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Antimicrobial Agents and Chemotherapy, March 2000, p. 640-646, Vol. 44, No. 3
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

Alexei Brooun, Songhua Liu, and Kim Lewis*

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.

* 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.

Antimicrobial Agents and Chemotherapy, March 2000, p. 640-646, Vol. 44, No. 3
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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