Polymerase chain reaction (PCR) identification of Aspergillus niger and Aspergillus tubingensis based on the calmodulin gene.

Aspergillus niger and A. tubingensis, species belonging to section Nigri, are commonly found in plant products and processed food, such as grapes, cereals, coffee and derivatives. These two species are very difficult to differentiate by classical morphological criteria and some isolates are known to produce ochratoxin A. The exact identification of these two species is very important to avoid overestimation of toxicological contamination and related risks. A PCR-based identification and detection assay was developed as tool to identify A. niger and A. tubingensis, using molecular differences obtained by sequencing the calmodulin gene. Two pairs of species-specific primers were designed and empirically evaluated for PCR identification of A. niger and A. tubingensis. Species-specific PCR products generated by each primer set were 505 bp (A. tubingensis) and 245 bp (A. niger) in length, which could be potentially useful for a multiplex PCR assay. The sensitivity of this assay was about 10 pg DNA in 25 µl PCR reaction volume, using pure total DNA of the two species. The method described in this study represents a rapid and reliable procedure to assess the presence in food products of two ochratoxigenic species of section Nigri.