Aflatoxigenic fungi contaminating maize (zea mays l.) and the potential of biological control using atoxigenic aspergillus species

Abstract

Aflatoxin contamination of maize is a major risk to health and well-being of Kenyan people. Aflatoxin-producing moulds viz: Aspergillus flavus, A. parasiticus and A. nomius can infect grains from pre-harvest stages in the field through to post-harvest stages in the stores. In Kenya, deaths have resulted from dramatic outbreaks of aflatoxin poisoning. Existing strategies for control of aflatoxigenic moulds mainly based on moisture reduction have been ineffective in the warm and humid areas. Similarly, using solar driers is expensive for the small scale farmers. The aim of this study was to control the growth of aflatoxigenic Aspergillus species in maize and maize feeds using atoxigenic strains. Aspergillus moulds were isolated from 113 maize samples and 113 soil samples obtained randomly from maize farms and rural households in Makueni, Nyeri Central, Bungoma South, Moiben and Ugunja districts. Out of 174 isolates 82.3% originated from maize and 17.7% from soil. Makueni district had the highest Aspergillus incidence at 58.1% followed by Nyeri Central (12.6%), Moiben (10.3%), Ugunja (10.3%) and Bungoma South (8.6%). The isolates were identified to species level using cultural, morphological and microscopic characteristics resulting to 10 species namely; Aspergillus flavus (78.5% occurrence), A. versicolor (8.0%), A. parasiticus (3.4%), A. clavatus (2.3%), A. sydowii (2.3%), A. fumigatus (2.3%), A. glaucus (1.1%), A. nidulans (1.1%), A. candidus (0.6%) and A. wentii (0.6%). Their toxigenicity was established through yellow pigment formation in PDA media and colour changes upon exposure to 27% ammonia solution in YES media. A total of 6.9% of the isolates were aflatoxigenic while 93.1% were atoxigenic. The atoxigenic strains were screened for antimould activity against aflatoxigenic strains and those found to be inducing the largest inhibition zones and therefore most effective were; UM082 A. parasiticus (19.9 mm), NM084 A. flavus (18.2 mm), UM127 A. flavus (17.8 mm), BM071 A. flavus (17.7 mm), BM092 A. fumigatus (17.5 mm), and NM083 A. flavus (17.3 mm). These atoxigenic strains were most effective on five aflatoxigenic Aspergillus strains namely; T-RS013 A. flavus, T-RM023-2 A. flavus, T-RS162 A. versicolor, T-RS016 A. versicolor and T-BS203 A. versicolor. These results show that atoxigenic Aspergillus species can inhibit the growth and sporulation of aflatoxigenic moulds thereby minimizing subsequent aflatoxin production in maize. These atoxigenic strains or their products can be used in food preservation systems to minimize aflatoxin poisoning.