Unraveling the significance of calcium as a biofilm promotion signal for Bacillus licheniformis strains isolated from dairy products.

Bacillus licheniformis, a quick and strong biofilm former, is served as a persistent microbial contamination in the dairy industry. Its biofilm formation process is usually regulated by environmental factors including the divalent cation Ca2+. This work aims to investigate how different concentrations of Ca2+ change biofilm-related phenotypes (bacterial motility, biofilm-forming capacity, biofilm structures, and EPS production) of dairy B. licheniformis strains. The Ca2+ ions dependent regulation mechanism for B. licheniformis biofilm formation was further investigated by RNA-sequencing analysis. Results revealed that supplementation of Ca2+ increased B. licheniformis biofilm formation in a dose-dependent way, and enhanced average coverage and thickness of biofilms with complex structures were observed by confocal laser scanning microscopy. Bacterial mobility of B. licheniformis was increased by the supplementation of Ca2+ except the swarming ability at 20 mM of Ca2+. The addition of Ca2+ decreased the contents of polysaccharides but promoted proteins production in EPS, and the ratio of proteins/polysaccharides content was significantly enhanced with increasing Ca2+ concentrations. RNA-sequencing results clearly indicated the variation in regulating biofilm formation under different Ca2+ concentrations, as 939 (671 upregulated and 268 downregulated) and 951 genes (581 upregulated and 370 downregulated) in B. licheniformis BL2-11 were induced by 10 and 20 mM of Ca2+, respectively. Differential genes were annotated in various KEGG pathways, including flagellar assembly, two-component system, quorum sensing, ABC transporters, and related carbohydrate and amino acid metabolism pathways. Collectively, the results unravel the significance of Ca2+ as a biofilm-promoting signal for B. licheniformis in the dairy industry.

inhibitory effects of citral, cinnamaldehyde, and tea polyphenols on mixed biofilm formation by foodborne Staphylococcus aureus and Salmonella enteritidis.

Biofilms are significant hazards in the food industry. In this study, we investigated the effects of food additive such as citral, cinnamaldehyde, and tea polyphenols on mixed biofilm formation by foodborne Staphylococcus aureus and Salmonella serotype Enteritidis. The adhesion rates of mixed strains in sub-MIC of additives were determined by a microtiter plate assay and bacterial communication signal autoinducer 2 (AI-2) production via a bioluminescence reporter Vibrio harveyi BB170. The structure of mixed biofilm was analyzed using scanning electron microscopy. The effect of the disinfectants hydrogen peroxide, sodium hypochlorite, and peracetic acid was tested on the mixed biofilm. Our results demonstrated that citral, cinnamaldehyde, and tea polyphenols were able to significantly inhibit mixed biofilm formation, while citral could reduce the synthesis of AI-2. Conversely, we observed a significant increase in AI-2 mediated by cinnamaldehyde. Tea polyphenols at lower concentrations induced AI-2 synthesis; however, AI-2 synthesis was significantly inhibited at higher concentrations (300 m g/ml). Food additives inhibited the adhesion of mixed bacteria on stainless steel chips and increased the sensitivity of the mixed biofilm to disinfectants. In conclusion, citral, cinnamaldehyde, and tea polyphenols had strong inhibitory effects on mixed biofilm formation and also enhanced the effect of disinfectant on mixed biofilm formation. This study provides a scientific basis for the application of natural food additives to control biofilm formation of foodborne bacteria.

Measurement of the complex refractive index of tissue-mimicking phantoms and biotissue by extended differential total reflection method.

Refractive index of biotissue is a useful optical parameter in the biomedical field. An extended differential total reflection method is introduced to determine the complex refractive index. The real part is directly determined by differential of the reflectance curve, and the imaginary part is obtained from nonlinear fitting. The method is verified by a series of tissue-mimicking phantoms, porcine muscle and porcine adipose.