Citation:
Sahib Alama, Hamid Ullah Shaha and Naresh Magan.
Water availability affects extracellular hydrolytic enzyme production by
Aspergillus flavus and Aspergillus parasiticus. World Mycotoxin Journal, 2009, volume 2, number 3, pp313-322
Abstract:
The objectives of this study were to examine the effect of different water
activities (a(w); 0.99, 0.96 and 0.94) and time (up to 120 h) on quantitative
and specific enzyme production during germination and initial growth of
Aspergillus flavus and A. parasiticus strains at 25 degrees C. This is an
important early indicator of potential for aflatoxin production under conducive
conditions. Qualitative API ZYM generic enzyme strips were used to identify key
hydrolytic enzymes produced. Subsequently, the temporal effects of a(w) on the
total/specific activity of the key 4-5 hydrolytic enzymes were determined using
4-nitrophenyl substrates in a 96-well microtitre plate assay. The main enzymes
produced by germinating conidia of A. flavus were esterase, lipase, acid
phosphatase, beta-glucosidase and N-acetyl-beta-D-glucosaminidase, while for A.
parasiticus these were alkaline phosphatase, lipase, acid phosphatase and beta-
fucosidase for both total (mu mol 4-nitrophenol/min/g) and specific activity
(nmol beta-nitrophenol/min/mu g protein). There were significant increases in
the specific activity of all these enzymes of germinating spores of A. flavus
(0-120 h) except for beta-glucosidase which was maximum at 72 h. The total/
specific activities of the enzymes produced by A. flavus were maximum at 0.99 a
(w) with the exception of acid phosphatase and N-acetyl-beta-D-glucosaminidase
at 0.94 a(w) For A. parasiticus, maximum total activity occurred at 0.99 aw for
fucosidase activity, while specific activity was found to be higher at lower a
(w) levels. These enzymes are important in early colonisation of food matrices
by these species and single factors (a(w) time) and two-way interactions were
all statistically significant for the enzymes assayed for both species. These
enzymes could be used as an early and rapid indicator of the activity of
Aspergillus section flavi species and suggests that rapid infection may occur
over a wide range of a(w) conditions.
