Component of an enzyme that catalyzes the reduction of free flavins (FMN, FAD and riboflavin) by NADH; the reduced flavins produced by this reaction likely spontaneously react with oxygen, yielding hydrogen peroxide. Is responsible for the major H(2)O(2) production in L.johnsonii in the presence of oxygen. Cannot use NADPH instead of NADH as the electron donor. a reduced flavin + NAD(+) = an oxidized flavin + 2 H(+) + NADH Requires LJ_0548 for activity, but the exact composition of the enzyme is unclear. Cells lacking both LJ_0548 and LJ_0549 completely lack the NADH-dependent flavin reductase activity detected in wild-type strain, and show a 40-fold reduction of hydrogen peroxide formation upon exposure to oxygen. Reductase activity and H(2)O(2) production in this mutant can only be restored by in trans complementation of both genes. Belongs to the NADH-dependent flavin reductase family.