Mechanistic insights into the synergistic interactions between macrolides and beta-lactams in bacterial infections

β-Lactams and macrolides are two cornerstone classes of antimicrobials in clinical practice. Traditionally, this combination was considered potentially antagonistic. However, accumulating evidence from basic and clinical research has begun to challenge this view and indicates that the two classes may exert complementary and synergistic effects in specific clinical contexts. This review first describes the mechanisms of action and resistance of both antibiotic classes, and then presents clinical evidence supporting the use of combination therapy. The greatest clinical benefit of combination therapy is observed in adults with severe community-acquired pneumonia (CAP). By contrast, results from non-severe CAP or pediatric populations have been inconsistent. We then systematically review potential synergistic mechanisms, including complementary antibacterial targets, macrolide penetration facilitated by β-lactam-induced cell-wall injury, suppression of bacterial repair and stress responses, biofilm disruption, immunomodulation, and quorum-sensing interference. We also classify the strength of evidence for each proposed mechanism. Finally, we compare the pharmacokinetic/pharmacodynamic (PK/PD) profiles of azithromycin, clarithromycin, and erythromycin and assess their potential impact on combination efficacy. We also discuss safety concerns, including adverse effects, drug interactions, and selection for resistance, along with strategies to address these issues. It should be emphasized that most mechanistic evidence derives from in vitro or animal studies, and only a minority comes from clinical research. The combination should be used with strict attention to indications, and its benefits should not be extrapolated indiscriminately to other conditions. We conclude that β-lactam-macrolide combination therapy demonstrates promising synergy in severe CAP and other specific infections. The multi-level mechanistic framework lends biological plausibility to the observed synergy, but biological synergy does not guarantee clinical benefit in all patients. Only when used in the right patients, at the right time, and with the right drug combination can this therapeutic strategy produce clinical benefit.