Comparison of the in vitro activities of BMS-284756 and four fluoroquinolones against Streptococcus pneumoniae

F. J. Boswell, J. M. Andrews and R. Wise,*

Department of Microbiology, City Hospital NHS Trust, Birmingham B18 7QH, UK

Sir,

BMS-284756 is a novel des-fluoro(6) quinolone, without the C-6 position fluorine characteristic of the existing fluoroquinolones. BMS-284756 has a broad spectrum of in vitro activity against both Gram-positive and Gram-negative bacteria.1 Resistance to fluoroquinolones in Streptococcus pneumoniae may be due to a number of stepwise mutations in the quinolone-resistance determining regions of the subunits of DNA gyrase and topoisomerase IV.2,3 Low-level fluoroquinolone resistance also occurs in S. pneumoniae due to the increased expression of the multidrug efflux pumps.4 In this communication we compare the in vitro activity of BMS-284756 (Bristol-Myers Squibb, New York, NY, USA), norfloxacin (Merck, Sharpe & Dohme Ltd, Hoddesdon, UK) with and without reserpine, an efflux pump inhibitor (10 mg/L) (Sigma–Aldrich Co. Ltd, Poole, UK),4 sparfloxacin (Aventis Pharma, Cedex, France), moxifloxacin and ciprofloxacin (Bayer AG, Wuppertal, Germany) against 53 clinical strains of S. pneumoniae demonstrating efflux-mediated fluoroquinolone resistance. The efflux phenotype was defined as a four-fold or greater reduction in norfloxacin MIC in the presence of reserpine and elevated ethidium bromide and acriflavine MICs.4 Four genetically defined strains, R6, R6N (over expression of PmrA), ATCC 49619 and 1N27 (efflux mutant from ATCC 49619), were also studied.4 The in vitro susceptibilities were determined using the standard agar plate dilution method recommended by the BSAC.5 Briefly, IsoSensitest agar (Oxoid, Basingstoke, UK) was employed supplemented with 5% defibrinated horse blood (Tissue Culture Services, Botolph Claydon, UK). Reserpine was freshly prepared and the media containing it was used immediately. A final inoculum of 104 cfu was employed using a multipoint inoculator (Mast, Bootle, UK). Plates were incubated at 35–37°C for 18–24 h in an atmosphere enriched with 4–6% CO2. The MIC was defined as the lowest antimicrobial concentration inhibiting bacterial growth (the presence of one or two colonies was ignored).

BMS-284756 (MIC90 0.12 mg/L) was the most active compound studied, moxifloxacin (MIC90 0.5 mg/L) and sparfloxacin (MIC90 1 mg/L) had similar activities to each other, and ciprofloxacin (MIC90 8 mg/L) and norfloxacin (MIC90 64 mg/L) were the least active against pneumococci (TableGo). The eight-fold reduction in norfloxacin MIC90 observed in the presence of reserpine is consistent with the efflux phenotype of these strains. Generally, the MIC90s of BMS-284756, moxifloxacin and sparfloxacin for the effluxing strains were same or within one dilution step of the MICs of the non-effluxing strains (R6 and ATCC). However the MIC90s of ciprofloxacin and norfloxacin for the effluxing strains were eight- and 16-fold higher, respectively, than the MICs of the non-effluxing strains (R6 and ATCC). This small study has shown that not all fluoroquinolones are equally affected by resistance mechanisms and either efflux pumps may be substrate specific or the rate of accumulation of fluoroquinolones is different.6 The BMS-284756 MIC90 (0.12 mg/L) for the efflux-mediated fluoroquinolone-resistant S. pneumoniae strains is within one dilution step of the BMS-284756 MIC90 (0.06 mg/L) reported for the wild-type susceptible population (T. M. Weller, J. M. Andrews, G. Jevons & R. Wise, unpublished results). Thus BMS-284756 appears to be unaffected by PmrA-mediated efflux.


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Table. In vitro activity (mg/L)
 

Notes

* Corresponding author. Tel: +44-121-507-4255; Fax: +44-121-551-7763; E-mail: r.wise{at}bham.ac.uk Back

References

1 . Fung-Tomc, J. C., Minassian, B., Kolek, B., Huczko, E., Aleksunes, L., Stickle, T. et al. (2000). Antibacterial spectrum of a novel des-fluoro(6) quinolone, BMS-284756. Antimicrobial Agents and Chemotherapy 44, 3351–6.[Abstract/Free Full Text]

2 . Pan, X.-S. & Fisher, L. M. (1996). Involvement of topoisomerase IV and DNA gyrase as ciprofloxacin targets in Streptococcus pneumoniae. Antimicrobial Agents and Chemotherapy 40, 2321–6.[Abstract]

3 . Tanovic, J., Perichon, B., Duval, J. & Courvalin, P. (1996). Contribution of mutations in gyrA and parC genes to fluoroquinolone resistance mutants of Streptococcus pneumoniae obtained in vivo and in vitro. Antimicrobial Agents and Chemotherapy 40, 2505–10.[Abstract]

4 . Gill, M. J., Brenwald, N. P. & Wise, R. (1999). Identification of an efflux pump gene, pmrA, associated with fluoroquinolone resistance in Streptococcus pneumoniae. Antimicrobial Agents and Chemotherapy 43, 187–9.[Abstract/Free Full Text]

5 . Working Party of the British Society for Antimicrobial Chemotherapy. (1991). A guide to sensitivity testing. Journal of Antimicrobial Chemotherapy 27, Suppl. D, 1–50.[ISI][Medline]

6 . Gill, M. J. & Wise, R. (1998). Activity of grepafloxacin against pneumococci resistant to fluoroquinolones by a putative efflux mechanism. In Program and Abstracts of the Thirty-eighth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Diego, CA, 1998. Abstract C-52, p. 83. American Society for Microbiology, Washington, DC.