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Antimicrobial Agents and Chemotherapy, March 2000, p. 640-646, Vol. 44, No. 3
Biotechnology Center, Tufts University,
Medford, Massachusetts 02155
Received 12 August 1999/Returned for modification 10 November
1999/Accepted 16 December 1999
Bacterial biofilms show enormous levels of antibiotic resistance,
but little is known about the underlying molecular mechanisms. Multidrug resistance pumps (MDRs) are responsible for the extrusion of
chemically unrelated antimicrobials from the bacterial cell. Contribution of the MDR-mediated efflux to antibiotic resistance of
Pseudomonas aeruginosa biofilms was examined by using
strains overexpressing and lacking the MexAB-OprM pump. Resistance of P. aeruginosa biofilms to ofloxacin was dependent on the
expression of MexAB-OprM but only in the low concentration range.
Unexpectedly, biofilm resistance to ciprofloxacin, another substrate of
MexAB-OprM, did not depend on the presence of this pump. Dose-dependent
killing indicated the presence of a small "superresistant" cell
fraction. This fraction was primarily responsible for very high
resistance of P. aeruginosa biofilms to quinolones.
Bacterial cells recovered from a biofilm and tested under nongrowing
conditions with tobramycin exhibited higher resistance levels than
planktonic cells but lower levels than cells of an intact biofilm.
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
A Dose-Response Study of Antibiotic Resistance in
Pseudomonas aeruginosa Biofilms
*
Corresponding author. Mailing address: Biotechnology
Center, Tufts University, 4 Colby St., Medford, MA 02155. Phone: (617) 627-3731. Fax: (617) 627-3993. E-mail: klewis{at}tufts.edu.
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