Uncovering Salmonella dry surface biofilm

Food scientists at the National University of Singapore (NUS) have revealed key characteristics of the foodborne pathogen Salmonella dry surface biofilm (DSB), a previously overlooked type of biofilm that commonly exists in dry food processing environments.

Bacterial biofilm formation in the food industry is a significant concern, affecting food safety, production efficiency, economic loss, and consumer trust. Most existing research on foodborne pathogenic biofilms has focused on wet surface biofilms (WSBs), as it is generally believed that microorganisms cannot survive under dry conditions, let alone form biofilms. However, recent recalls and outbreaks of Salmonella associated with low-moisture food (LMF) have provided mounting evidence that pathogens can persist in dry conditions.

A research team led by Assistant Professor LI Dan from the Department of Food Science and Technology at NUS has provided a detailed characterisation of the Salmonella DSBs examining their morphology, single-cell landscape, and response to sanitisation techniques in comparison to traditional WSBs. The efforts of this research offer valuable insights for managing dry biofilms in the food industry.

The research findings were published in the journal Applied and Environmental Microbiology* by the American Society for Microbiology (ASM).

The study explored the structural and functional characteristics of Salmonella Typhimurium DSB using advanced microscopy techniques, including confocal laser scanning, transmission electron, and scanning electron microscopy. These analyses revealed unique features of DSB compared to WSB. Specifically, DSB cells are surrounded by a dense, compact capsule that likely contributes to their higher antimicrobial resistance, while WSB cells display distinct plasma and outer membrane layers. Additionally, DSBs exhibit a “sandwich-like” vertical structure, with intact cells at the core and damaged cells on the outer layer.

To understand functional diversity within the biofilm, single-cell transcriptomics was employed, identifying bacterial clusters with active antioxidative and virulence functions—highlighting potential safety concerns for LMFs. The team also developed a water-free antibiofilm strategy tailored for the LMF industry. This method combines the flavonoid morin with 70% isopropyl alcohol, offering an effective sanitisation method without requiring water.

Confocal laser scanning microscopy of Salmonella wet biofilm (WSB) and dry surface biofilm (DSB). The DSB displays a distinctive “sandwich-like” vertical structure, as revealed in the cross-sectional views on the right and bottom of the image. Intact (green) cells are protected in the inner layer, while damaged (red) cells on the outer layer experience desiccation stress. This structure demonstrates the resilience of inner microbial cells in dry conditions, challenging the belief that low-moisture environments fully inhibit foodborne microorganisms.

Prof Li said, “The outcomes of this research are of critical importance, offering several pivotal insights. By detailing the structural and functional properties of Salmonella DSB, we illuminate how these biofilms resist desiccation and sanitisation, which are prevalent challenges in LMF environments. The application of single-cell RNA sequencing allows us to uncover the heterogeneity within DSB populations. These findings challenge the perception of dehydrated biofilm cells as uniformly dormant and underscore the persistent threat posed by these pathogens.

“With the advent of flavonoid-based sanitisers, the food industry can adopt more effective, eco-friendly, and scalable solutions for biofilm management. Implementing such strategies could significantly reduce contamination events, enhance consumer trust, and minimise financial losses associated with recalls and outbreaks. Taken together, these outcomes reaffirm our commitment to translating scientific discoveries into actionable solutions for food safety and public health,” added Prof Li.

Reference

Lin ZJ; Liang ZQ; He S; Chin FWL; Huang DJ; Hong Y; Wang X; Li D*, “Salmonella dry surface biofilm: morphology, single-cell landscape, and sanitization” Applied and Environmental Microbiology Volume: 90 Issue: 11 DOI: 10.1128/aem.01623-24 Published: 2024.  

* The paper was selected in the spotlight list as “Articles of Significant Interest in This Issue” in November 2024 (https://journals.asm.org/doi/10.1128/aem.02224-24).