In vivo pathogenicity and resistance to phagocytosis of Streptococcus equistrains with different levels of capsule expression

The glossy non-encapsulated strain of Steptococcus equi, NCTC 9682, was com pared with the matt strain Hidaka/95/2 which expresses a medium sized capsu le and with the mucoid CF32 which expresses a large sized capsule in phagoc ytosis assays and for virulence in inoculated horses. The three strains, NC TC 9682, Hidaka /95/2 and CF32 produced 2.0, 3.1, and 5.3 mg/g wet cells re spectively after 3 h incubation, but similar amounts of M-like proteins, cy totoxin and mitogen. NCTC 9682 showed no resistance to phagocytosis by equi ne neutrophils regardless of the presence of opsonin while strains Hidaka / 95/2 and CF32 showed almost complete resistance to phagocytosis. Furthermor e, NCTC 9682 produced no clinical disease although it infected the guttural pouch and caused seroconversion. Typical strangles with guttural pouch inv asion was observed in all horses infected with encapsulated strains. (C) 19 99 Elsevier Science B.V. All rights reserved.