PHENOTYPIC VARIATION AS A SIGNIFICANT WITHIN-STRAIN FACTOR IN COMPARISONS OF BACTERIA BASED ON PYROLYSIS MASS-SPECTROMETRY

Three separate colonies of a single strain of Staphylococcus aureus we re each subcultured ten times under identical conditions to form three source groups (labelled M1, M2 and M3), each of which was sampled 30 times. All 90 samples were examined by pyrolysis-mass spectrometry (Py -MS). When the total data from M1, M2 and M3 were compared in canonica l variate analysis, all differences were contained within the 95% conf idence limit. However, significant differences between the groups were observed when data from fewer than six subcultures per group were use d. These results suggest that phenotypic variation due to growth withi n a single bacterial strain prior to pyrolysis can produce sufficient significant differences on Py-MS analysis to prevent reliable strain d efinition.A similar experiment was performed using three serum specime ns (two of which were identical) as the source groups. The significanc e/non-significance of any differences remained consistent, whatever am ount of data used in the comparisons. The use of components of the bac terial cell in preference to the whole cell and, thereby, the avoidanc e of growth prior to Py-MS is suggested as an alternative to the use o f multiple subcultures as a method of controlling the effect of phenot ypic variation and improving the reproducibility of the technique.