title: Prolonged exposure of methicillin-resistant Staphylococcus aureus (MRSA) COL strain to increasing concentrations of oxacillin results in a multidrug-resistant phenotype.
abstract: Our previous studies demonstrated that exposure of a bacterium to increasing concentrations of an antibiotic would increase resistance to that antibiotic as a consequence of activating efflux pumps. This study utilises the same approach; however, it employs the methicillin-resistant Staphylococcus aureus (MRSA) COL strain, which is highly resistant to oxacillin (OXA). MRSA COL was adapted to 3200 mg/L of OXA. Changes in resistance to other antibiotics were evaluated and efflux pump activity during the adaptation process was determined. MRSA COL was exposed to stepwise two-fold increases of OXA. At the end of each step, minimum inhibitory concentration determination for erythromycin (ERY) and other antibiotics was conducted. Reserpine (RES) was employed to evaluate whether resistance to ERY was dependent on efflux pump activity. Efflux pump activity was also evaluated using the ethidium bromide (EB) assay. DNA typing of the products of each culture step was conducted to assess purity. Serial exposure of MRSA COL to increasing concentrations of OXA resulted in increased resistance to ERY, which could be eliminated with RES. Evaluation of efflux pump activity by the EB method indicated increased efflux activity. Resistance to ERY was accompanied by resistance to kanamycin, amikacin, ofloxacin, norfloxacin, ciprofloxacin and rifampicin. This is the first time that a multidrug-resistant phenotype has been experimentally produced as a consequence of exposure of the organism to an antibiotic to which it is initially highly resistant.
authors.affiliation: Unit of Mycobacteriology, Institute of Hygiene and Tropical Medicine, Universidade Nova de Lisboa, Rua da Junqueira 96, 1349-008 Lisbon, Portugal.
authors.forename: Ana
authors.initials: A
authors.lastname: Martins
authors.forename: Isabel
authors.initials: I
authors.lastname: Couto
authors.forename: Lone
authors.initials: L
authors.lastname: Aagaard
authors.forename: Marta
authors.initials: M
authors.lastname: Martins
authors.forename: Miguel
authors.initials: M
authors.lastname: Viveiros
authors.forename: Jette E
authors.initials: JE
authors.lastname: Kristiansen
authors.forename: Leonard
authors.initials: L
authors.lastname: Amaral
mesh_terms: D000900:Anti-Bacterial Agents / Q000008:administration & dosage*; D001426:Bacterial Proteins / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D024901:Drug Resistance, Multiple, Bacterial; D004996:Ethidium; D006801:Humans; D066298:In Vitro Techniques; D026901:Membrane Transport Proteins / Q000378:metabolism; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests; D010068:Oxacillin / Q000008:administration & dosage*; D010641:Phenotype; D013203:Staphylococcal Infections / Q000188:drug therapy / Q000382:microbiology; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; D004996:Ethidium; D010068:Oxacillin