Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/30/2010
Publication Date: 3/1/2011
Citation: Mehl, H.L., Cotty, P.J. 2011. Sequence of host contact influences the outcome of competition among Aspergillus flavus isolates during host tissue invasion. Applied and Environmental Microbiology. 77(5):1691-1697. Interpretive Summary: Aflatoxins are carcinogenic mycotoxins produced by Aspergillus flavus. Currently, aflatoxin contamination of crops is managed in part by application of biocontrol strains of A. flavus that do not produce aflatoxin (called atoxigenic strains). Atoxigenic strains reduce aflatoxin contamination by competitive exclusion of aflatoxin producers. However, factors contributing to displacement of aflatoxin producers during crop infection are unknown. We examined the influence of initial host contact on the ability of isolates to competitively displace subsequent isolates. An advantage was conferred when an isolate contacted maize kernel surfaces one hour prior to a second isolate compared to simultaneous inoculation of the isolates. However, this advantage decreased with inoculum concentration, suggesting competition for limited resources (i.e. attachment sites, nutrients) as a mechanism for preemptive exclusion. Data suggest both timing and density of application are critical for success of biocontrol strains. Atoxigenic strains of A. flavus adapted to early sporulation and dispersal to susceptible crop components should be selected to maximize both ability to displace aflatoxin producers and ability to reduce aflatoxin contamination of target crops.
Technical Abstract: Biological control of aflatoxin contamination by Aspergillus flavus is achieved by competitive exclusion of aflatoxin producers by atoxigenic strains. However, factors dictating the extent to which competitive displacement occurs during host infection are unknown. The role of preemptive exclusion in determining the outcome of competition between pairs of A. flavus isolates coinfecting maize kernels was examined. Isolate success during tissue invasion and reproduction was assessed by quantification of isolate-specific single nucleotide polymorphisms using pyrosequencing. Isolates were either inoculated simultaneously or an isolate was inoculated one hour prior to a second isolate. Isolates varied in competitive ability, but compared to simultaneous inoculation, an advantage was conferred to an isolate when it contacted the host before the second isolate. This advantage decreased with conidial concentration suggesting competition for limited resources (i.e. attachment sites, nutrients) on the kernel surface as a mechanism for preemptive exclusion. Coating kernel surfaces with dead conidia did not reduce the ability of the first isolate to outcompete the second. Prior washing of kernels with either hexane or ethanol did not influence the magnitude of the advantage conferred to the first isolate. However, washing with the surfactant Tween-80 significantly increased the extent to which the second isolate was excluded. Removal of attachment sites or nutrients by surfactant may increase competition for a limiting resource, thereby increasing advantage conferred to the first isolate to contact the host. Preemptive exclusion occurred prior to conidia germination and maize kernel colonization. These results have important implications for both biocontrol and epidemiology of aflatoxin contamination.