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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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A. flavusgrown in PMS and GMS media responded differently to the initial spore densities. (A) Higher concentrations of peptone inhibited AF productions in A. flavus A3.2890. P4, PMS media with the initial spore density of 104 spores/ml; P6, PMS media with the initial spore density of 106 spores/ml; G4, cultured in GMS media with the initial spore density of 104 spores/ml; G6, cultured in GMS media with the initial spore density of 106 spores/ml; P4+, PMS media with 15% peptone, cultured with the initial spore density of 104 spores/ml; P6+, PMS media with 15% peptone, cultured with the initial spore density of 106 spores/ml, St, AF standard. (B) Higher concentrations of peptone promoted mycelial growths. The total mycelium dry weights were measured after a 3-day culture, with initial spore densities of 104 or 106 spores/ml. (C) No direct correlations between AF productions and pH changes. In GMS media the pH was gradually decreased during the 55-hr culture, where a higher initial spore density led to faster acidification of the medium. In PMS media the pH was increased during culture, where a higher initial spore density led to rapid alkalization of the medium. Note that increased peptone concentrations did not cause a significant change in the pH of PMS media (P6 and P6+).
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Figure 3: A. flavusgrown in PMS and GMS media responded differently to the initial spore densities. (A) Higher concentrations of peptone inhibited AF productions in A. flavus A3.2890. P4, PMS media with the initial spore density of 104 spores/ml; P6, PMS media with the initial spore density of 106 spores/ml; G4, cultured in GMS media with the initial spore density of 104 spores/ml; G6, cultured in GMS media with the initial spore density of 106 spores/ml; P4+, PMS media with 15% peptone, cultured with the initial spore density of 104 spores/ml; P6+, PMS media with 15% peptone, cultured with the initial spore density of 106 spores/ml, St, AF standard. (B) Higher concentrations of peptone promoted mycelial growths. The total mycelium dry weights were measured after a 3-day culture, with initial spore densities of 104 or 106 spores/ml. (C) No direct correlations between AF productions and pH changes. In GMS media the pH was gradually decreased during the 55-hr culture, where a higher initial spore density led to faster acidification of the medium. In PMS media the pH was increased during culture, where a higher initial spore density led to rapid alkalization of the medium. Note that increased peptone concentrations did not cause a significant change in the pH of PMS media (P6 and P6+).

Mentions: To examine if the lack of AF production in PMS media with high initial spore densities is caused by rapid mycelial growth, and consequent depletion of nutrients, the peptone concentration in media from the original 5% was increased to 15% to see if AF production could be restored. We observed, conversely, that mycelia cultured with increased peptone concentration showed greatly reduced AF production, regardless of initial spore densities (104 or 106 spores/ml) (Figure 3A, P4+ and P6+). We then examined the mycelial growth in media with 5%, 10% and 15% peptone, and observed increased mycelium dry weights when the peptone concentrations were increased (Figure 3B), suggesting that high concentrations of peptone promoted mycelial growth and at the same time inhibited AF biosynthesis. For each of the peptone concentrations, it was observed that cultures with higher initial spore densities showed an increase in mycelial growth. Taken together, these studies revealed that high concentrations of peptone promoted mycelial growths but inhibited AF production, suggesting that A. flavus grown in the peptone medium is able to sense the peptone concentrations and is able to shift between fast growth and AF production.


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)

A. flavusgrown in PMS and GMS media responded differently to the initial spore densities. (A) Higher concentrations of peptone inhibited AF productions in A. flavus A3.2890. P4, PMS media with the initial spore density of 104 spores/ml; P6, PMS media with the initial spore density of 106 spores/ml; G4, cultured in GMS media with the initial spore density of 104 spores/ml; G6, cultured in GMS media with the initial spore density of 106 spores/ml; P4+, PMS media with 15% peptone, cultured with the initial spore density of 104 spores/ml; P6+, PMS media with 15% peptone, cultured with the initial spore density of 106 spores/ml, St, AF standard. (B) Higher concentrations of peptone promoted mycelial growths. The total mycelium dry weights were measured after a 3-day culture, with initial spore densities of 104 or 106 spores/ml. (C) No direct correlations between AF productions and pH changes. In GMS media the pH was gradually decreased during the 55-hr culture, where a higher initial spore density led to faster acidification of the medium. In PMS media the pH was increased during culture, where a higher initial spore density led to rapid alkalization of the medium. Note that increased peptone concentrations did not cause a significant change in the pH of PMS media (P6 and P6+).
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Figure 3: A. flavusgrown in PMS and GMS media responded differently to the initial spore densities. (A) Higher concentrations of peptone inhibited AF productions in A. flavus A3.2890. P4, PMS media with the initial spore density of 104 spores/ml; P6, PMS media with the initial spore density of 106 spores/ml; G4, cultured in GMS media with the initial spore density of 104 spores/ml; G6, cultured in GMS media with the initial spore density of 106 spores/ml; P4+, PMS media with 15% peptone, cultured with the initial spore density of 104 spores/ml; P6+, PMS media with 15% peptone, cultured with the initial spore density of 106 spores/ml, St, AF standard. (B) Higher concentrations of peptone promoted mycelial growths. The total mycelium dry weights were measured after a 3-day culture, with initial spore densities of 104 or 106 spores/ml. (C) No direct correlations between AF productions and pH changes. In GMS media the pH was gradually decreased during the 55-hr culture, where a higher initial spore density led to faster acidification of the medium. In PMS media the pH was increased during culture, where a higher initial spore density led to rapid alkalization of the medium. Note that increased peptone concentrations did not cause a significant change in the pH of PMS media (P6 and P6+).
Mentions: To examine if the lack of AF production in PMS media with high initial spore densities is caused by rapid mycelial growth, and consequent depletion of nutrients, the peptone concentration in media from the original 5% was increased to 15% to see if AF production could be restored. We observed, conversely, that mycelia cultured with increased peptone concentration showed greatly reduced AF production, regardless of initial spore densities (104 or 106 spores/ml) (Figure 3A, P4+ and P6+). We then examined the mycelial growth in media with 5%, 10% and 15% peptone, and observed increased mycelium dry weights when the peptone concentrations were increased (Figure 3B), suggesting that high concentrations of peptone promoted mycelial growth and at the same time inhibited AF biosynthesis. For each of the peptone concentrations, it was observed that cultures with higher initial spore densities showed an increase in mycelial growth. Taken together, these studies revealed that high concentrations of peptone promoted mycelial growths but inhibited AF production, suggesting that A. flavus grown in the peptone medium is able to sense the peptone concentrations and is able to shift between fast growth and AF production.

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.

Show MeSH