An invasive infection with an unusual spaB-possessing Erysipelothrix rhusiopathiae in a human

Erysipelothrix rhusiopathiae is a zoonotic pathogen that causes erysipelas in a variety of animals. In humans, in contrast to the cutaneous form called erysipeloid, which is an occupational disease and common in individuals who handle raw meat and fish, invasive systemic infections are unusual. E. rhusiopathiae expresses an immunogenic surface protein, Spa (surface protective antigen), which is involved in virulence. Among the antigenically different Spa proteins (SpaA, B and C), which are mostly associated with serovars, SpaA is by far the most prevalent in E. rhusiopathiae isolates from diseased animals. However, the Spa type has not been examined for human isolates, and it is unknown whether SpaB- or SpaC-possessing isolates can cause disease in humans. A Gram-positive, rod-shaped bacterium isolated from a case of human pyogenic spondylitis was analysed. The bacterium was identified as E. rhusiopathiae by a routine biochemical test and MS, and ultimately confirmed by an E. rhusiopathiae-specific PCR assay. Spa typing by sequencing revealed the SpaB type, and the serovar of the strain was identified as untypeable by a conventional agar gel precipitation test, but determined to be serovar 6 by a serotyping PCR assay. Sequence analysis of the serovar-defining chromosomal region revealed that the isolate displayed the same gene organization as the serovar 6 reference strain, but the region was disrupted by an insertion sequence element, suggesting that the isolate originated from a serovar 6 strain. These results highlight that unusual, spaB-possessing E. rhusiopathiae strains can potentially pose serious risks to humans.