Prospecting for the incidence of genes involved in ochratoxin and fumonisin biosynthesis in Brazilian strains of Aspergillus niger and Aspergillus welwitschiae

Abstract

Aspergillus niger “aggregate” is an informal taxonomic rank that represents a group of species from the section Nigri. Among A. niger “aggregate” species Aspergillus niger sensu stricto and its cryptic species Aspergillus welwitschiae (=Aspergillus awamori sensu Perrone) are proven as ochratoxin A and fumonisin B2 producing species. A. niger has been frequently found in tropical and subtropical foods. A. welwitschiae is a new species, which was recently dismembered from the A. niger taxon. These species are morphologically very similar and molecular data are indispensable for their identification. A total of 175 Brazilian isolates previously identified as A. niger collected from dried fruits, Brazil nuts, coffee beans, grapes, cocoa and onions were investigated in this study. Based on partial calmodulin gene sequences about one-half of our isolates were identified as A. welwitschiae. This new species was the predominant species in onions analyzed in Brazil. A. niger and A. welwitschiae differ in their ability to produce ochratoxin A and fumonisin B2. Among A. niger isolates, approximately 32% were OTA producers, but in contrast only 1% of the A. welwitschiae isolates revealed the ability to produce ochratoxin A. Regarding fumonisin B2 production, there was a higher frequency of FB2 producing isolates in A. niger (74%) compared to A. welwitschiae (34%). Because not all A. niger and A. welwitschiae strains produce ochratoxin A and fumonisin B2, in this study a multiplex PCR was developed for detecting the presence of essential genes involved in ochratoxin (polyketide synthase and radHflavin-dependent halogenase) and fumonisin (α-oxoamine synthase) biosynthesis in the genome of A. niger and A. welwitschiae isolates. The frequency of strains harboring the mycotoxin genes was markedly different between A. niger and A. welwitschiae. All OTA producing isolates of A. niger and A. welwitschiae showed in their genome the pks and radH genes, and 95.2% of the nonproducing isolates did not contain these genes. The α-oxoamine synthase gene was detected in 100% and 36% of the A. niger and A. welwitschiae isolates, respectively. The loss of ochratoxin A production in A. niger and A. welwitschiae is highly associated with gene deletions within the ochratoxin biosynthetic gene cluster. The loss of fumonisin production in A. welwitschiae is associated with gene deletions within the fumonisin biosynthetic gene cluster, but this is not the case with A. niger.