Spiramycin: a reappraisal of its antibacterial activity.

Spiramycin was less active than erythromycin in vitro against sensitive strains of Staphylococcus aureus, but was as effective against staphylococcal infections in mice when the drugs were administered immediately after infection: spiramycin was relatively more effective in prophylactic experiments when the antibiotics were administered at 4 or at 6 hr before infection. Strains of staphylococci which had been habituated to either spiramycin or erythromycin in vitro showed complete cross-resistance to both antibiotics in vitro, but the majority of a number of erythromycin-resistant strains isolated from patients were sensitive to spiramycin in vitro. Spiramycin was effective against a number of such strains in vivo, but erythromycin had little effect against these infections. Investigation into the distribution of the antibiotics in the serum and tissues of mice showed that spiramycin was maintained in tissues (lung, liver, kidney, spleen and heart) at higher concentrations and for a longer period than erythromycin, although erythromycin was initially present in serum in higher concentrations. It is suggested that the efficacy of spiramycin against experimental infections is due to its persistence in the animal body.