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Antimicrobial Agents and Chemotherapy, November 2009, p. 4809-4815, Vol. 53, No. 11
0066-4804/09/$08.00+0     doi:10.1128/AAC.00269-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Comparison of Methods To Test Antibiotic Combinations against Heterogeneous Populations of Multiresistant Pseudomonas aeruginosa from Patients with Acute Infective Exacerbations in Cystic Fibrosis{triangledown}

Juliet E. Foweraker,1* Christian R. Laughton,1 Derek F. Brown,2 and Diana Bilton1,{dagger}

Department of Microbiology, Papworth Hospital, Papworth Everard, Cambridge CB3 8RE, United Kingdom,1 Clinical Microbiology and Public Health Laboratory, Addenbrookes Hospital, Cambridge CB2 2QW, United Kingdom2

Received 26 February 2009/ Returned for modification 18 May 2009/ Accepted 14 August 2009

Multiresistant Pseudomonas aeruginosa isolates can chronically infect patients with cystic fibrosis. Acute infective exacerbations are treated with combinations of two antipseudomonal antibiotics. Patients may respond clinically even if the bacteria are resistant, possibly due to antimicrobial synergy. The challenge for testing for synergy in vitro is that there is no standardized method, and the antibiotic susceptibility in a population of P. aeruginosa isolates in a single sputum sample can vary. We therefore compared (i) antibiotic combinations with different examples of resistant bacteria from the same sputum sample and (ii) the results of synergy testing by different methods. Antibiotic synergy was tested by using resistant P. aeruginosa isolates recovered from sputum samples taken just before the start of treatment for an acute infective exacerbation. Several examples of each morphotype of P. aeruginosa were tested by cidal checkerboard, time-kill curve, and multiple-combination bactericidal testing. The isolates were typed by pulsed-field gel electrophoresis (PFGE). The results were compared with the clinical and microbiological responses to 14 days of antibiotic treatment. Forty-four resistant isolates from nine patients were tested. Some P. aeruginosa isolates with the same morphotype and PFGE pulsotype had different results by synergy testing. There was a poor correlation between the results of the different methods of synergy testing, and no one method would have predicted the response to treatment in all patients. The in vitro effects of antibiotic combinations against different isolates from the same sputum sample can vary, and the results depend on the methodology used. The role of combination testing for the treatment of antibiotic-resistant P. aeruginosa in acute exacerbations of chronic infection in patients with cystic fibrosis needs to be reviewed.

* Corresponding author. Mailing address: Department of Microbiology, Papworth Hospital, Papworth Everard, Cambridge CB3 8RE, United Kingdom. Phone: 44 1480 830541. Fax: 44 1480 364780. E-mail: juliet.foweraker{at}papworth.nhs.uk

{triangledown} Published ahead of print on 24 August 2009.

{dagger} Present address: Department of Cystic Fibrosis, Royal Brompton Hospital, London SW3 6NP, United Kingdom.

Antimicrobial Agents and Chemotherapy, November 2009, p. 4809-4815, Vol. 53, No. 11
0066-4804/09/$08.00+0     doi:10.1128/AAC.00269-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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