Azithromycin regulates bacterial virulence and immune response in a murine model of ceftazidime-treated Pseudomonas aeruginosa acute pneumonia

Pseudomonas aeruginosa (PA) remains one of the leading causes of nosocomial acute pneumonia. The array of virulence factors expressed by PA and the intense immune response associated with PA pneumonia play a major role in the severity of these infections. New therapeutic approaches are needed to overcome the high resistance of PA to antibiotics and to reduce the direct damage to host tissues. Through its immunomodulatory and anti-virulence effects, azithromycin (AZM) has demonstrated clinical benefits in patients with chronic PA respiratory infections. However, there is relatively little evidence in PA acute pneumonia. We investigated the effects of AZM, as an adjunctive therapy combined with ceftazidime (CAZ), in a murine model of PA acute pneumonia. We observed that the combined therapy (i) reduces the weight loss of mice 24 h post-infection (hpi), (ii) decreases neutrophil influx into the lungs at 6 and 24 hpi, while this effect is absent in a LPS-induced pneumonia or when PA is pretreated with antibiotics and mice do not receive any antibiotics, and that (iii) AZM, alone or with CAZ, modulates the expression of PA quorum sensing regulators and virulence factors (LasI, LasA, PqsE, PhzM, ExoS). Our findings support beneficial effects of AZM with CAZ on PA acute pneumonia by both bacterial virulence and immune response modulations. Further investigations are needed to clarify the exact underlying mechanisms responsible for the reduction of the neutrophils influx and to better discriminate between direct immunomodulatory properties of AZM, and indirect effects on neutrophilia resulting from bacterial virulence modulation.