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Aspergillus flavus grown in peptone as the carbon source exhibits spore density- and peptone concentration-dependent aflatoxin biosynthesis.

Yan S, Liang Y, Zhang J, Liu CM - BMC Microbiol. (2012)

Bottom Line: Metabolomic studies revealed that, in addition to inhibited AF biosynthesis, mycelia grown in peptone media with a high initial spore density showed suppressed fatty acid biosynthesis, reduced tricarboxylic acid (TCA) cycle intermediates, and increased pentose phosphate pathway products.Additions of TCA cycle intermediates had no effect on AF biosynthesis, suggesting the inhibited AF biosynthesis was not caused by depleted TCA cycle intermediates.This switching ability may offer Aspergillus species a competition advantage in natural ecosystems, producing AFs only when self-population is low and food is scarce.

View Article: PubMed Central - HTML - PubMed

Affiliation: Practaculture College, Gansu Agricultural University, Lanzhou 730070, China.

ABSTRACT

Background: Aflatoxins (AFs) are highly carcinogenic compounds produced by Aspergillus species in seeds with high lipid and protein contents. It has been known for over 30 years that peptone is not conducive for AF productions, although reasons for this remain unknown.

Results: In this study, we showed that when Aspergillus flavus was grown in peptone-containing media, higher initial spore densities inhibited AF biosynthesis, but promoted mycelial growth; while in glucose-containing media, more AFs were produced when initial spore densities were increased. This phenomenon was also observed in other AF-producing strains including A. parasiticus and A. nomius. Higher peptone concentrations led to inhibited AF production, even in culture with a low spore density. High peptone concentrations did however promote mycelial growth. Spent medium experiments showed that the inhibited AF production in peptone media was regulated in a cell-autonomous manner. mRNA expression analyses showed that both regulatory and AF biosynthesis genes were repressed in mycelia cultured with high initial spore densities. Metabolomic studies revealed that, in addition to inhibited AF biosynthesis, mycelia grown in peptone media with a high initial spore density showed suppressed fatty acid biosynthesis, reduced tricarboxylic acid (TCA) cycle intermediates, and increased pentose phosphate pathway products. Additions of TCA cycle intermediates had no effect on AF biosynthesis, suggesting the inhibited AF biosynthesis was not caused by depleted TCA cycle intermediates.

Conclusions: We here demonstrate that Aspergillus species grown in media with peptone as the sole carbon source are able to sense their own population densities and peptone concentrations to switch between rapid growth and AF production. This switching ability may offer Aspergillus species a competition advantage in natural ecosystems, producing AFs only when self-population is low and food is scarce.

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Mycelial growth and AF production ofA. flavuscultured with different initial spore densities. (A, B) Mycelial growth curves of A. flavus A3.2890 in 50 ml GMS (A) or PMS (B) media initiated with 104 (dotted line) or 106 spores/ml (solid line). The mycelium dry weights were measured during a period of 5 days. (C, D) Effects of PMS spent media on AF productions. (C) One ml fresh GMS (G0) or PMS (P0) media, or spent media (P4 and P6) were added to GMS media inoculated with 106 spores/ml. (D) Five ml fresh GMS (G0) or PMS (P0), or spent media (P4 and P6) were added to GMS media inoculated with 106 spores/ml. AF contents were measured after cultured at 28°C for 3 days. The spent media were prepared from 3-day PMS cultures with the initial spore densities of 104 (P4) or 106 (P6) spores/ml. All data were the mean ± SD of 3 HPLC measurements from mixed three independent samples.
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Figure 2: Mycelial growth and AF production ofA. flavuscultured with different initial spore densities. (A, B) Mycelial growth curves of A. flavus A3.2890 in 50 ml GMS (A) or PMS (B) media initiated with 104 (dotted line) or 106 spores/ml (solid line). The mycelium dry weights were measured during a period of 5 days. (C, D) Effects of PMS spent media on AF productions. (C) One ml fresh GMS (G0) or PMS (P0) media, or spent media (P4 and P6) were added to GMS media inoculated with 106 spores/ml. (D) Five ml fresh GMS (G0) or PMS (P0), or spent media (P4 and P6) were added to GMS media inoculated with 106 spores/ml. AF contents were measured after cultured at 28°C for 3 days. The spent media were prepared from 3-day PMS cultures with the initial spore densities of 104 (P4) or 106 (P6) spores/ml. All data were the mean ± SD of 3 HPLC measurements from mixed three independent samples.

Mentions: To exclude the possibility that the reduced AF production in PMS media initiated with high initial spore densities was caused by inhibited fungal growth, mycelium dry weights were determined during a five-day culture period. A. flavus cultured in GMS media with an initial density of 104 or 106 spores/ml showed a similar growth curve, with a continuous increase in dry weight during the five-day incubation. Higher initial spore density led to slightly faster mycelial growth, and an increased mycelium dry weight (Figure 2A). A. flavus cultured with 104 spores/ml in PMS media showed a similar growth curve to that in GMS media with the same spore density (Figure 2B). However, a much sharper exponential growth phase was observed in the first two days in PMS culture initiated with 106 spores/ml (Figure 2B). The mycelium dry weight reached the maximum level on the 4th day and decreased significantly afterwards, suggesting no inhibition of growth in the high density PMS culture. Instead, A. flavus cultured in PMS media with a high initial spore density grew faster and degenerated earlier (Figure 2B).


