In-vitro activity of gatifloxacin, a novel fluoroquinolone, compared with that of ciprofloxacin against Legionella spp.

J Antimicrob Chemother 1999; 44: 295–297

Susan L. Pendland*, Karen J. Losnedahl and Christopher A. Schriever

The University of Illinois at Chicago, Department of Pharmacy Practice, Microbiology Research Laboratory, 833 South Wood Street, Chicago, IL 60612, USA

Sir,

Gatifloxacin, also known as BMS-206584, AM-1155 and CG-5501, is a novel 8-methoxy fluoroquinolone with activity against a broad-spectrum of bacteria. In the present study we have evaluated the in-vitro activity of this agent against Legionella spp. and compared it with that of the older fluoroquinolone, ciprofloxacin, which has previously been shown to be active in vitro against these pathogens and to be effective when used as treatment of patients with infections caused by them. 1,2

Gatifloxacin was obtained from Bristol-Myers Squibb (Wallingford, CT, USA) and was prepared according to the manufacturer's instructions with dimethylsulphoxide (DMSO) as the solvent and sterile distilled water as the diluent. Ciprofloxacin, which was obtained from United States Pharmacopeia (Rockville, MD, USA), was prepared according to guidelines of the National Committee for Clinical Laboratory Standards (NCCLS). 3 The organisms used in the study were provided by the University of Illinois Hospital (Chicago, IL, USA), Northwestern University Hospital (Chicago, IL, USA), Abbott Laboratories (Chicago, IL, USA), the Centers for Disease Control (Atlanta, GA, USA) and the American Type Culture Collection (ATCC) (Rockville, MD, USA), and included 41 non-replicate clinical isolates and five ATCC strains (Legionella pneumophila33152, Legionella bozemanii 33217, Legionella dumoffii 33279, Legionella gormanii 33297 and Legionella longbeachae 33462). Of the clinical isolates, 34 were identified as L. pneumophila and the remaining seven as L. bozemanii (n= 2), L. longbeachae (n= 2) and one each of L. dumoffii, Legionella erythra and Legionella micdadei.

MICs were determined by an agar dilution method recommended by the NCCLS. 3 The medium used was buffered charcoal yeast extract (BCYE{alpha}) (Oxoid-Unipath, Ogdensburg, NY, USA) but, as MICs determined with this medium are often falsely high because of inactivation of the antibiotics by the charcoal, 4,5 we also carried out susceptibility testing with a less inhibitory medium, buffered starch yeast extract (BSYE); 6 the antibiotic concentrations in the media ranged from 0.0005 to 8 mg/L in BCYE{alpha} and 0.0005–0.5 mg/L in BSYE. Inocula were prepared by suspending organisms in sterile distilled water and adjusting the turbidity with a spectrophotometer at 625 nm so that it was equivalent to that of a 0.5 McFarland standard. A replicator device was used to inoculate approximately 8 µL of each suspension on to the surfaces of the BSYE plates, giving final inocula of approximately 10 6 cfu/spot. The suspensions were then diluted 1:100 with sterile distilled water and similarly inoculated on to the surfaces of the BCYE{alpha} plates, giving final inocula of approximately 10 4 cfu/spot. Control plates of each medium containing DMSO and sterile distilled water and blood agar plates (to ensure the purity of the inocula) were included. All tests were performed in duplicate and the plates were incubated at 35°C for 48 h in a humidified environment. The MIC was taken as the lowest concentration of each antibiotic that allowed no growth or only a barely visible haze.

While all 46 strains grew well on BCYE{alpha} agar, seven (two of L. pneumophila and five of the non-pneumophila Legionella spp.) failed to grow on BSYE, even when the higher inoculum was used. The use of DMSO as a solvent for gatifloxacin did not affect the growth of any of the isolates.

The MICs of gatifloxacin and ciprofloxacin are summarized in the Table. The MIC90s of both quinolones for the L. pneumophila and non-pneumophila Legionella spp. strains were 1 mg/L on BCYE{alpha} and 0.03 mg/L on BSYE. The lower MICs obtained with the non-charcoal-containing medium are believed to more accurately reflectthe susceptibilities of Legionella spp. isolates. Unfortunately, however, BSYE and other less antagonistic media do not support the growth of all strains. 4,5


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Table. MICs of gatifloxacin and ciprofloxacin for Legionella spp. strains, as determined with two different media
 
The data reported here demonstrate that gatifloxacin is active in vitro against Legionella spp. (both L. pneumophila and non-pneumophila Legionella spp.), the MIC90s of this agent being identical to those of ciprofloxacin. On the basis of MIC breakpoints of <=2 mg/L for gatifloxacin 7 and <=1 mg/L for ciprofloxacin, 8 all of the isolates were considered highly susceptible to both agents when MICs were determined with BSYE. When susceptibility testing was performed with BCYE{alpha}, all of the strains were susceptible to gatifloxacin and all but one strain of L. pneumophila,which exhibited intermediate susceptibility (MIC, 2 mg/L), were susceptible to ciprofloxacin. In a study recently published in this Journal, Croco et al., who used the Etest method with supplemented BCYE{alpha} agar, reported the MIC90 of gatifloxacin for 103 Legionella spp. isolates to be 0.38 mg/L (range 0.125–0.5 mg/L). 9 This value is approximately three-fold lower than those obtained in the present study with the agar dilution method and the same medium, but >10-fold higher than those obtained with BSYE.

In the light of the good in-vitro activity of gatifloxacin against Legionella spp. isolates, as demonstrated by a range of susceptibility testing methods, we believe that this antibiotic warrants further investigation as treatment of patients with infections caused by these organisms.

Acknowledgments

This work was supported by a grant from Bristol-Myers Squibb.

Notes

* Corresponding author. Tel: +1-312-996-8639; Fax: +1-312-413-1797; E-mail: Pendland{at}uic.eduPendland@uic.edu Back

References

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7 . Nakashima, M., Uematsu, T., Kosuga, K., Kusajima, H., Ooie, T., Masuda, Y. et al. (1995). Single- and multiple-dose pharmacokinetics of AM-1155, a new 6-fluoro-8-methoxy quinolone, in humans. Antimicrobial Agents and Chemotherapy 39, 2635 –40.[Abstract]

8 . National Committee for Clinical Laboratory Standards. (1999). Performance Standards for Antimicrobial Susceptibility Testing—Ninth Informational Supplement: Approved Standard M100-S9. NCCLS, Wayne, PA.

9 . Croco, M. A. T., Biedenbach, D. J., Pfaller, M. A., Doern, G. V. & Jones, R. N. (1984). In-vitro activities of gatifloxacin, sparfloxacin and trovafloxacin against 103 strains of Legionellaspp. Journal of Antimicrobial Chemotherapy 42, 672–3.[Free Full Text]