Errors associated with determining the susceptibilities of staphylococci to trimethoprim by the Vitek GPS-AK card

J Antimicrob Chemother 1999; 44: 293–294

Ian C. Carmichael*, Valerie Godfrey and Gary Nicholson

Department of Medical Microbiology, Level 6, Medical School, Ninewells Hospital, Dundee DD1 9SY, UK

Sir,

The Vitek system (bioMérieux, Hazelwood, MO, USA) was recently introduced into our laboratory as the principal means of identifying clinical isolates and performing susceptibility testing. However, concerns about the system's ability to accurately detect susceptibility to trimethoprim arose when the number of strains of methicillin-resistant Staphylococcus aureus (MRSA) resistant to this agent increased. Although trimethoprim is not normally regarded as first-line treatment of patients with infections caused by MRSA, it has been used successfully in combination with various other agents as oral therapy 1,2 and may also have some value as monotherapy, owing to its in-vitro activity against most staphylococci, including MRSA. 3 Trimethoprim has the added advantages of being inexpensive, being available as an oral formulation, being well absorbed and having an excellent safety profile. In common with antibiotics in general, any apparent increase in the incidence of resistance to trimethoprim will limit its potential as treatment and the investigations described here were undertaken in an attempt to identify an explanation for this phenomenon.

The susceptibilities of 100 non-replicate clinical isolates of MRSA to trimethoprim were determined in duplicate by three different methods. Vitek GPS-AK cards were inoculated, susceptibilities read according to the manufacturer's instructions and susceptibility categories assigned on the basis of MIC breakpoints recommended by the National Committee for Clinical Laboratory Standards (NCCLS), i.e. susceptible, MIC <=2 mg/L and resistant, MIC >=4 mg/L. 4 The Stokes' disc diffusion method was performed according to a method described previously 5 with DST agar (Oxoid, Basingstoke, UK) supplemented with 2% lysed horse blood, discs (Oxoid) containing 2.5 µg of trimethoprim and inocula with turbidities equivalent to that of a 0.5McFarland standard. The Oxford strain of S. aureus wasused as the control and the zones of inhibition were read after overnight incubation at 37°C. Susceptibility was defined as a zone of inhibition greater than or equal to that of the control or not >3 mm smaller and resistance as a zone of inhibition >3 mm smaller than that of the control. Finally, MICs were determined by an agar dilution method recommended by the NCCLS; 4 an MIC breakpoint of <=2 mg/L was used to define susceptibility. 6

Eighty-seven of the 100 isolates were identified as susceptible to trimethoprim by both the disc diffusion and agar dilution methods; MICs ranged from 0.25 to 1 mg/L. Of these 87, 36 (41%) were classified as susceptible and 32 (37%) as resistant with the Vitek card. Susceptibility testing of the remaining 19 strains by this method was inconclusive, the isolates being susceptible on one of the duplicate testings and resistant on the other, i.e. either 1 mg/L or 2 mg/L and >=4 mg/L respectively. Thirteen isolates were found to be resistant by all three methods. Preliminary investigations with coagulase-negative staphylococci and methicillin-susceptible strains of S. aureus revealed patterns similar to that observed with the MRSA strains (data not shown).

In summary, the susceptibilities of 100 clinical isolates of MRSA to trimethoprim, as determined by the disc diffusion and agar dilution methods, were identical, with 87 strains being categorized as susceptible. The Vitek GPS-AK card, on the other hand, incorrectly designated 32 of these 87 strains as resistant—a major error rate of 37%. Moreover, the results of susceptibility testing of a further 19 strains were not reproducible and, therefore, inconclusive. These observations suggest that the Vitek GPS-AK card is an unreliable method of determining the susceptibilities of staphylococci to trimethoprim.

Notes

* Corresponding author. Tel: +44-1382-660111. Back

References

1 . Rahman, M. (1998). Alternatives to vancomycin in treating methicillin-resistant Staphylococcus aureus infections. Journal of Antimicrobial Chemotherapy 41, 325–8.[Free Full Text]

2 . Chambers, H. F. (1997). Methicillin resistance in staphylococci: molecular and biochemical basis and clinical implications. Clinical Microbiology Reviews 10, 781–91.[Abstract]

3 . Lorian, V. (1996). Susceptibility to antibiotics. In Antibiotics in Laboratory Medicine, 4th edn (Lorian, V., Ed.), p. 1008. Williams & Wilkins, Baltimore, MD, USA.

4 . National Committee for Clinical Laboratory Standards. (1992). Performance Standards for Antimicrobial Susceptibility Testing—Fourth Informational Supplement: Approved Standard M100-S4. NCCLS, Villanova, PA.

5 . Scott, A. C. (1989). Laboratory control of antimicrobial therapy. In Practical Medical Microbiology, 13th edn (Collee, J. G., Duguid, J. P., Fraser, A. G. & Marmion, B. P., Eds), pp. 161–81. Churchill Livingstone, Edinburgh, UK.

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





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