Melissococcus plutonius is an important pathogen that causes
European foulbrood (EFB) in honeybee larvae. Recently, we discovered a group of M.
plutonius strains that are phenotypically and genetically distinct from other
strains. These strains belong to clonal complex (CC) 12, as determined by multilocus
sequence typing analysis, and show atypical cultural and biochemical characteristics
in vitro compared with strains of other CCs tested. Although EFB is
considered to be a purely intestinal infection according to early studies, it is unknown
whether the recently found CC12 strains cause EFB by the same pathomechanism. In this
study, to obtain a better understanding of EFB, we infected European honeybee
(Apis mellifera) larvae per os with a
well-characterized CC12 strain, DAT561, and analyzed the larvae histopathologically.
Ingested DAT561 was mainly localized in the midgut lumen surrounded by the peritrophic
matrix (PM) in the larvae. In badly affected larvae, the PM and midgut epithelial cells
degenerated, and some bacterial cells were detected outside of the midgut. However, they
did not proliferate in the deep tissues actively. By immunohistochemical analysis, the PM
was stained with anti-M. plutonius serum in most of the DAT561-infected
larvae. In some larvae, luminal surfaces of the PM were more strongly stained than the
inside. These results suggest that infection of CC12 strain in honeybee larvae is
essentially confined to the intestine. Moreover, our results imply the presence of
M. plutonius-derived substances diffusing into the larval tissues in
the course of infection.