Gingival isolates of oral Fusobacterium nucleatum strains (gram-negative anaerobic fusiform bacilli) have shown the characteristic ability to hemagglutinate a variety of erythrocytes (RBC) of human and animal origin. Other members of the genus tested (F. necrophorus, F. varium, and F. mortiferum) displayed little if any ability to hemagglutinate RBC. The hemagglutination (HA) activity could be observed in the F. nucleatum strains with the whole cells and in most instances with sonicated preparations of the organisms. The HA activity was observed in cell wall preparations of the organism and appeared dependent upon a heat-labile protein component of the cell wall. In decreasing order, the RBC that would hemagglutinate with the smallest concentration of HA preparations were rabbit, monkey, human, sheep, horse, and ox. No differences in HA activity of the preparations with cells from the various human blood types were noted. Absorption of the HA preparation of one strain with human cells removed HA moiety was bound to the cells via a Ca2+ binding site interaction since ethylenediaminetetraacetic acid and ethylene glycol-bis-N,N'-tetraacetic acid inhibited binding, and HA could be reestablished by the addition of Ca2+ but not Mg2+. Rabbit antisera to the F. nucleatum strains inhibited HA activity when tested with the HA preparation in the standard test, whereas anti-Leptotrichia buccalis sera or normal rabbit sera had no effect. A tanned-cell passive HA test with rabbit anti-F. nucleatum sera displayed reactivity between the homologous strains but little reactivity with the other Fusobacterium species tested.
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