Aspergillus flavus grown in peptone as the carbon source exhibits spore density- and peptone concentration-dependent aflatoxin biosynthesis.

Yan S, Liang Y, Zhang J, Liu CM - BMC Microbiol. (2012)

Mycelial growth and AF production ofA. flavuscultured with different initial spore densities. (A, B) Mycelial growth curves of A. flavus A3.2890 in 50 ml GMS (A) or PMS (B) media initiated with 104 (dotted line) or 106 spores/ml (solid line). The mycelium dry weights were measured during a period of 5 days. (C, D) Effects of PMS spent media on AF productions. (C) One ml fresh GMS (G0) or PMS (P0) media, or spent media (P4 and P6) were added to GMS media inoculated with 106 spores/ml. (D) Five ml fresh GMS (G0) or PMS (P0), or spent media (P4 and P6) were added to GMS media inoculated with 106 spores/ml. AF contents were measured after cultured at 28°C for 3 days. The spent media were prepared from 3-day PMS cultures with the initial spore densities of 104 (P4) or 106 (P6) spores/ml. All data were the mean ± SD of 3 HPLC measurements from mixed three independent samples.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3412747&req=5

Figure 2: Mycelial growth and AF production ofA. flavuscultured with different initial spore densities. (A, B) Mycelial growth curves of A. flavus A3.2890 in 50 ml GMS (A) or PMS (B) media initiated with 104 (dotted line) or 106 spores/ml (solid line). The mycelium dry weights were measured during a period of 5 days. (C, D) Effects of PMS spent media on AF productions. (C) One ml fresh GMS (G0) or PMS (P0) media, or spent media (P4 and P6) were added to GMS media inoculated with 106 spores/ml. (D) Five ml fresh GMS (G0) or PMS (P0), or spent media (P4 and P6) were added to GMS media inoculated with 106 spores/ml. AF contents were measured after cultured at 28°C for 3 days. The spent media were prepared from 3-day PMS cultures with the initial spore densities of 104 (P4) or 106 (P6) spores/ml. All data were the mean ± SD of 3 HPLC measurements from mixed three independent samples.
Mentions: To exclude the possibility that the reduced AF production in PMS media initiated with high initial spore densities was caused by inhibited fungal growth, mycelium dry weights were determined during a five-day culture period. A. flavus cultured in GMS media with an initial density of 104 or 106 spores/ml showed a similar growth curve, with a continuous increase in dry weight during the five-day incubation. Higher initial spore density led to slightly faster mycelial growth, and an increased mycelium dry weight (Figure 2A). A. flavus cultured with 104 spores/ml in PMS media showed a similar growth curve to that in GMS media with the same spore density (Figure 2B). However, a much sharper exponential growth phase was observed in the first two days in PMS culture initiated with 106 spores/ml (Figure 2B). The mycelium dry weight reached the maximum level on the 4th day and decreased significantly afterwards, suggesting no inhibition of growth in the high density PMS culture. Instead, A. flavus cultured in PMS media with a high initial spore density grew faster and degenerated earlier (Figure 2B).

Bottom Line: Metabolomic studies revealed that, in addition to inhibited AF biosynthesis, mycelia grown in peptone media with a high initial spore density showed suppressed fatty acid biosynthesis, reduced tricarboxylic acid (TCA) cycle intermediates, and increased pentose phosphate pathway products.Additions of TCA cycle intermediates had no effect on AF biosynthesis, suggesting the inhibited AF biosynthesis was not caused by depleted TCA cycle intermediates.This switching ability may offer Aspergillus species a competition advantage in natural ecosystems, producing AFs only when self-population is low and food is scarce.

View Article: PubMed Central - HTML - PubMed

Affiliation: Practaculture College, Gansu Agricultural University, Lanzhou 730070, China.

ABSTRACT

Background: Aflatoxins (AFs) are highly carcinogenic compounds produced by Aspergillus species in seeds with high lipid and protein contents. It has been known for over 30 years that peptone is not conducive for AF productions, although reasons for this remain unknown.

Results: In this study, we showed that when Aspergillus flavus was grown in peptone-containing media, higher initial spore densities inhibited AF biosynthesis, but promoted mycelial growth; while in glucose-containing media, more AFs were produced when initial spore densities were increased. This phenomenon was also observed in other AF-producing strains including A. parasiticus and A. nomius. Higher peptone concentrations led to inhibited AF production, even in culture with a low spore density. High peptone concentrations did however promote mycelial growth. Spent medium experiments showed that the inhibited AF production in peptone media was regulated in a cell-autonomous manner. mRNA expression analyses showed that both regulatory and AF biosynthesis genes were repressed in mycelia cultured with high initial spore densities. Metabolomic studies revealed that, in addition to inhibited AF biosynthesis, mycelia grown in peptone media with a high initial spore density showed suppressed fatty acid biosynthesis, reduced tricarboxylic acid (TCA) cycle intermediates, and increased pentose phosphate pathway products. Additions of TCA cycle intermediates had no effect on AF biosynthesis, suggesting the inhibited AF biosynthesis was not caused by depleted TCA cycle intermediates.

Conclusions: We here demonstrate that Aspergillus species grown in media with peptone as the sole carbon source are able to sense their own population densities and peptone concentrations to switch between rapid growth and AF production. This switching ability may offer Aspergillus species a competition advantage in natural ecosystems, producing AFs only when self-population is low and food is scarce.

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