Comparison of growth,nutritional utilisation patterns,and niche overlap indices of toxigenic and atoxigenic Aspergillus flavus strains

The effects of temperatures (20–30 °C) and water activity (0.90–0.99 a_w) on the lag phase duration, mycelial growth, and nutritional utilisation patterns of two toxigenic (AFL1⁺ & AFL2⁺) and three atoxigenic (AFL1⁻, AFL2⁻, & AFL3⁻) Aspergillus flavus strains were evaluated in vitro. Both temperature and a_w and their interactions had a significant influence on the growth and nutritional utilisation patterns (p < 0.05). There were no significant differences between toxigenic and atoxigenic strains in terms of lag phase prior to growth and mycelial growth rates. Based on carbon source (CS) utilisation patterns, toxigenic and atoxigenic strains' niche size was greater at higher temperatures and in wetter conditions. Additionally, based on niche overlap indices (NOIs), regardless of temperature, when water was freely available, atoxigenic and toxigenic strains co-existed. However, under moisture stress, the nutritional competitiveness was variable. Temporal carbon utilisation sequences (TCUS) of toxigenic and atoxigenic strains were compared. At 0.99 a_w most CS sources were utilised by the strains and the time to detection (TTD) of each strain was shortest on monosaccharides at the same level of a_w. Conversely, under moisture stress the least number of CS was utilised. The current study has demonstrated that carbon utilisation patterns are equally important as are other determinants of competitiveness and that growth rate alone is not a key attribute which determines competitiveness.     

Effect of environmental factors on in vitro and in situ interactions between atoxigenic and toxigenic A. flavus strains and control of aflatoxin contamination of maize

Abstract

The potential of three atoxigenic strains from different geographical origins in Africa were examined for in vitro and in situ competitiveness against two toxigenic strains of Aspergillus flavus and subsequent inhibition of aflatoxin B₁ (AFB₁) production under different environmental conditions. Temperature, water activity (a _w) and substrate influenced the types of interaction between the three AFL^? and two AFL⁺ strains. The competitiveness and AFB₁ reduction ability of the three atoxigenic strains when interacting with the two toxigenic strains were evaluated by inoculation of 100, 25:75, 50:50, 75:25 and 100% ratios of mixed spore suspensions in vitro on malt extract and milled maize agars over 28 days and in situ on stored maize grain for 14 days, respectively at 0.99, 0.96 and 0.90 a _w. For all the treatments, the effect of a _w and inoculum ratio and their interaction was highly significant. Toxin inhibition was >80% in vitro at both 0.99 and 0.96 a _w. In situ AFB₁ reduction was influenced by the toxigenic strain assayed, a _w and the inoculum ratio. Where control was achieved, it was more variable at 0.96 a _w, while with more stringent water stress conditions (0.90 a _w) the percentage inhibition was up to 77.2%. The study shows the importance of including environmental factors in screening and identifying effective atoxigenic strains for control of AFs (aflatoxins).     

Mycotoxigenic fungi and mycotoxins associated with stored maize from different regions of Lesotho

Samples of stored maize from villages located in five different agroecological zones (southern lowlands, northern lowlands, Senqu river valley, foothills and mountains) of Lesotho were collected in 2009/10 and 2010/11 and assessed for contamination with toxigenic fungi. The water activity of all samples collected during the two seasons was <0.70. The total fungal populations of the maize from different regions in the two seasons was not significantly different (p?>?0.05). Fusarium verticillioides, F. proliferatum and F. subglutinans predominated in different regions in both seasons based on molecular analyses. In the 2009/10 season, the isolates of these species all produced FB₁, while in the 2010/11 season, very few produced FB₁. A. flavus isolates (2009/10) were recovered from mountains and Senqu river valley samples while the 2010/11 isolates were predominantly from the foothills and northern lowlands. The mountain isolates of Aspergillus section Flavi produced the highest levels of AFB₁ (20 mg kg^?1). Aspergillus parasiticus was only isolated from the foothills, Senqu river valley and southern lowlands samples, and the AFB₁ levels produced ranged from ‘none detected’ to 3.5 mg kg^?1. The Aspergillus ochraceous isolates were least frequently encountered in both seasons. In the 2009/10 season, the isolates from the northern lowlands produced ochratoxin A (OTA) in culture. No isolates of A. niger from different regions in both seasons produced any OTA. Multi-mycotoxin analyses of the maize samples were done for a range of mycotoxins. At least one sample from each region in both seasons was FB₁-positive. FB₁ levels for 2010/11 samples (7–936 μg kg^?1) were higher than in the 2009/10 season (2–3 μg kg^?1). In both seasons, the mountains registered the highest levels of FB₁. Deoxynivalenol (DON) was recovered from all the samples analysed, with the highest mean contamination of 1,469 μg kg^?1 in samples from the northern lowlands. Moniliformin (MON) was detected from all agroecological zones in the two seasons (5–320 μg kg^?1 in 2009/10; 15–1,205 μg kg^?1 in 2010/11). Emerging toxins such as fusaproliferin (FUS) and beauvericin (BEA) were also detected. OTA was not detected in any of the samples analysed. Only one 2009/10 sample in the Senqu river valley was positive for AFB₁. This is the first report on toxigenic fungi and multi-mycotoxin contamination of maize samples from subsistence farmers’ stores in different agroecological zones of Lesotho.     

Mokola virus involved in a human contact (South Africa)

Isolations of Mokola virus (MOKV) are rare, but in South Africa and Zimbabwe this genotype 3 lyssavirus variant has been occasionally found in domestic mammals (cats and a dog) with a total of 17 virus isolates (South Africa 10, Zimbabwe 7) having been recovered during the past 30 years. We report the identification of a MOKV isolate involved in a human contact in Grahamstown (Eastern Cape, South Africa) and a genetic comparison with previously characterized isolates. This reported MOKV case was in a previously immunized cat. While the continual recovery of MOKV isolates in domestic cats is speculative of the existence of a reservoir host species among bats or rodents, the lack of protection with currently used vaccines is discussed and the need for biologicals with a wider spectrum of protection against this lyssavirus variant is highlighted.