title: Antimicrobial Activity of Fosfomycin-Tobramycin Combination against Pseudomonas aeruginosa Isolates Assessed by Time-Kill Assays and Mutant Prevention Concentrations.
abstract: The antibacterial activity of fosfomycin-tobramycin combination was studied by time-kill assay in eight Pseudomonas aeruginosa clinical isolates belonging to the fosfomycin wild-type population (MIC = 64 μg/ml) but with different tobramycin susceptibilities (MIC range, 1 to 64 μg/ml). The mutant prevention concentration (MPC) and mutant selection window (MSW) were determined in five of these strains (tobramycin MIC range, 1 to 64 μg/ml) in aerobic and anaerobic conditions simulating environments that are present in biofilm-mediated infections. Fosfomycin-tobramycin was synergistic and bactericidal for the isolates with mutations in the mexZ repressor gene, with a tobramycin MIC of 4 μg/ml. This effect was not observed in strains displaying tobramycin MICs of 1 to 2 μg/ml due to the strong bactericidal effect of tobramycin alone. Fosfomycin presented higher MPC values (range, 2,048 to >2,048 μg/ml) in aerobic and anaerobic conditions than did tobramycin (range, 16 to 256 μg/ml). Interestingly, the association rendered narrow or even null MSWs in the two conditions. However, for isolates with high-level tobramycin resistance that harbored aminoglycoside nucleotidyltransferases, time-kill assays showed no synergy, with wide MSWs in the two environments. glpT gene mutations responsible for fosfomycin resistance in P. aeruginosa were determined in fosfomycin-susceptible wild-type strains and mutant derivatives recovered from MPC studies. All mutant derivatives had changes in the GlpT amino acid sequence, which resulted in a truncated permease responsible for fosfomycin resistance. These results suggest that fosfomycin-tobramycin can be an alternative for infections due to P. aeruginosa since it has demonstrated synergistic and bactericidal activity in susceptible isolates and those with low-level tobramycin resistance. It also prevents the emergence of resistant mutants in either aerobic or anaerobic environments.
authors.affiliation: Servicio de Microbiología, Hospital Universitario Ramón y Cajal, Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Madrid, Spain Red Española de Investigación en Patología Infecciosa (REIPI), Instituto de Salud Carlos III, Madrid, Spain.
authors.forename: María
authors.initials: M
authors.lastname: Díez-Aguilar
authors.affiliation: Servicio de Microbiología, Hospital Universitario Ramón y Cajal, Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Madrid, Spain Red Española de Investigación en Patología Infecciosa (REIPI), Instituto de Salud Carlos III, Madrid, Spain mariaisabel.morosini@salud.madrid.org.
authors.forename: María Isabel
authors.initials: MI
authors.lastname: Morosini
authors.affiliation: Servicio de Microbiología, Hospital Universitario Ramón y Cajal, Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Madrid, Spain CIBER en Epidemiología y Salud Pública (CIBERESP), Instituto de Salud Carlos III, Madrid, Spain Unidad de Resistencia a Antibióticos y Virulencia Bacteriana asociada al Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain.
authors.forename: Ana P
authors.initials: AP
authors.lastname: Tedim
authors.affiliation: Servicio de Microbiología, Hospital Universitario Ramón y Cajal, Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Madrid, Spain CIBER en Epidemiología y Salud Pública (CIBERESP), Instituto de Salud Carlos III, Madrid, Spain.
authors.forename: Irene
authors.initials: I
authors.lastname: Rodríguez
authors.affiliation: Department of Clinical Microbiology, Istanbul Medical Faculty, Istanbul University, Istanbul, Turkey.
authors.forename: Zerrin
authors.initials: Z
authors.lastname: Aktaş
authors.affiliation: Servicio de Microbiología, Hospital Universitario Ramón y Cajal, Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Madrid, Spain Red Española de Investigación en Patología Infecciosa (REIPI), Instituto de Salud Carlos III, Madrid, Spain Unidad de Resistencia a Antibióticos y Virulencia Bacteriana asociada al Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain.
authors.forename: Rafael
authors.initials: R
authors.lastname: Cantón
mesh_terms: D000890:Anti-Infective Agents / Q000494:pharmacology*; D004357:Drug Synergism; D005578:Fosfomycin / Q000494:pharmacology*; D008826:Microbial Sensitivity Tests; D009154:Mutation; D011550:Pseudomonas aeruginosa / Q000187:drug effects / Q000235:genetics*; D014031:Tobramycin / Q000494:pharmacology*
chemical_list: D000890:Anti-Infective Agents; D005578:Fosfomycin; D014031:Tobramycin