Direct evidence for antipseudomonal activity of macrolides: exposure-dependent bactericidal activity and inhibition of protein synthesis by erythromycin, clarithromycin, and azithromycin

Abstract

Several previous investigators have reported that long-term administration of certain macrolides is efficacious in patients with persistent pulmonary Pseudomonas aeruginosa infections, even though the clinically achievable concentrations of these medications are far below their MICs. In the present study, we examined how sub-MICs of macrolide antibiotics affect the viability of and protein synthesis in several strains of P. aeruginosa. We report that 48 h, but not 12 or 24 h, of growth on agar containing a clinically achievable concentration of azithromycin (0.5 microgram/ml, 1/128 the MIC) significantly reduces the viability of strain PAO-1. Similar effects were seen with erythromycin and clarithromycin at 2 micrograms/ml (1/128 and 1/64 the respective MICs), whereas josamycin, oleandomycin, ceftazidime, tobramycin, minocycline, and ofloxacin had no effect on viability, even following 48 h of incubation with concentrations representing relatively high fractions of their MICs. The bactericidal activity of azithromycin seen following 48 h of incubation was not limited to strain PAO-1 but was also seen against 13 of 14 clinical isolates, including both mucoid and nonmucoid strains. Although viability was not decreased prior to 48 h, we found that 4 micrograms of azithromycin per ml inhibits protein synthesis after as little as 12 h and that protein synthesis continues to decrease in a time-dependent manner. We likewise found that P. aeruginosa accumulates azithromycin intracellulary over the period from 12 to 36 h. These results suggested that sub-MICs of certain macrolides are bactericidal to P. aeruginosa when the bacteria are exposed to these antibiotics for longer periods. Exposure-dependent intracellular accumulation of the antibiotic and inhibition of protein synthesis may partially account for the antipseudomonal activity of macrolides over relatively prolonged incubation periods.

Suppression of Pseudomonas aeruginosa quorum-sensing systems by macrolides: a promising strategy or an oriental mystery?

“…13 These data may explain, at least in part, the relatively higher concentrations of macrolides at the site of infection, in contrast to the lower serum levels. Furthermore, accumulation of macrolide was demonstrated not only in host cells but also in bacteria in a long incubation period, 14 a fi nding which may be associated with sub-MIC effects on pathogens and, perhaps, clinical effi cacy. These data suggests that macrolide antibiotics potentially exhibit antibacterial activity, not only through direct bactericidal and bacteriostatic effects but also by the suppression of bacterial virulence factors and the modulation of host defense systems.…”

Buccal Adherence of Pseudomonas aeruginosa in Patients with Cystic Fibrosis under Long-Term Therapy with Azithromycin

“…Although not bactericidal for P. aeruginosa, macrolides inhibit the synthesis of pseudomonal virulence factors [8,9], impair bacterial defence mechanisms and enhance bactericidal effects of quinolones [10,11]. Clinical benefit in DPB may also be due to modification of host responses.…”

Longterm azithromycin therapy for three patients with chronic lower respiratory tract infections

“…AZM has much stronger effects than EM or CAM in reducing the virulence factors produced by P. aeruginosa. [7][8][9][10] Furthermore, as shown in Fig. 1, AZM also exerted much stronger effects in reducing elastase production by P. aeruginosa than did CAM.…”