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Mechanism by which ammonium bicarbonate and ammonium sulfate inhibit mycotoxigenic fungi
D A DePasquale1, T J Montville
1Department of Food Science, Cook College, Rutgers State University, New Brunswick 08903-0231.
Abstract:
In this study we examined the mechanism by which ammonium bicarbonate inhibits mycotoxigenic fungi. Elevated extracellular pH, alone, was not responsible for the antifungal activity. Although conidia of Penicillium griseofulvum and Fusarium graminearum had internal pH (pHi) values as high as 8.0 in buffer at an external pH (pHo) of 9.5, their viability was not markedly affected. The pHi values from conidia equilibrated in glycine-NaOH-buffered treatments without ammonium bicarbonate or ammonium sulfate were similar to values obtained from buffered treatments containing the ammonium salts. Thus, inhibition did not appear to be directly related to increased pHi. Ammonium sulfate in buffered media at pH greater than or equal to 8.7 was as inhibitory as ammonium bicarbonate, but was completely ineffective at pH less than or equal to 7.8. The hypothesis that free ammonia caused the fungal inhibition was tested by using ammonium sulfate as a model for ammonium bicarbonate. Viability, expressed as log CFU/ml, and percent germination of P. griseofulvum and F. graminearum decreased dramatically as the free ammonia concentration increased. Germination rate ratios (the germination rate in buffered ammonium sulfate divided by the germination rate in buffer alone) decreased linearly as the free ammonia concentration increased, further establishing NH3 as the toxic agent. Ammonium bicarbonate inhibits fungi because the bicarbonate anion supplies the alkalinity necessary to establish an antifungal concentration of free ammonia.
Insights
Ammonium bicarbonate inhibits fungi by increasing internal pH. Ammonium sulfate showed similar antifungal activity, indicating free ammonia is the toxic agent responsible for inhibiting fungal growth and germination.
Area of Science:
- Mycology
- Agricultural Science
- Biochemistry
Background:
- Mycotoxigenic fungi pose significant threats to agriculture and food safety.
- Understanding the mechanisms of antifungal agents is crucial for developing effective control strategies.
Purpose of the Study:
- To elucidate the inhibitory mechanism of ammonium bicarbonate against mycotoxigenic fungi.
- To determine the role of extracellular and intracellular pH in ammonium bicarbonate's antifungal activity.
- To identify the specific chemical species responsible for fungal inhibition.
Main Methods:
- Comparative analysis of fungal viability and germination rates under varying pH conditions.
- Equilibration of fungal conidia in buffered solutions with and without ammonium salts.
- Correlation of fungal inhibition with measured concentrations of free ammonia (NH3).
Main Results:
- Elevated extracellular pH alone did not cause significant fungal inhibition.
- Internal pH of fungal conidia did not directly correlate with inhibition.
- Ammonium sulfate exhibited antifungal activity comparable to ammonium bicarbonate at pH ≥ 8.7, but not at pH ≤ 7.8.
- Fungal viability and germination decreased linearly with increasing free ammonia concentrations.
Conclusions:
- Ammonium bicarbonate's antifungal activity is primarily mediated by the generation of free ammonia (NH3).
- The bicarbonate anion facilitates the creation of an alkaline environment necessary for producing toxic levels of free ammonia.
- Free ammonia, not elevated pH, is the direct toxic agent inhibiting mycotoxigenic fungi.