Growth and aflatoxin production by Aspergillus parasiticus when in the presence of Streptococcus lactis

Streptococcus lactis was grown with Aspergillus parasiticus in modified APT broth. Three inoculation procedures were used: (a) S. lactis was grown 3 days, then conidia of A. parasiticus were added (SLAP), (b) both organisms were added simultaneously (ST) and (c) A. parasiticus was grown 3 days, then S. lactis was added (APSL). At 3, 6 and 10 days of incubation, contents of flasks were analyzed for growth of each organism, pH of broth and aflatoxin content. S. lactis did not survive past 3 days when grown alone. In ST cultures, S. lactis grew to the same extent as in the control at 3 days; it remained viable at a low level through 10 days. In APSL cultures, S. lactis growth was inhibited at 3 days but the bacterium survived through 7 days (10 days of mold growth) at reduced numbers. At 3 days there were no appreciable differences in growth of A. parasiticus. At 6 days, in ST and SLAP cultures, growth of the mold was inhibited, while in the APSL culture growth increased over that in the control. At 10 days, growth of mold was somewhat increased over the control in all test conditions. The pH of broth in the A. parasiticus control and APSL culture was 6 at 3 days, dropped to 4.5–4.6 at 6 days and rose to 7 by 10 days. In ST and SLAP cultures, the pH was at 4.1 at 3 days and rose to pH 7 by 10 days. Aflatoxin (B₁ plus G₁) content was lowest at 3 days and increased at 6 days. Between 6 and 10 days two patterns were observed. In APSL and SLAP cultures, aflatoxin content decreased, while it increased in the ST culture. These patterns occurred when aflatoxin content was expressed on a total or per gram of dried mycelium basis. At 3 days the amounts of aflatoxin B₁ and G₁ were approximately equal. Between 3–6 days the amount of G₁ increased more rapidly than that of B₁. Between 6 and 10 days in the ST culture, the amount of G₁ increased at a slower rate than that of B₁ while in SLAP and APSL cultures, the amount of G₁ decreased more rapidly than that of B₁. When a different lot of the same medium was used, aflatoxin production was greatly reduced. The pH of broth at all test conditions rose through the incubation period.