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Journal of Clinical Microbiology, October 2005, p. 5085-5090, Vol. 43, No. 10
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.10.5085-5090.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Antibiotic Susceptibilities of Pseudomonas aeruginosa Isolates Derived from Patients with Cystic Fibrosis under Aerobic, Anaerobic, and Biofilm Conditions

Dominic Hill,1 Barbara Rose,1 Aniko Pajkos,1 Michael Robinson,2 Peter Bye,2 Scott Bell,3 Mark Elkins,2 Barbara Thompson,4 Colin MacLeod,4 Shawn D. Aaron,5 and Colin Harbour1*

Department of Infectious Diseases and Immunology, University of Sydney, Sydney, Australia,1 Department of Respiratory Medicine, Royal Prince Alfred Hospital, Sydney, Australia,2 Department of Thoracic Medicine, The Prince Charles Hospital, Brisbane, Queensland, Australia,3 Department of Microbiology, Royal Prince Alfred Hospital, Sydney, Australia,4 The Ottawa Health Research Institute, The Ottawa Hospital, Ottawa, Canada5

Received 21 December 2004/ Returned for modification 5 April 2005/ Accepted 1 August 2005

Recent studies have determined that Pseudomonas aeruginosa can live in a biofilm mode within hypoxic mucus in the airways of patients with cystic fibrosis (CF). P. aeruginosa grown under anaerobic and biofilm conditions may better approximate in vivo growth conditions in the CF airways, and combination antibiotic susceptibility testing of anaerobically and biofilm-grown isolates may be more relevant than traditional susceptibility testing under planktonic aerobic conditions. We tested 16 multidrug-resistant isolates of P. aeruginosa derived from CF patients using multiple combination bactericidal testing to compare the efficacies of double and triple antibiotic combinations against the isolates grown under traditional aerobic planktonic conditions, in planktonic anaerobic conditions, and in biofilm mode. Both anaerobically grown and biofilm-grown bacteria were significantly less susceptible (P < 0.01) to single and combination antibiotics than corresponding aerobic planktonically grown isolates. Furthermore, the antibiotic combinations that were bactericidal under anaerobic conditions were often different from those that were bactericidal against the same organisms grown as biofilms. The most effective combinations under all conditions were colistin (tested at concentrations suitable for nebulization) either alone or in combination with tobramycin (10 µg ml–1), followed by meropenem combined with tobramycin or ciprofloxacin. The findings of this study illustrate that antibiotic sensitivities are dependent on culture conditions and highlight the complexities of choosing appropriate combination therapy for multidrug-resistant P. aeruginosa in the CF lung.

* Corresponding author. Mailing address: Department of Infectious Diseases and Immunology, Blackburn Blg DO6, University of Sydney, NSW Sydney 2006, Australia. Phone: 61 2 93514334. Fax: 61 2 93514731. E-mail: charbour{at}infdis.usyd.edu.au.

Journal of Clinical Microbiology, October 2005, p. 5085-5090, Vol. 43, No. 10
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.10.5085-5090.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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