Toxigenic potential of Aspergillus species occurring on maize kernels from two agro-ecological zones in Kenya

Abstract

Two agro-ecological zones in Kenya were selected to compare the distribution in maize of Aspergillus spp. and their toxigenicity. These were Nandi County, which is the main maize growing region in the country but where no human aflatoxicoses have been reported, and Makueni County where most of the aflatoxicosis cases have occurred. Two hundred and fifty-five households were sampled in Nandi and 258 in Makueni, and Aspergillus was isolated from maize. Aspergillus flavus and A. parasiticus isolates were tested for the presence of aflD and aflQ genes. Positive strains were induced to produce aflatoxins on yeast extract sucrose and quantified using liquid chromatography-tandem mass spectrometry (LCMSMS). Aspergillus flavus was the most common contaminant, and the incidence of occurrence in Nandi and Makueni was not significantly different (82.33% and 73.26%, respectively). Toxigenic strains were more prevalent than non-toxigenic strains. All the toxigenic strains from Makueni were of the S-type while those from Nandi belonged to the l-type. Quantitative differences in aflatoxin production in vitro between isolates and between strains were detected with S strains producing relatively larger amounts of total aflatoxins, B toxins and lower values for G toxins. This was in accord with the frequent aflatoxicosis outbreaks in Makueni. However some L strains produced considerable amounts of B toxins. Given the widespread distribution of toxigenic strains in both regions, the risk of aflatoxin poisoning is high when favorable conditions for toxin production occur.

Figure 1

Aspergillus isolates grown on Aspergillus flavus parasiticus agar (AFPA) medium to identify members belonging to Section Flavi; (a) Positive isolate (b) Negative isolate.

Figure 2

Agarose gel electrophoretic pattern of polymerase chain reaction (PCR) products expressing aflD and aflQ genes. (a) Aspergillus flavus isolates from Makueni county : M—molecular weight 100 bp ladder (Promega); -C—Negative control 3VM482dg; -C—Negative control 1VM291br; +C—Positive control 1VM118g; +C— (2VM963Lg); 1—2M1090; 2—2VM983br; 3—1VM147g; 4—1VM97yg; 5—3VM566g; 6—2VM902br; 7—1VM83y; 8—3VM566Lg; 9—1VM403g; 10—2VM964yg; 11—1VM132g; 12—1VM408dg; 13—1VM239g; 14—1VM79g; 15—3VM482dg; 16—1VM328g; 17—1VM175g; 18—3VM671g; 19—1VM39wy; 20—2VM966yg; 21—2M1223yg; 22—2VM882g; 23—2VM890g; 24—2VM892g; 25—1VM80sg. (b) Aspergillus flavus isolates from Nandi county: M—molecular weight 100 bp ladder; 1—BM69YG; 2-BM38YG; 3—BM73YG; 4—BMG; 5—BM1YG; 6—BM10YG; 7—BMYG

Figure 3

Colonies of non-aflatoxigenic and aflatoxigenic strains of Aspergillus flavus observed under ultra violet light. Strains were cultivated in Yeast Extract Sucrose (YES) medium and photographed on the third day of incubation at 28 °C.

Figure 4

Frequency of toxigenic strains of Aspergillus flavus in two zones in Kenya.