Phenotypic characterization of E. aerogenes strains.

E. aerogenes isolates were recovered periodically from two patients undergoing chemotherapy with imipenem—P1 to P5 from patient P and G1 to G7 from patient G. ATCC15038 was used as a reference strain. MICs were determined by broth microdilution in MHII broth as described in Materials and Methods. For clarity, strains have been classified as R for resistant and S for susceptible. Detailed MICs are presented in supplementary information (S1 Table). Different classes of antibiotics were tested: phenicols (chloramphenicol), quinolones (nalidixic acid, fleroxacin and norfloxacin), macrolides (erythromycin), carbapenems (imipenem and ertapenem), cephalosporins (cefepime, ceftazidime and cefotaxime), tetracycline and polymyxin B. Gentamicin is often the last option to treat MDR strains. For this reasons, we chose not to include this antibiotic in our assays but we refer to MIC values obtained from the healthcare facilities.a Sampling sites: U, urine; V, vagina; S, Sputum; B, blood; T, trachea.b The presence of porins and efflux pump proteins were determined immunoblots using specific antibodies directed against Escherichia coli OmpF, OmpC, AcrA, AcrB or TolC.*Mass spectrometry analysis indicated that porin-plus isolates only express Omp36 whereas strain ATCC15038 expresses both Omp36 and Omp35 (S1 Fig and S2 Table).Phenotypic characterization of E. aerogenes strains.