Serum obtained from a patient histopathologically diagnosed as intestinal spirochetosis was investigated serodiagnostically by agglutination test. B. aalborgi which is a human intestinal spirochete reacted strongly with the human serum, while B. pilosicoli which has potential pathogenicity to humans reacted with the serum, but as strongly and its titer was different than the other three species. On the other hand, intestinal spirochetes (Matsumoto isolates) were isolated from the biopsy samples

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... of the patient. The morphological, biochemical, and genetic characteristics of the isolates were very similar to those of B. aalborgi. Furthermore, the protein profiles of the Matsumoto isolates were also similar to those of B. aalborgi but were different than those of B. pilosicoli and B. hyodysenteriae. The reaction profiles of the Matsumoto isolates in immunoblotting were relatively similar to those of B. aalborgi except for a 74 kDa band but were different from those of B. pilosicoli and B. hyodysenteriae. Therefore, we identified the Matsumoto isolates as B. aalborgi and diagnosed the patient with a B. aalborgi infection. KEY WORDS: Brachyspira aalborgi, human, intestinal spirochete.