Mechanisms of Vibrio Biofilm Dispersal from Microplastics Under Simulated Human Environmental Conditions

Biofilms consist of structured communities of bacteria encapsulated within a self-produced polymeric matrix. They are formed as a survival strategy for bacteria like Vibrio parahaemolyticus and Vibrio vulnificus in various environments, including marine settings. These structures are inherently designed to be resistant to environmental stresses and antimicrobial agents, which complicate efforts to minimize their impact on human health.  To this end, a new study published in Frontiers in Microbiology and conducted by Ryan Leighton, Liyan Xiong, Gracie Anderson, Grace Astarita, Guoshuai Cai, Robert Sean Norman and led by Professor Alan Decho from the Department of Environmental Health Sciences at the University of South Carolina, the researchers investigated the dynamics of Vibrio biofilm dispersal from microplastics and its implications on human health. They studied also how different environmental conditions such as nutrient deprivation, changes in temperature and pH, and exposure to simulated human gastric and intestinal fluids affect the dispersal behavior of Vibrio parahaemolyticus and Vibrio vulnificus from MPs.

The team primarily assessed biofilm development and subsequent dispersal of V. parahaemolyticus and V. vulnificus on three types of commonly found marine microplastics: low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS). In their experiments, they simulated environmental conditions (temperature, pH, nutrient levels) and human digestive system conditions (using simulated gastric fluid (SGF) and simulated intestinal fluid (SIF)) to examine how these factors influence biofilm dispersal. The also included in their studies experiments that mimic environmental stressors that biofilms might encounter before and after ingestion by humans. This included sudden changes in temperature, pH adjustments to mimic gastric and intestinal environments, and nutrient starvation conditions to simulate the transition from a nutrient-rich to a nutrient-poor environment. The team used SGF and SIF to replicate the conditions within the human stomach and intestines, respectively. They also introduced a Human Plasma-Like Medium (HPLM) to simulate the nutrient composition of human blood plasma, to examine how such conditions affect biofilm behavior compared to traditional laboratory media. The researchers employed various assays to quantify biofilm biomass, assess the viability of dispersed cells, and measure the concentration of the signaling molecule cyclic-di-GMP, which plays a crucial role in biofilm formation and dispersal.

The authors found that both V. parahaemolyticus and V. vulnificus demonstrated the ability to disperse from all types of microplastic surfaces under almost all tested conditions, which showcase the robust mechanism for survival and spread in the different tested environments. They also found that brief incubation in elevated temperature, altered pH levels, and nutrient deprivation effectively triggered enhanced biofilm dispersal. Interestingly, V. parahaemolyticus showed resilience to low pH conditions, which suggests it may withstand the acidic environment of the human stomach better than V. vulnificus.

Moreover, they observed exposure to SGF, SIF, and HPLM affected the biofilm biomass and dispersal of both Vibrio species differently, which indicates that the composition of the surrounding medium plays a significant role in their lifecycle transitions. Specifically, SGF led to reduced biofilm viability and dispersal, while SIF and HPLM supported biofilm formation and viability to various extents. These experiments revealed that stress conditions, particularly those simulating the human digestive system, influenced the levels of cyclic-di-GMP. High concentrations of this molecule were associated with increased biofilm formation, while its reduction was linked to enhanced dispersal.

In summary, Professor Decho and his team provided critical insights into how Vibrio species manage to disperse from biofilms on microplastics under conditions that mimic their transition from marine environments to the human digestive system. The findings showcase the complexities of microbial adaptation to environmental and physiological stressors and demonstrate the potential risks posed by microplastic pollution as a vector for pathogenic bacteria. Future studies should focus on trying to understand these mechanisms which ultimately lead to develop new strategies to mitigate the health risks associated with the ingestion of contaminated seafood.