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One hundred and forty-eight isolates of bacteria from 20 intraoral odontogenic abscesses were tested for their susceptibility to spiramycin and metronidazole alone or in combination. All isolates, except Rothia spp. (one), Enterococcus avium (three), Haemophilus parainfluenzae (one) and Staphylococcus aureus (one) were sensitive to spiramycin and/or metronidazole. Among the anaerobes, spiramycin as well as metronidazole showed good antimicrobial activity against species of Prevotella, Eubacterium, Peptostreptococcus, Bacteroides and Porphyromonas. All the aerobes were resistant to metronidazole. A potential synergic effect was found in 17% of the clinical isolates tested, as the MICs of spiramycin and metronidazole for 25 isolates (21 isolates of Eubacterium, two of lactobacilli, one strain of Peptostreptococcus sp. and one Clostridium clostridiiforme) were significantly reduced by the addition of the other antibiotic.

Macrolides are actively concentrated by leucocytes. The dose-effect responses of spiramycin (Sp) and erythromycin (Er) on phytohaemagglutinin (PHA) and pokeweed mitogen (PWM) stimulated human mononuclear leucocytes (MNL) were studied. Cell viability was not altered at any antibiotic concentration (1-100 mg/l). Both Sp and Er showed dose-related inhibition of the proliferative response of PHA and PWM stimulated MNL. Very marked effects were observed at high antibiotic concentrations and the effects observed at low concentrations (1-10), although small, were also significant. Similar results were observed for the mitogen PWM. A decrease in tritiated thymidine (3H-TdR) incorporation occurred only if Sp and Er were added during the first 8 h of culture. Sp and Er also induced a decrease in tritiated uridine (3H-UdR) uptake. These data suggest that Sp and Er interfered with an early event in the cell cycle. However Sp did not affect PHA binding to MNL. The clinical significance of these findings is discussed.

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