In order to confirm and extend the genus and species characterization
of the environmental isolates, protein extracts were
separated on one dimensional SDS PAGE gels. M. luteus ATCC 4698
(and luteus ATCC 49732) were used as reference strains. Selected
gel bands were excised from electrophoretic bands and digested
with trypsin. The MALDI TOF spectra were very similar among all
environmental isolates (including the 2 strains [ASO3-C10 and
ASO3-C17] that displayed aberrant fermentation) and the ATCC
reference strains (ATCC 4698), suggesting their identity as M. luteus
(see Table 1 for list of experimental ions). However, the mass
spectrum of ATCC 4698 was the only strain to additionally display
the prominent m/z 1623.9 peak. The mass 1623.9 in the other reference was not present in M. luteus ATCC 49732. Mass 1623.9 was presumably generated by mutation (K)GVLDVQGVEYEIFR(L)
(ATCC 4698) to (K)GVLDVKGAEYEIFR(L) (SK 58). The presence of
the additional lysine (SK58) generates an additional trypsin
cleavage site, producing two peptides. These results are consistent
with M. luteus not being a homogenous species. It has been
proposed that M. luteus has 3 biovars [9] based on 16S rRNA
sequence comparisons.