Chandre Van de Merwe | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Chandre Van de Merwe. This seminar is open to the general public to attend.

Link for online participants: https://ualberta-ca.zoom.us/j/96878013258?pwd=Qnd1R2dTTnJKMUhFTFlTMkY5aXFwUT09

Thesis Topic: Pressure resistance of Escherichia coli and Listeria monocytogenes

PhD with Drs. Lynn McMullen and Michael Gaenzle.

Seminar Abstract:

High pressure processing (HPP) is a non-thermal processing technology aimed to inactivate bacteria and improve the shelf life of food. Commercial parameters for ready-to-eat (RTE) meat products are 600 MPa for 3-7 min at cold or room temperatures.  Many regulatory agencies require a 5-log reduction of pathogenic bacteria when foods are treated with HPP. The inactivation of pathogens is dependent on several factors including the food matrix. The aim of the research was to investigate impact of carbohydrate and fat on pressure resistance of Escherichia coli in beef and yogurt. To determine the effect of fat, four ground beef and five yogurt samples with different fat content were inoculated with resistant or sensitive strains of E. coli and treated at 600 MPa, 20 or 30°C for 3 min. To eliminate the effects of adiabatic heating, sample temperatures were adjusted prior to pressure treatment. The impact of HHP was dependent on strain and fat content in ground beef. When samples were cooled prior to treatment to account for adiabatic temperature changes, E. coli was more resistant to HPP in low fat (3%) ground beef compared to resistance in high fat (35.7%) ground beef. In yogurt, the impact of the fat content on treatment lethality was limited. The reduction of cell counts for different strains ranged from less than 2 to more than 8 log CFU/g, indicating that lethality of HHP is highly strain dependent.

To reduce the prevalence of pathogenic microorganisms such as L. monocytogenes and E. coli O157:H7 in food processing facilities, extensive cleaning and sanitation practices are used. Unfortunately, bacteria form biofilms, a complex network for cells surrounded by extracellular components that not only allows for sessile cells to attach to surfaces but also increases resistance to sanitizers. The aim of this research was to investigate if sessile cells of L. monocytogenes in single-, dual- and multi-species biofilms on RTE chicken meat and in broth solutions are more resistant to HPP compared to planktonic cells. Sessile cells were transferred onto RTE chicken meat using methods that mimic the transfer from a meat slicer onto the product. The RTE chicken meat and broth samples were treated at 600 MPa for 3 mins at 20 °C. L. monocytogenes cells in broth were more sensitive to high pressure compared to planktonic cells on meat. When using the method that mimics the transfer from slicer to meat, there was no difference in resistance between sessile and planktonic cells. This is the first study to investigate the resistance of L. monocytogenes biofilms to HPP.

L. monocytogenes biofilms are believed to contribute to its long-term persistence in food processing facilities. Once the organism establishes itself in the food processing facility it can contaminate foods products through cross contamination from the environment. The final aim of this research was to use comparative genomics with total of 398 Listeria genomes to determine if there was a correlation between source of isolation and the ecology of different strains of L. monocytogenes. Strains of L. monocytogenes belonging to phylogenic clade lineage II was more likely to persist in food processing facilities compared to strains from lineage I. Furthermore, an overall trend was observed in which a higher proportion of lineage II strains were isolated from food processing facilities and meat. The pressure resistance strains isolated from meat and dairy processing facilities were evaluated and resistance between lineage I and II was compared. At 400 MPa, lineage I strains were more resistant to HPP compared to lineage II strains but there was no difference when treated at 500 MPa or greater. The different contamination routes and phenotypic response to intervention methods observed by L. monocytogenes from lineage I and II demonstrate the need for different control measures to minimize the spread and contamination of this pathogen in food processing facilities. With optimization, HHP could be a potential post processing intervention to inactive L. monocytogenes and E. coli O157:H7.