Non-specific, agar medium-related peaks can result in false positive Mycoplasma alkalescens and Mycoplasma arginini identification by MALDI-TOF MS

MALDI-TOF MS is a fast and accurate tool to identify Mycoplasma species in liquid media. However, when trying to identify presumptive Mycoplasma bovis (M. bovis) colonies from solid medium (the "direct transfer method") a surprisingly high occurrence of M. arginini and M. alkalescens identification was observed. It was hypothesized that agar medium components are associated with false positive identification with Mycoplasma spp., as M. bovis colonies are very small and grow into the agar. The objective of this study was to determine whether complete modified pleuropneumonia-like organism (PPLO) agar (supplemented with horse serum, sodium pyruvate, technical yeast extract, ampicillin sodium salt and colistin) and the separate components, result in false identification as Mycoplasma spp. by MALDI-TOF MS. A total of 100 samples were examined, of which 33% of the modified PPLO agar spots were identified as M. alkalescens (16%) and M. arginini (17%)), albeit with relatively low score values ( < 1.85). No false identification of M. bovis was obtained. Several medium components (unsupplemented PPLO agar, horse serum and colistin) resulted in spectra with peaks showing close matches with peaks present in the M. alkalescens and M. arginini database spectra. This study shows that the direct transfer method should be interpreted with caution, and one should strive to pick as little as possible agar when sampling Mycoplasma-like colonies from solid medium containing PPLO agar, horse serum and/or colistin.