Biodiversity of Aspergillus species in some important agricultural products

Abstract

The genus Aspergillus is one of the most important filamentous fungal genera. Aspergillus species are used in the fermentation industry, but they are also responsible of various plant and food secondary rot, with the consequence of possible accumulation of mycotoxins. The aflatoxin producing A. flavus and A. parasiticus, and ochratoxinogenic A. niger, A. ochraceus and A. carbonarius species are frequently encountered in agricultural products. Studies on the biodiversity of toxigenic Aspergillus species is useful to clarify molecular, ecological and biochemical characteristics of the different species in relation to their different adaptation to environmental and geographical conditions, and to their potential toxigenicity. Here we analyzed the biodiversity of ochratoxin producing species occurring on two important crops: grapes and coffee, and the genetic diversity of A. flavus populations occurring in agricultural fields. Altogether nine different black Aspergillus species can be found on grapes which are often difficult to identify with classical methods. The polyphasic approach used in our studies led to the identification of three new species occurring on grapes: A. brasiliensis, A. ibericus, and A. uvarum. Similar studies on the Aspergillus species occurring on coffee beans have evidenced in the last five years that A. carbonarius is an important source of ochratoxin A in coffee. Four new species within the black aspergilli were also identified in coffee beans: A. sclerotioniger, A. lacticoffeatus, A. sclerotiicarbonarius, and A. aculeatinus. The genetic diversity within A. flavus populations has been widely studied in relation to their potential aflatoxigenicity and morphological variants L- and S-strains. Within A. flavus and other Aspergillus species capable of aflatoxin production, considerable diversity is found. We summarise the main recent achievements in the diversity of the aflatoxin gene cluster in A. flavus populations, A. parasiticus and the non-toxigenic A. oryzae. Studies are needed in order to characterise the aflatoxin biosynthetic genes in the new related taxa A. minisclerotigenes and A. arachidicola.

Keywords: Aspergillus Sect. Nigri; Sect. Flavi; aflatoxins; grapes; ochratoxin A; polyphasic identification coffee beans.

Fig. 1.

AFLP dendrogram evidencing molecular biodiversity of representative black aspergilli isolated from grape in Europe.

Fig. 2.

Phylogenetic tree based on calmodulin sequence data of Aspergillus section Nigri. Numbers above branches are bootstrap values. Only values above 70 % are indicated. ^* Strains were labelled using accession number of ITEM, Culture Collection of Agri-Food Important Toxigenic Fungi, ISPA-CNR, Bari, Italy.

Fig. 3.

A. Arabica coffee. Ripen cherries on tree. B. depulped cherries. C. dried parchment coffee beans in drying yard. D-F. direct plating of parchment coffee beans on MEA and DG18. G-I. direct plating of green coffee beans on MEA and DG18.

Fig. 4.

Robusta coffee. A. Ripe cherries on tree. B. dried cherries. C. dried coffee beans in drying yard. D-F. direct plating of coffee cherries on MEA and DG18. G-I. direct plating of green coffee beans on MEA and DG18.

Fig. 5.

Neighbour-joining tree based on phylogenic analysis of the partial β-tubulin gene sequences of black aspergilli recovered from Thai coffee.

Fig. 6.

Comparison of relative average abilities (expressed as Luminescence Unit from Fluorescence detector: LU) to produce ochratoxins of A. carbonarius and A. niger.

Fig. 7.

Causes of A. flavus diversity.