenotypic key reactions, as well as commercially available systems, allow identification of the frequently encountered human species. If accurate identification of uncommon isolates is a concern, molecular methods, such as 16S ribosomal RNA gene PCR/sequencing, are necessary. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry can also be helpful. Besides infections with only one infectious agent, all mentioned genera are often found in mixed infections. The accompanying bacteria and the clinical presentation might guide the antibiotic choice. In general, it can be said that empirical treatment of infections with the fastidious gram-negative rods based on the clinical picture sometimes fails due to antibiotic resistance. Susceptibility testing should be performed on all isolates according to the CLSI or EUCAST guidelines in order to adjust treatment if necessary.