European foulbrood (EFB) is an important infectious disease of honeybee larvae, but its pathogenic mechanisms are still poorly understood. The causative agent, Melissococcus plutonius , is a fastidious organism, and microaerophilic to anaerobic conditions and the addition of potassium phosphate to culture media are required for growth. Although M. plutonius is believed to be remarkably homologous, in addition to M. plutonius isolates with typical cultural characteristics, M. plutonius -like organisms, with characteristics seemingly different from those of typical M. plutonius , have often been isolated from diseased larvae with clinical signs of EFB in Japan. Cultural and biochemical characterization of 14 M. plutonius and 19 M. plutonius -like strain/isolates revealed that, unlike typical M. plutonius strain/isolates, M. plutonius -like isolates were not fastidious, and the addition of potassium phosphate was not required for normal growth. Moreover, only M. plutonius -like isolates, but not typical M. plutonius strain/isolates, grew anaerobically on sodium phosphate-supplemented medium and aerobically on some potassium salt-supplemented media, were positive for β-glucosidase activity, hydrolyzed esculin, and produced acid from L-arabinose, D-cellobiose, and salicin. Despite the phenotypic differences, 16S rRNA gene sequence analysis and DNA-DNA hybridization demonstrated that M. plutonius -like organisms were taxonomically identical to M. plutonius . However, by pulsed-field gel electrophoresis analysis, these typical and atypical ( M. plutonius -like) isolates were separately grouped into two genetically distinct clusters. Although M. plutonius is known to lose virulence quickly when cultured artificially, experimental infection of representative isolates showed that atypical M. plutonius maintained the ability to cause EFB in honeybee larvae even after cultured in vitro in laboratory media. Because the rapid decrease of virulence in cultured M. plutonius was a major impediment to elucidation of the pathogenesis of EFB, atypical M. plutonius discovered in this study will be a breakthrough in EFB research.

ABSTRACT. Streptococcus suis is an important pathogen for both swine and humans. In this study, we genotyped 105 S. suis isolates from porcine endocarditis in East Japan on the basis of profiles of capsular serotype-specific, virulence-associated and pilus-associated genes. The most common genotype was cps2J/mrp+/epf−/sly−/sbp2−/sep1−/sgp1+ (76.19%), followed by nt(non-typeable)/mrp+/epf−/sly−/ sbp2−/sep1−/sgp1+ (7.62%) and cps2J/mrp+/epf+/sly+/sbp2+/sep1−/sgp1− (7.62%). The representative isolates of mrp+/epf−/sly−/ sbp2−/sep1−/sgp1+ were classified into ST28 complex, a clonal complex previously referred to as ST27 complex, whereas those of mrp+/epf+/sly+/sbp2+/sep1−/sgp1− were classified into ST1 complex by multilocus sequence typing. Because the majority of human clinical isolates were assigned to ST1 and ST28 complexes, most isolates from porcine endocarditis investigated in this study may have the potential to cause S. suis infection in humans.

No abstract