Natural approach of using nisin and its nanoform as food bio-preservatives against methicillin resistant Staphylococcus aureus and E.coli O157:H7 in yoghurt.

BACKGROUND: Natural antimicrobial agents such as nisin were used to control the growth of foodborne pathogens in dairy products. The current study aimed to examine the inhibitory effect of pure nisin and nisin nanoparticles (nisin NPs) against methicillin resistant Staphylococcus aureus (MRSA) and E.coli O157:H7 during the manufacturing and storage of yoghurt. Nisin NPs were prepared using new, natural, and safe nano-precipitation method by acetic acid. The prepared NPs were characterized using zeta-sizer and transmission electron microscopy (TEM). In addition, the cytotoxicity of nisin NPs on vero cells was assessed using the 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The minimum inhibitory concentrations (MICs) of nisin and its nanoparticles were determined using agar well-diffusion method. Further, fresh buffalo's milk was inoculated with MRSA or E.coli O157:H7 (1 × 106 CFU/ml) with the addition of either nisin or nisin NPs, and then the inoculated milk was used for yoghurt making. The organoleptic properties, pH and bacterial load of the obtained yoghurt were evaluated during storage in comparison to control group. RESULTS: The obtained results showed a strong antibacterial activity of nisin NPs (0.125 mg/mL) against MRSA and E.coli O157:H7 in comparison with control and pure nisin groups. Notably, complete eradication of MRSA and E.coli O157:H7 was observed in yoghurt formulated with nisin NPs after 24 h and 5th day of storage, respectively. The shelf life of yoghurt inoculated with nisin nanoparticles was extended than those manufactured without addition of such nanoparticles. CONCLUSIONS: Overall, the present study indicated that the addition of nisin NPs during processing of yoghurt could be a useful tool for food preservation against MRSA and E.coli O157:H7 in dairy industry.

Assessing Listeria monocytogenes growth kinetics in rice pudding at different storage temperatures.

Rice pudding is a popular artisanal dairy dessert highly consumed in the main rice-producing countries, including Egypt. This study aimed to evaluate and model the growth of Listeria monocytogenes in rice pudding dessert stored at different temperatures (4-25 °C) over its shelf-life. Lab-scale rice pudding samples were prepared following a traditional Egyptian recipe and inoculated with a three-strain cocktail of L. monocytogenes (ca. 3 × 102 cfu/g). Inoculated rice pudding samples (pH = 6.67 ± 0.06 and aw = 0.99 ± 0.002) were stored at different isothermal conditions (4, 8, 12, 18, and 25 °C) and microbiologically analysed for up to 30 days for pathogen quantification by plate count methodology. Global regression analysis was used to fit the Baranyi model coupled with the Ratkowsky model to growth data, relating L. monocytogenes concentrations (N, log cfu/g) with storage temperature (°C) and times (d). Model validation was performed using published independent data. L. monocytogenes growth potential increased by increasing storage temperature. The estimated Ratkowsky model parameters were b = 0.0819 ± 0.0017 log cfu/d·°C and Tmin = -3.28 ± 0.20 °C. The indices RMSE = 0.39 and R2adj = 0.97 indicated a good agreement between the experimental data and the model predictions. The estimated maximum growth rate (µmax) values ranged between 0.355 and 5.363 log cfu/d from 4 to 25 °C. The model was successfully validated using published L. monocytogenes Scott A and California strains growth data in rice pudding samples stored at 5, 12 and 22 °C, as evidenced by the assessed statistical indices. The predictive model developed and validated in this study will aid in decision-making regarding the microbiological safety of rice pudding dessert with respect to L. monocytogenes growth, considering a wide range of storage temperatures.

Antifungal and antitoxin effects of propolis and its nanoemulsion formulation against Aspergillus flavus isolated from human sputum and milk powder samples.

