The food fermentation fungus Aspergillus oryzae is a source of natural antimicrobials against Listeria monocytogenes

Abstract

Listeria monocytogenes is a highly adaptable foodborne pathogen that causes multiple foodborne illness outbreaks annually despite stringent food safety measures. The ubiquitous presence of L. monocytogenes in agricultural production environments provides easy routes of contamination to the human food production chain. The remarkable resilience of L. monocytogenes in harsh food processing and preservation conditions presents further challenges to controlling this pathogen in food and food processing plants. Furthermore, there is an increasing consumer demand for natural antimicrobials in food. Aspergillus oryzae is a food fermentation fungus with a generally recognized as safe status and is a workhorse in biotechnology applications. In this study, we examined the antimicrobial activity of Aspergillus oryzae fermentates and extracts toward L. monocytogenes, both in laboratory cultures and contaminated milk. Aspergillus oryzae-derived antimicrobials can be obtained in 2 culture conditions, which we term natural products 1 and 2 (NP1 and NP2). Laboratory cultures of L. monocytogenes were effectively and rapidly killed by both NP1 and NP2 extracts. In contaminated milk, the NP1 extract was bactericidal, whereas the NP2 extract was bacteriostatic. Nevertheless, the NP2 extract was heat stable, retaining antimicrobial activity even after boiling. Profiling L. monocytogenes transcriptional response to a subinhibitory level of NP2 fermentate, we observed significant shifts in amino acid metabolism and iron uptake, suggesting that these pathways can be tackled to increase the efficacy of NP2. Taken together, A. oryzae fermentates and extracts are promising candidates for natural antimicrobial treatments in food and food processing environments.