Meta-analysis found in vitro synergy. Especially in A baumanii. Mechanism unclear
Synergy testing can be done on isolate if available
Can not necessarily infer in vivo synergy (Beta-lactam/aminoglycoside synergy is demonstrated in vitro but NOT in vivo for example).
Systematic Review and Meta-Analysis of In Vitro Synergy of Polymyxins and Carbapenems. Antimicrobial Agents and Chemotherapy. October 2013 Volume 57 Number 10. p.5104 – 5111.