BACKGROUND AND AIM: Aspergillus flavus causes human and animal diseases through either inhalation of fungal spores or ingestion of mycotoxins as aflatoxins produced in human and animal feed as secondary metabolites. This study was aimed to detect the incidence of A. flavus and its aflatoxins in human sputum and milk powder samples and explore the efficacy of pure propolis (PP) and propolis nanoemulsion (PNE) as natural decontaminants against fungal growth and its released aflatoxins. MATERIALS AND METHODS: A. flavus was isolated by mycological culture and identified macroscopically and microscopically. Coconut agar medium and thin-layer chromatography (TLC) were used to qualitatively detect aflatoxins in the isolated strains. Toxins were extracted from toxigenic strains by the fast extraction technique. The quantitative detection of toxin types was explored by high-performance liquid chromatography (HPLC). PNE was prepared by a novel method using natural components and characterized by Fourier-transform infrared spectroscopy, Zetasizer, and transmission electron microscopy. The effects of PP and PNE on A. flavus growth and its toxin were determined by the well-diffusion method and HPLC. RESULTS: The mycological culture showed that 30.9% and 29.2% of sputum and milk powder samples were positive for A. flavus, respectively. TLC confirmed the production of 61.8% and 63.2% aflatoxin by the isolated strains in sputum and milk powder, respectively. PP and PNE showed antifungal activity on A. flavus growth with mean±standard error (SE) inhibition zones of 27.55±3.98 and 39.133±5.32 mm, respectively. HPLC revealed positive contamination of toxin extracts with AFB1, AFB2, and AFG2 at 0.57±0.026, 0.28±0.043, and 0.1±0.05 mg/L, respectively. After treatment with PP and PNE, a significant decrease in AFB1, AFB2, and AFG2 concentrations was observed. CONCLUSION: This study suggested using propolis and its nanoformulation as antifungal and antitoxins in human medicine and the food industry to increase the food safety level and stop food spoilage.

Investigation of the effect of chitosan and silver nanoparticles on the antibiotic resistance of Escherichia coliO157:H7 isolated from some milk products and diarrheal patients in Sohag city, Egypt.

BACKGROUND AND AIM: Antimicrobial-resistant Escherichia coli O157:H7 causes serious diseases in humans, especially when circulated in their food. This study was designed to detect the presence of E. coli O157:H7 using the fliC H7 gene in some milk products as kareish cheese, labena, and yoghurt sold in Sohag city, Egypt, and among diarrheal patients admitted to governmental hospitals in Sohag and also to highlight the risk factors associated with their infection. In addition, the antimicrobial resistance and the effect of chitosan nanoparticles (CNP) and silver nanoparticles (SNP) on E. coli O157:H7 isolates obtained from both milk products and patients were investigated. MATERIALS AND METHODS: Microbiological culture methods and polymerase chain reaction were used for detecting E. coli O157:H7 in 150 milk products and 150 stool samples. Resistance against some antimicrobials that were used in the treatment of animals and humans was investigated using the disk diffusion technique. CNP and SNP at two concentrations (30 and 60 µg/mL) and average sizes of 25.1 and 26.5 nm, respectively, were identified by transmission electron microscopy. Their effect on E. coli O157:H7 isolates was examined using the well diffusion method. Risk factors for infection were investigated using statistical analysis. RESULTS: There were 11.3% and 14.7% of milk products and stool samples positive for E. coli O157:H7, respectively. These isolates exhibited high antimicrobial resistance to ampicillin, tetracycline, and gentamycin. CNP and SNP demonstrated inhibitory effects on E. coli O157:H7 growth, which significantly increased at high concentrations (60 µg/mL), with mean inhibition zones of 31.941±3.749 and 30.681±3.871 mm for CNP in milk products and patient isolates, respectively. The respective values for SNP were 33.588±3.675 mm and 32.500±2.444 mm, indicating a higher bactericidal effect than that of CNP. Regarding risk factors for infection, both young and elderly subjects and those in contact with infected persons and/or having chronic diseases were infected. CONCLUSION: CNP and SNP are suitable for both medical and agricultural applications for disease control and enhancement of food quality.