Microbiology Laboratory, Department of Chemical and Environmental Sciences, University of Limerick, Ireland
Sir,
The enterococci are a cause of nosocomial urinary tract infections (UTIs) and are generally associated with instrumentation or structural abnormalities of the urinary tract. Such infections may be treated with a number of antimicrobial agents including ampicillin, nitrofurantoin and vancomycin.1 Ciprofloxacin, considered to have only modest activity against enterococci,2 is not used as a drug of first choice but has been successfully employed in the treatment of enterococcal UTIs.3
A report by Perry et al.2 indicated problems associated with ciprofloxacin susceptibility testing with the enterococci. Following the Stokes/Comparative method of disc diffusion proposed by the BSAC Working Party,4 the authors indicated use of a ciprofloxacin 5 µg disc, as opposed to the recommended 1 µg disc, to be necessary for discrimination between susceptible and resistant enterococcal isolates.
New BSAC criteria for disc diffusion testing have since been developed including ciprofloxacin disc content, zone diameter breakpoints and corresponding MICs, specifically for urinary pathogens.5 These guidelines indicate MICs 8 mg/L and zones of inhibition
17/19 mm for resistant Gram-positive/negative isolates and MICs
4 mg/L and inhibition zones measuring
18/20 mm for susceptible Gram-positive/negative isolates.
In our laboratory we tested a total of 228 clinical isolates (all urinary unless otherwise stated) against ciprofloxacin in accordance with most recent BSAC guidelines.5 The isolates included both Gram-positive cocci [87 Enterococcus faecalis, eight Enterococcus faecium, three Enterococcus avium, 101 Staphylococcus aureus (urinary and surgical wound infection), one Staphylococcus hominis] and Gram-negative bacilli (18 Escherichia coli, three Pseudomonas aeruginosa, one Pseudomonas fluorescens, three Proteus mirabilis, one Klebsiella pneumoniae, one Klebsiella oxy-
toca, one Citrobacter freundi). Disc diffusion susceptibility testing was performed using the recommended 1 µg disc (Oxoid, Basingstoke, UK) with the addition of a 5 µg ciprofloxacin disc (Oxoid) for tests incorporating enterococci. Ciprofloxacin MICs were determined by broth macrodilution in IsoSensitest broth (Oxoid) with an inoculum of 5 x 105 cfu/mL.4
For non-enterococcal isolates, resistance categories from broth dilution and disc diffusion tests incorporating the ciprofloxacin 1 µg disc correlated well. Susceptibility testing of the 138 non-enterococcal isolates revealed a total of three major errors (identification of susceptible isolates as resistant) and one very major error (identification of a resistant isolate as susceptible) with disc diffusion testing, all of which were associated with S. aureus.
In contrast, analysis of results generated for the enterococci revealed disparity between MIC resistance categories and results from disc diffusion assays incorporating the 1 µg ciprofloxacin discs (Table). Of the 98 enterococci tested, 52 (53.1%) expressed MICs
4 mg/L (susceptible) with the remaining 46 (46.9%) isolates having MICs
8 mg/L (resistant) (range 16>128 mg/L).
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Significantly, testing of enterococcal isolates with a ciprofloxacin 5 µg disc produced excellent correlation between MIC and disc diffusion resistance categories, identifying all enterococcal isolates with MICs 4 mg/L as susceptible and 45 of 46 isolates with MICs
8 mg/L as resistant. The one E. faecalis isolate (MIC 32 mg/L) producing a zone of inhibition of 17 mm with the ciprofloxacin 1 µg disc measured a zone of 27 mm with the ciprofloxacin 5 µg disc indicating susceptibility and thus constituting a single very major error.
Despite the introduction of a revised BSAC disc diffusion susceptibility testing method, the results discussed here concur with those presented by Perry et al.,2 indicating that accurate discrimination of susceptible and resistant enterococcal isolates by disc diffusion requires the inclusion of a ciprofloxacin 5 µg disc. We acknowledge that the disc diffusion criteria provided are not necessarily intended for testing of enterococci, which may account for the errors associated with the test. Similarly, these criteria have not been specified for the other isolates examined in this study; however, correlation of the disc diffusion and broth dilution results for these isolates was excellent.
The antibiotic resistance profiles of uropathogens in both community- and hospital-acquired infections have been changing in recent years and antibiotic resistance has become a major concern.6 Quinolones have good activity against urinary pathogens and are valuable agents in the treatment of complicated UTIs.7 From an epidemiological standpoint inclusion of a ciprofloxacin 5 µg disc in susceptibility tests would provide an accurate insight into the level of ciprofloxacin resistance in the enterococci. Similarly, given the increasing incidence of multidrug-resistant bacteria and the possibility of requiring second choice antibiotics for effective therapy, it is imperative that antimicrobial susceptibility test results be as accurate as possible, enabling consideration of alternative therapy where necessary.
Acknowledgments
The authors would like to acknowledge the assistance afforded by the personnel of the Microbiology Laboratory at the Regional Hospital, Limerick. This work is funded by an Enterprise Ireland Strategic Research Grant (no. ST99/043).
Notes
Corresponding author. Tel: +353-61-202646; Fax: +353-61-202568; E-mail: catherine.adley{at}ul.ie
References
1 . Moellering, R. C., Jr (1992). Emergence of Enterococcus as a significant pathogen. Clinical Infectious Diseases 14, 11738.[ISI][Medline]
2 . Perry, D. J., Ford, M. & Gould, F. K. (1994). Susceptibility of enterococci to ciprofloxacin. Journal of Antimicrobial Chemotherapy 34, 2978.[ISI][Medline]
3 . Landman, D. & Quale, J. M. (1997). Management of infections due to resistant enterococci: a review of therapeutic options. Journal of Antimicrobial Chemotherapy 40, 16170.[Abstract]
4 . Working Party of the British Society for Antimicrobial Chemotherapy. (1991). A guide to sensitivity testing. Journal of Antimicrobial Chemotherapy 27, Suppl. D, 150.[ISI][Medline]
5 . Working Party of the British Society for Antimicrobial Chemotherapy. (2000). BSAC standardized disc sensitivity testing method. [Available on-line.] http://www.bsac.org.uk
6 . Chomarat, M. (2000). Resistance of bacteria in urinary tract infections. International Journal of Antimicrobial Agents 16, 4837.[ISI][Medline]
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Wise, R., Jarlier, V., Naber, K. G., Graninger, W., Nicolle, L. E., Hooton, T. et al. (2000). Progress in the management of urinary tract infections: discussion. Journal of Antimicrobial Chemotherapy 46, Suppl. 1, 635.