Determination of MICs in the routine laboratory

Pramod M. Shah,*

Klinikum der Johann Wolfgang Goethe-Universität, Frankfurt am Main, Germany

Sir,

In the age of automation and commercially available microtitre plates for MIC determination, more and more laboratories use these methods, although the report of the Working Party on Susceptibility Testing of the BSAC clearly shows that in the routine laboratory, an agar diffusion technique, if performed correctly, delivers results satisfactory for patient care.1 Post-commercially available MIC microtitre plates lack concentrations at the lower end of the panel. As a result, low MICs are reported as ‘<= xyz mg/L’, thus the ‘true’ MIC is often not determined. I randomly selected a recent publication from the Journal. Johnson et al.2 report a MIC range of <=0.06–2 and MIC50 and MIC90 of <=0.06 mg/L of penicillin for surveillance isolates of pneumococci from England and Wales. The MIC of penicillin was not determined for at least 90% of the strains.

When using commercially available MIC plates, one ends up working with an extended ‘breakpoint’ method, as the number of dilutions is generally limited. Routine methods should be designed to detect slight but clinically significant shifts in susceptibility patterns (see gradual shift in susceptibility to penicillin in Neisseria gonorrhoeae,3 or more recently in Streptococcus pneumoniae,4 or to ciprofloxacin in Salmonella5). If MIC determination is used, concentrations chosen should allow end-point determination. To my knowledge, PROTEKT (Prospective Resistance Organism Tracking and Epidemiology for the Ketolide Telithromycin) has initiated a programme to do just this, albeit for limited species of bacterial pathogen. At present, Etest is the one (if not only) commercially available test method that allows ‘true’ determination of MIC.

Would we accept as correct that the distance between two atoms was <=1 mm because the smallest distance the method employed could measure was 1 mm?

Notes

* Tel: +49-69-6301-6614; Fax: 49-69-6301-7717; E-mail: shah{at}em.uni-frankfurt.de Back

References

1 . Report of the Working Party on Susceptibility Testing of the British Society of Antimicrobial Chemotherapy. (2001). Antimicrobial susceptibility testing. Journal of Antimicrobial Chemotherapy 48, Suppl. S1, 1–102.[Abstract/Free Full Text]

2 . Johnson, A. P., Warner, M., George, R. C. & Livermore, D. M. (2001). Activity of moxifloxacin against clinical isolates of Streptococcus pneumoniae from England and Wales. Journal of Antimicrobial Chemotherapy 47, 411–5.[Abstract/Free Full Text]

3 . Thornsberry C., Gavan T. L. & Gerlach E. H. (1977). New developments in antimicrobial agent susceptibility testing. In Cumitech 6, (Sherris, J. C., Ed.). American Society for Microbiology, Washington, DC.

4 . Saah, A. J., Mallone, J. P., Tarpay, M. M., Thornsberry, C. T., Roberts, M. A. & Rhoades, E. R. (1980). Relative resistance to penicillin in the pneumococcus: a prevalence and case–control study. Journal of the American Medical Association 243, 1824–7.[ISI]

5 . Threlfall, E. J., Ward, L. R., Skinner, J. A., Smith, H. R. & Lacey, S. (1999). Ciprofloxacin-resistant Salmonella typhi and treatment failure. Lancet 353, 1590–1.[ISI][Medline]