The biodiversity of Aspergillus flavus in stored rice grain leads to a decrease in the overall aflatoxin B1 production in these species.

Aspergillus flavus is a significant fungus that poses a threat to food safety by producing mycotoxins in various crops. In this study, A. flavus isolates were obtained from storage rice collected from seven provinces in southern China, and their AFB1 production, biosynthesis genes presence, and diversity were detected. Results showed that 56 out of the 81 A. flavus isolates produced detectable levels of AFB1, and 71 isolates (87.6 %) possessed aflR gene in their AF synthesis gene cluster, while only 41 isolates (50.6 %) had the ver-1 gene present. Genetic diversity analysis using inter-simple sequence repeats (ISSR) markers revealed seven main clusters among the isolates and the genetic similarity coefficients of 81 A. flavus isolates ranged from 0.53 to 1.00. Additionally, coculture assays were conducted using two toxigenic and two atoxigenic isolates from the same grain depot to investigate the effect of intraspecific inhibition on AFB1 production and to assess the AFB1 contamination risk of storage rice. The in situ results demonstrated that the atoxigenic isolates effectively inhibited the AFB1 contamination of toxigenic isolates. These findings provide insight into the genetic diversity of A. flavus isolates populations and highlight the potential food safety hazards of them in stored rice grain in China.

Biosurfactant from Pseudomonas fragi enhances the competitive advantage of Pseudomonas but reduces the overall spoilage ability of the microbial community in chilled meat.

Biosurfactants from Pseudomonas spp. have been reported to exhibit antibacterial and anti-adhesive properties, but their role during meat spoilage remains unclear. In this study, the biosurfactant was isolated from an isolate of Pseudomonas fragi with strong spoilage potential, and its surface tension and emulsification ability were determined. The chemical and microbial characteristics of the biosurfactant-treated meat samples were periodically analyzed. The results demonstrated that the biosurfactant produced by P. fragi could reduce surface tension and showed good emulsification properties. For the in situ spoilage trials, biosurfactant from P. fragi changed the microbial diversity on meat, helping Pseudomonas establish a dominant position in the population. However, biosurfactant treatment caused chicken meat to exhibit a weaker spoilage state, as indicated by the growth of psychrophilic microorganisms, total volatile basic nitrogen (TVBN) and meat color. These results provide practical information for understanding the role of P. fragi biosurfactant during chilled meat storage.

New insights into the persistent effect of transient cinnamaldehyde vapor treatment on the growth and aflatoxin synthesis of Aspergillus flavus.

The antimicrobial effects of continuous treatment with essential oils (EOs) in both liquid and gaseous phases have been intensively studied. Due to their rapid volatility, the effects of EOs on microorganisms after transient treatment are also worth exploring. In this work, the persistent effects of cinnamaldehyde (CA) vapor on Aspergillus flavus were detected by a series of biochemical analyses. Transcriptome analysis was also conducted to study the gene expression changes between recovered and normal A. flavus. When CA vapor was removed, biochemical analyses showed that the oxidative stress induced by the antimicrobial atmosphere was alleviated, and almost all the damaged functions were restored apart from mitochondrial function. Remarkably, the suppressed aflatoxin production intensified, which was confirmed by the up-regulation of most genes in the aflatoxin synthetic gene cluster, the velvet-related gene FluG and the aflatoxin precursor acetyl-CoA. Transcriptomic analysis also demonstrated significant changes in secondary metabolism, energy metabolism, oxidative stress, and amino acid metabolism in the recovery group. Taken together, these findings provide new insights into the mechanisms underlying the response of A. flavus to CA vapor treatment and will guide the rational application of EOs.

The Temporal Dynamics of Sensitivity, Aflatoxin Production, and Oxidative Stress of Aspergillus flavus in Response to Cinnamaldehyde Vapor.

Cinnamaldehyde (CA), a natural plant extract, possesses notable antimicrobial properties and the ability to inhibit mycotoxin synthesis. This study investigated the effects of different concentrations of gaseous CA on A. flavus and found that higher concentrations exhibited fungicidal effects, while lower concentrations exerted fungistatic effects. Although all A. flavus strains exhibited similar responses to CA vapor, the degree of response varied among them. Notably, A. flavus strains HN-1, JX-3, JX-4, and HN-8 displayed higher sensitivity. Exposure to CA vapor led to slight damage to A. flavus, induced oxidative stress, and inhibited aflatoxin B1 (AFB1) production. Upon removal of the CA vapor, the damaged A. flavus resumed growth, the oxidative stress weakened, and AFB1 production sharply increased in aflatoxin-producing strains. In the whole process, no aflatoxin was detected in aflatoxin-non-producing A. flavus. Moreover, the qRT-PCR results suggest that the recovery of A. flavus and the subsequent surge of AFB1 content following CA removal were regulated by a drug efflux pump and velvet complex proteins. In summary, these findings emphasize the significance of optimizing the targeted concentrations of antifungal EOs and provide valuable insight for their accurate application.

Effects of different carbon sources on the oxidative stress tolerance of Aspergillus niger HY2 isolated from spoiled paddies.

Fungi often experience oxidative stress in response to the environment during growth. In this study, Aspergillus niger HY2, whose presence easily results in paddy mildew, was used to investigate the effects of different carbon sources on morphological development, reactive oxygen species (ROS) metabolites, and antioxidant enzymes activities. Morphological development mainly includes the changes in conidial production and colony diameter. ROS metabolites production include the content of hydrogen peroxide (H2O2), superoxide anion (O2·-), and malondialdehyde (MDA). The results indicated that A. niger HY2 adapted to H2O2 exposure by decreasing growth and increasing the activities of some antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). Different carbon sources also affected the expression of the developmental-specific gene flbA and the oxidative stress tolerance-related gene cat. When incubated with glucose, sucrose, and xylose as carbon sources, A. niger exhibited stronger oxidative stress tolerance, but when incubated with maltose as a carbon source, A. niger exhibited relatively poor oxidative stress tolerance. Our results can provide a theoretical basis for further understanding mechanisms of metabolic adaptation and developing targeted strategies to control the spoilage caused by A. niger.

Involvement of AprD in regulating biofilm structure, matrix secretion, and cell metabolism of meat-borne Pseudomonas fragi during chilled storage.

Pseudomonas fragi is by far one of the most threatening species in the spoilage of chilled meat that is stored under aerobic conditions. The membrane protein AprD is a well-established regulator controlling protease secretion in Pseudomonas spp. However, its exact roles in modulating metabolic pathways and spoilage potential of P. fragi at the molecular level remain undefined. Here, an in-frame deletion mutation of aprD was used to explore the impacts on their biofilm structure, matrix secretion, and cell metabolism. The results showed that ΔaprD formed relatively disorganized loose aggregation in biofilm, resulting in a thinner structure and more dead cells. Meanwhile, marked changes in the content of extracellular carbohydrates and proteins were observed. Furthermore, intracellular metabolomic profiling revealed the involvement of aprD in several cellular metabolic pathways, mostly including the carbohydrate pathway, amino acid pathway, and nucleotide pathway, while the characterization of extracellular metabolism clarified the variations in the spoilage-related metabolites (e.g., creatine, IMP, spermine, fatty acids, amino acids, and oligopeptides) could be highly correlated with aprD deletion. In this finding, we indicated that aprD could be responsible for cell reproduction and in situ spoilage potential of P. fragi NMC25 during chilled storage by controlling related metabolism and nutrients utilization. Thus, our results will contribute to an improved understanding of the regulatory mechanism of aprD gene in meat spoilage contaminated with P. fragi, which can be valuable to ensure the quality and safety of meat.