Tingting Gu wins the 2024 pre-doctoral research poster competition

Person poses with a peace sign in front of the EPI vertical banner that reads "understanding global emergence and the spread of infectious disease."

Listeria monocytogenes is one of the top five foodborne pathogens responsible for food safety risks that lead to illnesses. University of Florida College of Agricultural and Life Sciences Ph.D. candidate Tingting Gu explored the implications of Listeria’s symbiosis with other background microflora on food contact surfaces. Her research project, which is funded by the Center for Produce Safety, won first place in the pre-doctoral poster competition at EPI Research Day 2024.

Listeria can cause listeriosis, a serious infection which especially affects vulnerable populations like immunocompromised people, newborns, elderly and pregnant women,” Gu said.

As a consequence of decreased sanitation effect in those hard-to-reach areas in food processing facilities, Listeria has been found in fresh produce, dairy products and frozen meat products. Gu and her team simulated produce processing environments by testing the bacteria on stainless steel 304, which is the most widely used food contact surface found in facilities.

To investigate Listeria’s persistence under different environmental factors, the team developed cocktail biofilms – combinations of Listeria and other bacteria – with a foodborne pathogen commonly found in facilities, such as E. coli O157:H7, and Pseudomonas fluorescens and Ralstonia insidiosa, which are good biofilm formers that have been isolated in produce processing environments from a research study conducted by USDA.

Results revealed Listeria’s persistence is significantly influenced by symbiotic relationships with other microflora. In addition, symbiosis is significantly affected by environmental factors, including surface properties of food contact surfaces, microbial nutrition, and symbiotic species. The symbiosis implies utilization of indicator bacteria that may lead to increased or decreased Listeria-related risks.

“This phenomenon is very interesting,” Gu said. “My next step is to investigate why.”

Gu previously earned her master’s degree in pharmaceutical sciences from the University of Massachusetts Lowell before coming to UF to pursue a doctorate in food science. She pivoted toward studying foodborne pathogens to bridge her pharmaceutical science knowledge with her passion for public health, and she hopes to make a difference by bringing a new perspective with her contributions to the field.

“I realized how meaningful this work is and how great our team is as a whole,” Gu said. “This is the result of the whole team – my advisor and my other lab mates – that contributed so much to finish this project.”

Implications of symbiosis on listeria monocytogenes biofilm on food contact surfaces


  • Tingting Gu – Department of Food Science and Human Nutrition, Institute of Food and Agricultural Sciences, University of Florida
  • Boce Zhang – Department of Food Science and Human Nutrition, Institute of Food and Agricultural Sciences, University of Florida

We investigated Listeria monocytogenes biofilm formation in cocktail cultures under various conditions. We found L. monocytogenes biofilm formation is greatly affected by symbiotic microorganisms, and their symbiosis can be influenced by the environment.


L. monocytogenes can adhere to food contact surfaces and form highly persistent biofilms. L. monocytogenes biofilm can be significantly affected when it forms cocktail biofilms with other microorganisms, especially good biofilm formers. To investigate how this symbiosis affects L. monocytogenes’ persistence, the study developed cocktail biofilms of L. monocytogenes with other common isolates in produce processing facilities.


Stainless steel 304 coupons (SS304) as standard food contact surfaces were used with four types of noncoated surface topographies: 1) coupons with commercial native/bare finish (SS304-B), 2) coupons with commercial #4 brushed finish (SS304-4), 3) coupons modified with micropillars (SS304-Dot), and 4) coupons modified with microlines (SS304-Line). The study included coupons with and without Dursan coating, a previously identified fouling-resistant agent against monospecies L. monocytogenes biofilm. The study evaluated the symbiosis in dual-species cocktail biofilm (L. monocytogenes + Escherichia coli O157:H7; L. monocytogenes + Pseudomonas fluorescence; L. monocytogenes + Ralstonia insidiosa) and four-species cocktail biofilm (L. monocytogenes + E. coli O157:H7 + P. fluorescence + R. insidiosa). L. monocytogenes biofilms were cultivated in monospecies and cocktail species on different SS304 coupons for 7 days at 4 °C in lettuce juice extract to simulate the produce processing conditions.


Among dual-species cocktail biofilms, P. fluorescence had a strong synergistic effect on L. monocytogenes biofilm formation on all SS304 coupons. E. coli O157:H7 and R. insidiosa showed synergistic or antagonistic symbiosis with L. monocytogenes on different coupons, suggesting substrate properties can influence symbiotic relationships in cocktail biofilm. In the four-species cocktail biofilm, synergistic symbiosis with L. monocytogenes was consistently observed under all conditions. The results suggest that symbiosis in biofilm can be impacted by cocktail species, organic loads, surface chemistry, and surface topography.


Symbiosis is critical in L. monocytogenes biofilm formation and can have significant food implications. Synergistic symbiosis with L. monocytogenes should be mitigated to avoid negative food safety implications. Antagonistic symbiosis may indicate potential intervention strategies against L. monocytogenes biofilm.