The Impact of Strain Variability on the Inactivation Efficacy of Ultrasound Technology

Ultrasound is one of the promising emerging technologies. Ultrasound energy is generated when sound waves travel through a liquid medium. This movement in a liquid medium generates bubbles, and the compression and expansion of these bubbles produce the cavitation phenomenon (1). This cavitation phenomenon is responsible for the antimicrobial action associated with ultrasound exposure.

Cavitation Phenomenon and Antimicrobial Effects

The cavitation phenomenon occurs due to the compression and expansion of bubbles in the liquid medium, producing three main effects:

The first is a mechanical effect due to the compression-expansion cycles of the bubbles, therefore generating microjets and shear forcing that can physically disrupt the bacterial cell membranes, causing leakages of cellular components (2).
Another is the chemical effects generated through the production of radicals due to cavitation phenomenon. These radicals produced during ultrasound treatment; are able to enter into the cell due to the leakage in the cell membrane, which can cause damage to the DNA.
Next is the thermal effect; due to the expansion of the bubbles, there is a creation of localized hotspots, and the increasing cycles of the compression-expansion cycles of the bubbles make for the increase in temperature of the liquid medium during the US treatment, which further contributes to the killing effect of US treatment.

Challenges and Innovations in Ultrasound Technology

In addition to its inactivation potential, ultrasound (US) is valued for its ability to preserve the functional components of food, a challenge that conventional thermal processing often faces. However, US has not enjoyed commercial usage in the food industry because sonication alone requires a long treatment time and high energy density to achieve the required industrial benchmark for food processing (5 log reduction) (3). This has encouraged research into ultrasound-assisted processes where the US is combined with heat, chemicals, essential oil, and other non-thermal technologies to facilitate synergistic/additive effects and increase the killing rate in a shorter time.

In our study, we evaluated how strain variability affects the efficacy of ultrasound (US) treatment under dynamic temperature conditions. As strains we define the isolates of the same microbial species with different genotypic/phenotypic characteristics (4). Therefore, due to the distinct phenotypic and genotypic differences, variability in resistance to ultrasound treatment is crucial. In-depth studies on one strain may not be applicable to another strain within different species. Some studies have evaluated strain variability in growth and inactivation to thermal treatment (5, 6). Large variations in resistance and even growth under different conditions within a microbial species. Currently, existing studies on inactivation variability to US treatment are scarce; our study focused on a diverse pool of strains from very important food-associated microorganisms, which were important foodborne pathogens and food spoilage microorganisms.

Strains of Listeria monocytogenes, Saccharomyces cerevisiae, Lactiplantibacillus plantarum and Escherichia coli were exposed to ultrasound treatment and then the microbial population before and after ultrasound treatment were evaluated to assess resistance (7). The results of our studies showed that resistance to US treatment varied between the species we assessed. However, the magnitude of variability in resistance differed between the species, with L. plantarum and S. cerevisiae having the lowest and highest variability respectively. Our findings also show that there was no correlation between the strain’s origin and inactivation levels in S. cerevisiae, L. plantarum, and L. monocytogenes (7). However, the US resistant strains of E. coli shared a similar characteristic of possessing a transmissible locus of heat resistance. Identified resistant and sensitive strains for microorganisms provides valuable information that can be useful for further research concerning US technology. For instance, resistant strains will help in optimisation studies for US or US-assisted processes to ensure food safety and quality. In addition, comparison between sensitive and resistant strains of species will help identify the molecular targets of US and strain-specific differences responsible for susceptibility to US technology.

Finally, the presence of strain variability following US exposure indicates that the efficacy of US treatment is not only dependent on the species but also on the strains within a species. Therefore, process designs, predictive modelling for shelf-life, optimization, and quantitative risk assessment need to take strain variability into serious consideration, as controls developed for a sensitive strain might be counterproductive for a resistant strain in terms of ensuring food safety and quality.

References

Dai, Jinming, Mei Bai, Changzhu Li, Haiying Cui, and Lin Lin. “Advances in the mechanism of different antibacterial strategies based on ultrasound technique for controlling bacterial contamination in food industry.” Trends in Food Science & Technology 105 (2020): 211-222.
Cameron, Michelle, Lynn D. McMaster, and Trevor J. Britz. “Electron microscopic analysis of dairy microbes inactivated by ultrasound.” Ultrasonics Sonochemistry 15, no. 6 (2008): 960-964.
Lee, Hyoungill, Bin Zhou, Wei Liang, Hao Feng, and Scott E. Martin. “Inactivation of Escherichia coli cells with sonication, manosonication, thermosonication, and manothermosonication: Microbial responses and kinetics modeling.” Journal of Food Engineering 93, no. 3 (2009): 354-364.
Wiedmann, Martin. “Molecular subtyping methods for Listeria monocytogenes.” Journal of AOAC International 85, no. 2 (2002): 524-532.
Aryani, D. Chandra, H. M. W. Den Besten, W. C. Hazeleger, and M. H. Zwietering. “Quantifying variability on thermal resistance of Listeria monocytogenes.” International journal of food microbiology 193 (2015): 130-138.
Aryani, D. C., H. M. W. Den Besten, and M. H. Zwietering. “Quantifying variability in growth and thermal inactivation kinetics of Lactobacillus plantarum.” Applied and Environmental Microbiology 82, no. 16 (2016): 4896-4908.
Okafor, E.T., Pavli, F., Hummerjohann, J., & Valdramidis, V (2024). Determination of Microbial Strain Variability and Kinetics of Food-associated Microorganisms Following Ultrasound Treatment. (Manuscript submitted for Publication). Department of Food Sciences and Nutrition, University of Malta

Esther Tobechukwu Okafor

PhD Student | R&D Scientist | Food Safety Microbiologist

Verified

Esther Tobechukwu Okafor completed her BSc degree in Microbiology from the University of Nigeria (Nsukka). Through this, she acquired research competence in food microbiology and African beer fermentation. After her bachelor study, she worked in the food industry for about 4 years as a Food Safety and Quality assurance professional. Here, she was responsible for assuring food and water safety, microbiological analysis, continuous process improvement, and new product development. In 2019, she proceeded to the Netherlands to do a master’s degree in food safety (Specialisation: food microbiology) at Wageningen University (the Netherlands). During the masters’ thesis, she assessed the susceptibility of biofilms of Listeria monocytogenes to cleaning and disinfection regimes. Based on the findings of the master’s thesis, her interest in alternative antimicrobials was stimulated, and this led to a research internship at the Universidad Politecnica de Cartagena (Spain) on essential oils and their ability to induce antibiotic resistance. Currently, Esther is working as a PhD student at the University of Malta (Malta). The project aims to identify the molecular targets and the underlying mechanism for inactivation by Ultrasound technology for common food-borne pathogens and spoilage microorganisms.

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