Effect of dry sanitizing methods on Bacillus cereus biofilm

Salmonella dry surface biofilm: morphology, single-cell landscape, and sanitization

In this study, Salmonella Typhimurium dry surface biofilm (DSB) formation was investigated in comparison with wet surface biofilm (WSB) development. Confocal laser scanning microscopic analysis revealed a prominent green cell signal during WSB formation, whereas a red signal predominated during DSB formation. Electron microscopy was also used to compare the features of DSB and WSB. Overall, WSB was unevenly scattered over the surface, whereas DSB was evenly dispersed. In contrast to WSB cells, which have a distinct plasma membrane and outer membrane layer, DSB cells are contained in large capsules and compressed. Next, microbiome single-cell transcriptomics was used to investigate the functional heterogeneity of the Salmonella DSB microbiome, with nine clusters successfully identified. Although over 60% of the dried cells were metabolically inactive, the rest of the Salmonella cells still demonstrated specific antioxidative and virulence capabilities, suggesting a possible concern for low-moisture food (LMF) safety. Finally, because sanitization in LMF industries must be conducted without water, a list of 39 flavonoids was tested for their combined effect with 70% isopropyl alcohol (IPA) against DSB, and morin induced the greatest reduction in the green:red ratio from 3.67 to 0.43. Significantly higher reductions of Salmonella viability in DSB were achieved by 10-, 100-, 1,000-, and 10,000-µg/mL morin (1.69 ± 0.25, 3.21 ± 0.23, 4.32 ± 0.24, and 5.18 ± 0.16 log CFU/sample reductions) than 70% IPA alone (1.55 ± 0.20 log CFU/sample reduction) (P < 0.05), indicating the potential to be formulated as a dry sanitizer for the LMF industry.IMPORTANCEDSB growth of foodborne pathogens in LMF processing environments is associated with food safety, financial loss, and compromised consumer trust. This work is the first comprehensive examination of the characteristics of Salmonella DSB while exploring its underlying survival mechanisms. Furthermore, morin dissolved in 70% IPA was proposed as an efficient dry sanitizer against DSB to provide insights into biofilm control during LMF processing.

Characterization of a cell wall hydrolase with high activity against vegetative cells, spores and biofilm of Bacillus cereus

Bacillus cereus is a prevalent foodborne pathogen that induces food poisoning symptoms such as vomiting and diarrhea. Its capacity to form spores and biofilm enables it to withstand disinfectants and antimicrobials, leading to persistent contamination during food processing. Consequently, it is necessary to develop novel and efficient antimicrobial agents to control B. cereus, its spores, and biofilms. Peptidoglycan hydrolases have emerged as a promising and eco-friendly alternative owing to their specific lytic activity against pathogenic bacteria. Here, we identified and characterized a Lysozyme-like cell wall hydrolase Lys14579, from the genome of B. cereus ATCC 14579. Recombinant Lys14579 specifically lysed B. cereus without affecting other bacteria. Lys14579 exhibited strong lytic activity against B. cereus, effectively lysing B. cereus cell within 20 min at low concentration (10 μg/mL). It also inhibited the germination of B. cereus spores and prevented biofilm formation at 12.5 μg/mL. Moreover, Lys14579 displayed good antimicrobial stability with negligible hemolysis in mouse red blood cells and no cytotoxicity against RAW264.7 cells. Notably, Lys14579 effectively inhibited B. cereus in boiled rice and minced meat in a dose-dependent manner. Furthermore, bioinformatics analysis and point mutagenesis experiments revealed that Glu-47 was the catalytic site, and Asp-57, Gln-60, Ser-61 and Glu-63 were active-site residues related with the cell wall lytic activity. Taken together, Lys14579 could be a promising biocontrol agent against vegetative cells, spores, and biofilm of B. cereus in food industry.

Effectiveness of Ultra-High Irradiance Blue Light-Emitting Diodes in Inactivating Escherichia coli O157:H7 on Dry Stainless Steel and Cast-Iron Surfaces

The use of blue light-emitting diodes (LEDs) is emerging as a promising dry decontamination method. In the present study, LEDs emitting ultra-high irradiance (UHI) density at 405 nm (842 mW/cm2) and 460 nm (615 mW/cm2) were used to deliver high-intensity photoinactivation treatments ranging from 221 to 1107 J/cm2. The efficacy of these treatments to inactivate E. coli O157:H7 dry cells was evaluated on clean and soiled stainless steel and cast-iron surfaces. On clean metal surfaces, the 405 and 460 nm LED treatment with a 221 J/cm2 dose resulted in E. coli reductions ranging from 2.0 to 4.1 log CFU/cm2. Increasing the treatment energy dose to 665 J/cm2 caused further significant reductions (>8 log CFU/cm2) in the E. coli population. LED treatments triggered a significant production of intracellular reactive oxygen species (ROS) in E. coli cells, as well as a significant temperature increase on metal surfaces. In the presence of organic matter, intracellular ROS generation in E. coli cells dropped significantly, and treatments with higher energy doses (>700 J/cm2) were required to uphold antimicrobial effectiveness. The mechanism of the bactericidal effect of UHI blue LED treatments is likely to be a combination of photothermal and photochemical effects. This study showed that LEDs emitting monochromatic blue light at UHI levels may serve as a viable and time-effective method for surface decontamination in dry food processing environments.

