Differential analysis of secondary metabolites by LC-MS following strain improvement of Streptomyces lydicus AS 4.2501.

Metabolite variations in a high-yielding mutant and its parent strain were studied by comparative LC-MS analysis after strain improvement. Streptomyces lydicus AS 4.2501-P28, a propionate-resistant mutant isolated by the high-frequency screening method using the principle of eliminating precursor inhibition effects, showed an increase of 267% in streptolydigin titre over the starting strain. Culture extracts of this mutant and its parent strain were analysed in parallel by an LC-MS technique, including full scan and extracted-ion scan, ESI-MS (electrospray-ionization MS) detection, DAD (diode-array detection) and MS2 (tandem MS) measurement. The main metabolic variations were obviously found in intermediates, metabolites and biosynthetic pathways: two unknown metabolites with the molecular [M-H]- ions at m/z 423.3 and 687.2, corresponding to two branch pathways, were blocked in the mutant, and the accumulation of a significant intermediate at m/z 363.1 [M-H]- decreased dramatically in the mutant cultures, resulting in the overproduction of streptolydigin (an antibiotic that inhibits prokaryotic RNA polymerase) in the mutant. Ion fragmentations of the tandem-MS spectra provided experimental evidence for the structural characterization of the three compounds obtained. In comparison with the traditional methods, comparative LC-MS analysis was rapid, sensitive and suitable for characterizing intermediates, metabolites and pathways for elucidation of the metabolic alterations after the isolation of improved strains.