Introduction
Microwaves refer to electromagnetic waves that are non-ionizing and have frequencies that range from 0.3 to 300GHz. Microwaves produce two effects when irradiated on microorganisms: thermal and non-thermal effects. Thermal effects result from microwave energy absorption by cell molecules, triggering them to vibrate at a fast speed, thus having an overall heating effect. The non-thermal effect of microwaves is an idea that emanated from experiments where bacterial cultures were destroyed mainly by microwave-induced heating.
The General Effect of Microwaves on the Growth of Microorganisms
The effect microwaves will have on microorganism growth is primarily reliant on the radiation frequencies and the overall energy the microorganism absorbs. The application of microwaves at specific frequencies, for sufficiently long periods, and at high energies has a dominant thermal effect and kills the cells of yeast or bacteria (Zhen et al., 2022). Multiple trials using microwave radiation of numerous yeast and bacteria cultures in wet environments, for instance, water suspensions, failed to indicate additional killing of microbes by microwaves. This is compared to killing instigated at identical temperatures using conventional heating methods. Nonetheless, in dry environments, the microwave radiation-killing effect happened at significantly low rates and after prolonged irradiation.
This was most probably due to the low rate of transforming the energy of microwaves to heat. Some studies have even illustrated that the degree of microorganism killing was linked with the moisture content of the specimen under experiment (Esam, et al., 2021). In distinction, irradiating microorganisms with microwaves at lower temperatures than the thermal destruction level resulted in numerous effects, including enhanced growth and death. Some microorganisms showed differing effects. For example, in Escherichia coli, the pernicious impact of microwaves was dissimilar to that of hyperthermia.
Thermal Effects of Microwaves on the Growth of Microorganisms
Heating microorganisms beyond the thermal destruction temperature by microwaves kills them. Multiple studies have been conducted to establish the minimal microwave energy dose that can be utilized for sterilization and disinfection (Esam, et al., 2021) in a study that involved toothbrushes with contaminants of Streptococcus mutants, a bacteria that leads to dental caries, microwave radiation for five minutes exhibited complete sanitization of the toothbrushes (Esam, et al., 2021). It has also been demonstrated that microwaves are effective in bacteria reduction, for example, in formerly worn denatures. The frequency of microwaves at 18GHZ has been shown to have a robust deactivating effect on bacteria. After exposure to microwave at such a frequency three times consecutively, Escherichia coli and Staphylococcus aureus cultures were inactivated entirely. Exposure of Bacillus cereus spores and Escherichia coli cultures to a home microwave oven’s greatest power obliterates them after four and two minutes, respectively.
The microwave’s thermal killing effect is not selective, and microorganisms will be destroyed when supplied with an appropriate dose. The irradiation of cultures of Staphylococcus aureus, Bacillus subtilis, Candida albicans, and Staphylococcus aureus contaminated with dental resin with microwaves at 650 watts results in killing after 2 minutes. Even more vulnerable to microwave radiation than is the case with bacterial cultures is Candida albicans. Similar killing effects are observed in bacteria of identical species radiated for 6 minutes at 650 watts. In particular, C. albicans inoculums and S. aureus are destroyed entirely, significantly reducing the number of B. subtilis and P. aeruginosa. Microwave killing effect on microorganisms is non-selective and is synergistic with the effects of other killing agents that are non-selective. In a study that entailed the interaction of hydrogen peroxide, a disinfectant with microwave (Esam, et al., 2021). After 10 minutes of exposure and a maximum temperature of 60°C, the synergetic effect of killing Pseudomonas aeruginosa and Escherichia coli was attained.
Certain conditions may have the effect of reducing the harmful thermal impact that microwaves have on microorganisms. This includes increasing sodium chloride concentration in an extracellular medium. Cultures of Salmonella enteritidis, Staphylococcus aureus, Staphylococcus aureus, and Escherichia coli were injected into mashed potato preparation (Sohail, et al., 2022). Then, it was exposed to a microwave at 2450 MHz frequency and heated at 800 watts for one minute. The percentage of killed cells is inverse to the medium’s concentration of sodium chloride.
Non-Thermal Effects of Microwaves on the Growth of Microorganisms
At microwave frequencies of 835 and 640-41 MHz, there is an effect that appears resonant on the development of specific yeast such as Saccharomyces cerevisiae. This effect is not reliant on energy absorbed from the microwave. A conclusive result cannot be reached with a decrease or enhancement of 10 MHZ frequency of this microwave range. However, it is essential to understand the non-thermal mechanism microwaves have in microbes.
This will help in the future utilization of microwave technology or in avoiding the harmful effects it may have on humans living symbiotically with microbes. However, the limitation in the scope of studies has resulted in little knowledge (Esam, et al., 2021). One study involving the exposure to microwaves at 18 GHZ frequency of Escherichia coli culture at under 40°C temperature observed that cell membrane pores opened and there were temporary morphological fluctuations (Esam, et al., 2021). This effect appears to be electro-kinetic and is induced by increased cations and anion movements, resulting in the cell membrane’s localized structural disarrangements, triggering the appearance of pores.
Huge pores of membranes result in the leakage of crucial intracellular molecules out of the bacteria cells, which can result in death. Over 87% of the exposed cells remained viable; thus, the effect was rescindable. Microwaves had identical effects on cell membranes in another study (Esam, et al., 2021). The study entailed irradiation of the spores of Bacillus licheniformis with microwaves for 2 minutes at 2 KW and 2450 MHz frequency (Esam, et al., 2021). It resulted in spore cortex hydrolysis, bulging, and rupturing, including the inner membranes. This effect is caused by microwave non-thermal effects, as identical temperatures did not result in such fluctuation in the inner membrane and spore coat.
The application of microwave radiation with analogous characteristics to vegetative forms of Bacillus subtilis causes cell wall disruption, and with electron microscopy transmission, cytoplasmic proteins are detected. When Bacillus subtilis and Escherichia coli are radiated with microwave at 600 W, 2450MHz frequency, and at 40 degrees, it was observed that the cell wall swelled and cytoplasmic proteins were aggregated. These effects could not be ascribed to thermal fluctuations as other heat sources did not produce identical changes. The microwave’s irradiation disintegrating effects are time-dependent, with the cell wall damage intensity being proportionate to the total microwave energy that has been absorbed.
Conclusion
In conclusion, microwave radiation can substantially affect the growth of microbial cultures. These effects differ from enhancing microorganisms’ growth to killing them (Esam, et al., 2021). The effects’ extent and nature rely on the frequency of microwaves and the entire energy the microorganism absorbs. Seemingly low-frequency, low-energy microwaves boost microorganism growth (Esam, et al., 2021). On the other hand, high-frequency, high-energy microwaves have the effect of destroying microorganisms.
References
Esam, B. Y., Ali, M., Muhanad, A. A., Abdulmutalib, A. A., Amaal, M. A., Fatima, M. G., & SaraS, A. (2021). Evaluation the effect of microwave radiation on gram positive and negative bacteria. Journal of Chemistry and Nutritional Biochemistry, 39-45. Web.
Sohail, M., Juie, N. R., Eun, H. C., & Ihn, H. (2022). Microwave radiation and the brain: mechanisms, current status, and future prospects. International Journal of Molecular Sciences, 2-26. Web.
Zhen, Z., Jiahao, W., Yihe, H., & Long, W. (2022). Microwaves, a potential treatment for bacteria: a review. Front Microbiol, 10-12. Web.