Prevalence of Bacillus cereus in dairy powders focusing on its toxigenic genes and antimicrobial resistance

Bacillus cereus is a common environmental foodborne microorganism that is mainly found to harbor toxigenic genes with multiple antibiotic resistances and is linked to threatening the safety of dried milk in concern to powdered infant milk formula. In the current investigation, the mean value of B. cereus in 140 samples of powdered milk was 0.57 × 10² ± 0.182 × 10², 0.15 × 10² ± 0.027 × 10², 0.21 × 10² ± 0.035 × 10², and 0.32 × 10² ± 0.072 × 10² CFU/g in a percentage of 64.0 samples of whole milk powder, 43.3 of skim milk powder, 26.7 of powdered infant milk formula and 36.7 milk–cereal-based infant formula, respectively. The results revealed that B. cereus isolates were found to harbor toxigenic genes in the following percentages: 77.8, 2.0, 72.7, 16.2, and 67.7 for nhe, hbl, cytK, ces, and bceT, respectively. Despite all evaluated B. cereus strains were originated from dairy powders, they showed a significant difference (P < 0.05) in their harbored toxigenic cytK gene between whole and skim milk powders with powdered infant formula and milk–cereal-based infant formula, as well as between powdered infant formula and milk–cereal-based infant formula. All isolated B. cereus strains were resistant to cefoxitin, colistin sulfate, neomycin, trimethoprim–sulfamethoxazole, oxacillin, and penicillin. Based on the antimicrobial resistance of B. cereus strains to cephalothin, chloramphenicol, nalidixic acid, and tetracycline, there was a significant difference (P < 0.05) between powdered infant milk formula and whole milk powder strains. This survey is one of few studies proceeded in Egypt to determine the prevalence of toxigenic B. cereus strains in milk–cereal-based infant formula and powdered infant formula as well as skim milk powder.

The Use of Ozone as an Eco-Friendly Strategy against Microbial Biofilm in Dairy Manufacturing Plants: A Review

Managing spoilage and pathogenic bacteria contaminations represents a major challenge for the food industry, especially for the dairy sector. Biofilms formed by these microorganisms in food processing environment continue to pose concerns to food manufacturers as they may impact both the safety and quality of processed foods. Bacteria inside biofilm can survive in harsh environmental conditions and represent a source of repeated food contamination in dairy manufacturing plants. Among the novel approaches proposed to control biofilm in food processing plants, the ozone treatment, in aqueous or gaseous form, may represent one of the most promising techniques due to its antimicrobial action and low environmental impact. The antimicrobial effectiveness of ozone has been well documented on a wide variety of microorganisms in planktonic forms, whereas little data on the efficacy of ozone treatment against microbial biofilms are available. In addition, ozone is recognized as an eco-friendly technology since it does not leave harmful residuals in food products or on contact surfaces. Thus, this review intends to present an overview of the current state of knowledge on the possible use of ozone as an antimicrobial agent against the most common spoilage and pathogenic microorganisms, usually organized in biofilm, in dairy manufacturing plants.

Biofilm-associated heat resistance of Bacillus cereus spores in vitro and in a food model, Cheonggukjang jjigae

Bacillus cereus, a foodborne pathogen, is capable of forming spores and biofilms as methods to withstand environmental stresses. These bacterial structures are an issue for food safety as they aid the bacteria survive heat sterilisation processes of foods and food contact surfaces. This study was conducted to investigate the role of the biofilm structure in providing an extra layer of protection to spores against heat treatments. For this, heat resistance of B. cereus spores in intact biofilms was compared to that of planktonic spores in vitro and in a Cheonggukjang jjigae food model. Using methods developed in this study to measure the wet and dry heat resistance of spores in intact biofilms, it was found that B. cereus spores have significantly higher heat resistances when present in biofilms rather than as planktonic spores, and that dry heat is less effective than wet heat at killing spores in biofilms. In further detail, for wet heat treatments, spores in biofilms of the strain isolated from Cheonggukjang (Korean fermented whole soybean), B. cereus CH3, had generally higher wet heat resistances than the reference strain, B. cereus ATCC 10987, both in vitro and in the Cheonggukjang jjigae food model. However, the spores in biofilms of the two strains showed similar heat resistance to dry heat, with some exceptions, when biofilms were formed in vitro or in Cheonggukjang jjigae broth. Meanwhile, B. cereus ATCC 10987 spores in biofilms had higher or similar wet heat resistances in vitro compared to in Cheonggukjang jjigae broth. Wet heat resistances of B. cereus CH3 spores in biofilms were all statistically similar regardless of biofilm formation media (brain heart infusion and Cheonggukjang jjigae broths). For dry heat, spores in biofilms of both B. cereus strains were more heat resistant when biofilms were formed in the Cheonggukjang jjigae food model rather than in vitro. Altogether, heat resistances of spores in biofilms formed in vitro and in the food environment were found to be different depending on the tested B. cereus strain, but higher than planktonic spores in any case. This is the first study examining the heat resistance of B. cereus spores in intact biofilms matrices attached to the surface, both in vitro and in a food model. Therefore, this research is valuable to understand the protective effects of biofilms formed in food environments and to reduce the food safety risks associated with B. cereus.