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 were 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 canonical variate analysis, all differences were contained within the 95% confidence limit. However, significant differences between the groups were observed when data from fewer than six subcultures per group were used. These results suggest that phenotypic variation due to growth within a single bacterial strain prior to pyrolysis can produce sufficient significant differences on Py-MS analysis to prevent reliable strain definition. A similar experiment was performed using three serum specimens (two of which were identical) as the source groups. The significance/non-significance of any differences remained consistent, whatever amount of data used in the comparisons. The use of components of the bacterial cell in preference to the whole cell and, thereby, the avoidance of growth prior to Py-MS is suggested as an alternative to the use of multiple subcultures as a method of controlling the effect of phenotypic variation and improving the reproducibility of the technique.