Physiological studies of type cultures of Neurospora crassa, N. sitophila, and N. intermedia in terms of growth on bread, effects of heat flashes, water activity, food additives and viability of stored conidia revealed no differences in either environmental or substrate preferences among the species on which a classification could be based. 345 putative wild isolates of Neurospora were collected from a wide variety of sources - silage, Indonesian ontjom, wheat grains, flour, bread, crumpets, bakery air and bakery dust. These all grew more vigorously than the type cultures. They were compared with respect to morphological traits such as conidial colour and size, and degree of protoperitheciality, but these features again proved inadequate to distinguish between species. The wild isolates were crossed with tester strains which were derived or synthesized from isolates in the collection and authenticated with reference strains from the Fungal Genetics Stock Center and the Commonwealth Mycological Institute. The times required in these crosses for perithecium formation and ascospore ejection, and the perithecial pattern and percentage of dark-coloured ascospores produced were investigated. The latter proved to be an unambiguous and highly reproducible diagnostic character for species delineation. Neurospora crassa accounted for the bulk of the collection, conflicting with the popular view that N. sitophila is the principal Neurospora contaminant of bread. Only heterothallic strains of both mating types of N. crassa, N. sitophila, and N. intermedia were represented. Based on six characteristics (conidial colour, mating-type, protoperitheciality and percentage dark-coloured ascospores with the three species testers), the isolates of bakery origin were grouped into 112 individual strains. Such variability indicated that there was continual infection in these bakeries from a large population of strains estimated from analysis based upon the Poisson distribution to be about 170. Based on this finding, a model of the probable infection cycle of Neurospora in bakeries is described.
|Date of Award||1980|