Enhancement of host resistance to bacterial infections in normal and immunosuppressed mice with Actinobacillus suis.

A single intraperitoneal (ip) inoculation of heat-killed Actinobacillus suis ATCC 15,557 (AS 15,557) into normal and immunosuppressed (dexamethasone-treated) mice led to remarkable nonspecific resistance to ip challenge with lethal doses of opportunistic pathogens such as Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus and Candida albicans. The duration of this enhanced protective action and the minimal effective dose, in normal mice, induced by AS 15,557 were superior to those induced by other bacterial immunostimulants such as heat-killed Lavtobacillus casei YIT 9018 (LC 9018) and penicillin-treated Streptococcus pyogenes, Su (OK-432). In immunosuppressed mice; the reduced in vivo killing activity of peritoneal exudate cells (PECs) against P. aeruginosa infection was markedly augmented by ip injection of AS 15,557. The degree of PEC augmentation induced by AS 15,557 was higher than that induced by LC 9018 or by OK-432. The toxicity and histopathological changes associated with AS 15557 were very low, as compared with those by produced by LC 9018 and OK-432. The results suggest that AS 15,557, which showed a strong resistance-enhancing capacity against opportunistic bacterial infections, may be a useful bacterial immunostimulant.