COMPARATIVE ANTIMICROBIAL ACTIVITY AND POSTANTIBIOTIC EFFECT OF AZITHROMYCIN, CLARITHROMYCIN AND ROXITHROMYCIN AGAINST SOME RESPIRATORY PATHOGENS

Recent macrolide derivatives, roxithromycin, azithromycin and clarithr omycin show more favourable pharmacokinetic characteristics in compari son to old ones and some differences in antibacterial activity. With t he aim of improving our understanding of some aspects of their action against respiratory pathogens, we determined the MICs and MBCs of Stre ptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, M oraxella catarrhalis and Haemophilus influenzae. Azithromycin was the most active agent against Haemophilus influenzae and Moraxella catarrh alis, while clarithromycin was more active against Streptococcus pneum oniae, Streptococcus pyogenes and Staphylococcus aureus with MICs simi lar to those of erythromycin. The bactericidal activity of all tested derivatives was weak against Staphylococcus aureus (MBC/MIC ratio simi lar to 16) and against Moraxella catarrhalis (MBC/MIC ratio, 8-16), bu t good against Streptococcus pneumoniae, Streptococcus pyogenes and Ha emophilus influenzae (MBC/MIC ratio, 2-4). The determination of killin g curves in the presence of 2 MIC and 10 MIC of azithromycin, clarithr omycin and roxithromycin confirmed their weak bactericidal activity ag ainst Staphylococcus aureus and Moraxella catarrhalis as well as their effective activity against Streptococcus pyogenes and Streptococcus p neumoniae. Azithromycin showed the highest bactericidal activity again st Haemophilus influenzae. As expected, the three derivatives produced a quite prolonged PAE when exposed to 5 MIC for 1 h, ranging between 2-4 h. The bactericidal activity and the prolonged PAE of new macrolid es for the most common respiratory pathogens should assure a good clin ical activity in respiratory infections including those sustained by H aemophilus influenzae, which is less susceptible to erythromycin and o ther old macrolides.