Mutants of Streptococcus lactis subsp. diacetylactis lacking diacetyl reductase activity

Three strains of Streptococcus lactis subsp. diacetylactis, namely DRC-1, DRC-2 and DRC-3 which produced diacetyl up to 120 h of incubation were exposed to the ultraviolet irradiation as well as N-methyl-N'-nitro-N-nitrosoguanidine (NTG) to isolate mutants lacking diacetyl reductase activity. UV irradiation did not produce any isolate completely devoid of diacetyl reductase activity, though, 99.5% loss in activity could be achieved. NTG treatment proved to be more effective and seven survivors exhibiting complete loss of diacetyl reductase activity were recovered. These altered characteristics were retained on repeated subculturing.