IgM-enriched immunoglobulin improves colistin efficacy in a pneumonia model by Pseudomonas aeruginosa

We evaluated the efficacy of ceftazidime or colistin in combi nation with polyclonal IgM-enriched immunoglobulin (IgM-IG), in an experimental pneumonia model (C57BL/6J male mice) using two multidrug-resistant Pseudomonas aeruginosa strains, both ceftazidime-susceptible and one colistin-resistant. Pharmaco dynamically optimised antimicrobials were administered for 72 h, and intravenous IgM-IG was given as a single dose. Bacterial tissues count and the mortality were analysed. Ceftazidime was more effective than colistin for both strains. In mice infected with the colistin-susceptible strain, ceftazidime reduced the bacterial concentration in the lungs and blood (22.42 and 23.87 log10 CFU/ ml) compared with colistin (20.55 and 21.23 log10 CFU/ml, re spectively) and with the controls. Colistin plus IgM-IG reduced the bacterial lung concentrations of both colistin-susceptible and resistant strains (22.91 and 21.73 log10 CFU/g, respectively) and the bacteraemia rate of the colistin-resistant strain (244%). These results suggest that IgM-IG might be useful as an adjuvant to colistin in the treatment of pneumonia caused by multidrug resistant P. aeruginosa.