Co-culture of Cronobacter sakazakii and Staphylococcus aureus: Explore the influence of mixed biofilm formation and regulation of Cronobacter sakazakii biofilm formation genes.

Bacterial biofilm is a protective matrix composed of metabolites secreted by bacteria that envelop bacteria. By forming a biofilm, bacteria can considerably improve their environmental tolerance. In food-related processing environment, different types of microorganisms are often present in biofilms. The main contaminating strain in the powdered infant formula (PIF) processing environment, Cronobacter sakazakii and Staphylococcus aureus continues to pollute the PIF processing environment after biofilm production. This study selected Cronobacter sakazakii with a weak biofilm-forming ability as one of the test organisms. The coexistence of Cronobacter sakazakii and Staphylococcus aureus on the surface of production equipment was simulated to analyze the interaction. Biofilm formation in the co-culture group was significantly higher than the others. In-depth study of the effect of Staphylococcus aureus on the biofilm formation genes of Cronobacter sakazakii. Results show two bacteria can coexist on the surface of a metal device, forming a more compact hybrid biofilm structure. Under co-culture conditions, S. aureus increased bcsA and fliD expression in Cronobacter sakazakii, whereas decreased bcsC expression. Signaling molecules produced by Staphylococcus aureus (Autoinducer 2) significantly promoted the biofilm formation of Cronobacter sakazakii at the concentration of 0-500 ng/mL (0.099-0.177) and up-regulated the expression of bcsA, filD and flhD genes.

Effect of Thermostable Enzymes Produced by Psychrotrophic Bacteria in Raw Milk on the Quality of Ultra-High Temperature Sterilized Milk.

Ultra-high temperature sterilized milk (UHT) is a popular dairy product known for its long shelf life and convenience. However, protein gel aging and fat quality defects like creaming and flavor deterioration may arise during storage. These problems are primarily caused by thermostable enzymes produced by psychrotrophic bacteria. In this study, four representative psychrotrophic bacteria strains which can produce thermostable enzymes were selected to contaminate UHT milk artificially. After 11, 11, 13, and 17 weeks of storage, the milk samples, which were contaminated with Pseudomonas fluorescens, Chryseobacterium carnipullorum, Lactococcus raffinolactis and Acinetobacter guillouiae, respectively, demonstrated notable whey separation. The investigation included analyzing the protein and fat content in the upper and bottom layers of the milk, as well as examining the particle size, Zeta potential, and pH in four sample groups, indicating that the stability of UHT milk decreases over time. Moreover, the spoiled milk samples exhibited a bitter taste, with the dominant odor being attributed to ketones and acids. The metabolomics analysis revealed that three key metabolic pathways, namely ABC transporters, butanoate metabolism, and alanine, aspartate, and glutamate metabolism, were found to be involved in the production of thermostable enzymes by psychrotrophic bacteria. These enzymes greatly impact the taste and nutrient content of UHT milk. This finding provides a theoretical basis for further investigation into the mechanism of spoilage.

Comparative proteomics reveals the antibiotic resistance and virulence of Cronobacter isolated from powdered infant formula and its processing environment.

Cronobacter species are opportunistic foodborne pathogens that can cause neonatal meningitis, sepsis, and necrotizing enterocolitis. In this genus, certain level strains have high mortality to infant (Cronobacter sakazakii and Cronobacter malonaticus) and antibiotic tolerance. Cronobacter has strong environmental tolerance (acid resistance, high temperature resistance, UV resistance, antibiotic resistance, etc.) and can survive in a variety of environments. It has been isolated in various production environments and products in several countries. However, the relationships between Cronobacter antibiotic tolerance and virulence remain unclear, especially at the molecular level. In this study, 96 strains of Cronobacter were isolated from powdered infant formula and its processing environment and screened for antibiotic tolerance, and proteomic maps of the representative strains of Cronobacter with antibiotic tolerance were generated by analyzing proteomics data using multiple techniques to identify protein that are implicated in Cronobacter virulence and antibiotic resistance. The increase in antibiotic tolerance of Cronobacter had a certain increase in the production of enterotoxin and hemolysin. Only triple tolerated Cronobacter sakazakii decreased the utilization of sialic acid. A total of 16,131 intracellular proteins were detected in eight representative strains, and different proteomes were present in strains with different antibiotic tolerance, including 56 virulence-related proteins. Multiple virulence proteins regulated by unknown genes were also found in the eight isolated representative strains.