Experimental actinomycosis in mice induced by alginate gel particles containing Actinomyces israelii

The present study aimed to make an animal model for investigating chronic infection. Bacterial cells of Actinomyces israelii (ATCC 10048) were packed in alginate gel particles and injected intra-peritoneally into BALB/c mice. Actinomycotic lesions were induced efficiently in 9 mice out of 12 after 3 or 9 weeks. Actinomycotic lesions were not produced by injecting a bacterial suspension of A. israelii except in one animal. Neither did injection of gel particles without bacteria induce lesions. Bacteria survived in the lesions for at least 9 weeks after the injection, and serum IgG levels against the bacteria were significantly elevated in the animals, indicating that the bacteria were protected from immunological elimination and activated humoral immunity in the animals. Histopathological observation of the lesion by staining revealed that the bacteria in the lesions were acid-fast and seemed to become resistant to phagocytosis. The bacterial masses were surrounded by inflammatory cells, including polymorphonuclear leukocytes and macrophages, and were separated from the host cells by a fringe-like structure similar to the capsular structure of natural sulfur granules. The present study indicated that the use of an alginate gel was effective in inducing actinomycotic lesions in mice.