Antibacterial effect of light emitting diodes of visible wavelengths on selected foodborne pathogens at different illumination temperatures

https://doi.org/10.1016/j.ijfoodmicro.2013.07.018Get rights and content

Highlights

  • 461 and 521 nm LEDs produced a bactericidal effect at 10 and 15 °C but not 20 °C.

  • 642 nm LED did not show bactericidal or bacteriostatic effect at any temperature.

  • The Gram nature of the strains did not influence the susceptibility to the LEDs.

  • LEDs combined with chilling could be used as a novel food preservation technology.

Abstract

The antibacterial effect of light emitting diodes (LEDs) in the visible region (461, 521 and 642 nm) of the electromagnetic spectrum was investigated on Escherichia coli O157:H7, Salmonella typhimurium, Listeria monocytogenes and Staphylococcus aureus. The irradiances of the 461, 521 and 642 nm LEDs were 22.1, 16 and 25.4 mW/cm2, respectively. Bacterial cultures suspended in tryptic soy broth were illuminated by 10-watt LEDs at a distance of 4.5 cm for 7.5 h at 20, 15 and 10 °C. Regardless of the bacterial strains, bacterial inactivation was observed with the range of 4.6–5.2 log CFU/ml at 10 and 15 °C after illumination with the 461 nm LED, while illumination with the 521 nm LED resulted in only 1.0–2.0 log reductions after 7.5 h. On the other hand, no antibacterial effect was observed using the 642 nm LED treatment. The photodynamic inactivation by 461 and 521 nm LEDs was found to be greater at the set temperatures of 10 and 15 °C than at 20 °C. The D-values for the four bacterial strains at 10 and 15 °C after the illumination of 461 nm LED ranged from 1.29 to 1.74 h, indicating that there was no significant difference in the susceptibility of the bacterial strains to the LED illumination between 10 and 15 °C, except for L. monocytogenes. Regardless of the illumination temperature, sublethal injury was observed in all bacterial strains during illumination with the 461 and the 521 nm LED and the percentage of injured cells increased as the treatment time increased. Thus, the results show that the antibacterial effect of the LEDs was highly dependent on the wavelength and the illumination temperature. This study suggests the potential of 461 and 521 nm LEDs in combination with chilling to be used as a novel food preservation technology.

Introduction

The preservation of raw fruits, vegetables, fish and meat products is of prime importance to the food industry. This is not only because these foods are consumed widely throughout the world as part of a staple diet, but also because they are highly prone to contamination by microorganisms due to their high water activity and rich content of nutrients. The most common preservation method for these raw foods is refrigeration or chilling; however, it does not kill microorganisms, but simply inhibits or retards their growth. Moreover, some pathogenic bacteria such as Listeria monocytogenes and Yersinia enterocolitica, and other psychrotrophic bacteria can grow at low temperatures, threatening public health and shortening the shelf life of raw foods (Walker et al., 1990, Andersen et al., 1991). Thus, there is a need for the development of another hurdle for these raw foods which will be effective in eliminating or reducing microbial contamination and be environmentally friendly without compromising on the quality of foods and public health.

A light emitting diode (LED) is a semiconductor device that emits visible light when an electric current passes through it. LEDs can emit light within a very narrow wavelength spectrum and can be considered to be of a monochromatic wavelength (Held, 2009). This is an advantage over other traditional visible light sources as they are not able to produce monochromatic wavelengths. LEDs also have several other advantages over traditional visible light sources such as lower energy consumption and high durability. The size of the LED can be made to be very small which would be flexible to fit most designs and be easily implemented into existing systems without requiring special disposal methods at the end of its use (Mori et al., 2007, Hamamoto et al., 2007). For these reasons, LED technology has been widely applied to not only optics and electronics, but also agriculture and medicine.

LEDs bring about an antibacterial effect through a phenomenon known as photodynamic inactivation. Some intracellular molecules known as photosensitizers can produce reactive oxygen species (ROS) once they absorb light, which react with cellular constituents such as lipids, proteins and the DNA to bring about a cytotoxic effect (Luksiene, 2009). Based on this mechanism, the use of LEDs has recently received increased attention and its potential for clinical applications has been investigated as a novel technology for bacterial inactivation. Maclean et al. (2010) reported that a 405 nm LED system inactivated methicillin resistant Staphylococcus aureus (MRSA) in clinical environments such as the vascular ward, the burns unit and the intensive care unit, and observed reductions between 56 and 90% in the bacterial concentration on frequently touched contact surfaces. The studies conducted by Guffey and Wilborn, 2006a, Guffey and Wilborn, 2006b tested the effect of LEDs of 405 and 470 nm on the populations of Pseudomonas aeruginosa, Staphylococcus aureus and Propionibacterium acnes, and a wavelength dependent inactivation pattern was observed.

Although there have been some studies on using LEDs to produce a 405 nm wavelength for inactivating the bacteria aforementioned, little information is available on the antibacterial effect of other wavelengths. Thus, the objective of this study was to determine if wavelengths above the 405 nm region such as 461, 521 and 642 nm have an antibacterial effect on four common foodborne pathogens, Escherichia coli O157:H7, L. monocytogenes, Salmonella typhimurium and S. aureus. To see if temperature affects the inactivation of pathogens by LEDs, three different temperatures were tested during the LED illumination. Testing the effect of temperature on the inactivation would give insights into the potential for using chilling in combination with LEDs as a novel technology for food preservation. The sublethal injury of the surviving cells after the LED illumination was also evaluated in this study.

Section snippets

Bacterial cultures

The bacterial strains used in this study were E. coli O157:H7 (EDL 933), S. typhimurium (ATCC 14028), L. monocytogenes (BAA-679) and S. aureus (ATCC 6538). E. coli O157:H7 was obtained from Dr. Henry Mok at the Department of Biological Sciences at the National University of Singapore. The other bacterial strains were purchased from the American Type Culture Collection (ATCC; Manassas, Virginia, USA). To obtain working cultures from the frozen stock, vials were thawed at room temperature, and a

Characterization of the LED

Three LEDs – blue, green and red, were found to have intensity peaks at 461, 521 and 642 nm wavelengths, respectively (Fig. 2). The dosages received by the bacterial suspensions were calculated based on the measured irradiance after each time interval during the illumination (Table 1). After a 7.5-h illumination by the 461, 521 and 642 nm LEDs, the dosages received by the bacterial suspension were 596.7, 431.2, and 688.0 J/cm2, respectively, reflecting the fact that the red colored LED had a

Discussion

The influence of LED on photosynthesis in plants prompted investigators to study its effect in improving the nutritional values of fruits and vegetables. For example, Ma et al. (2012) reported that treatment with 660 nm LEDs on citrus fruits induced accumulation of β-cry which is the predominant carotenoid in Satsuma mandarins. In addition, it was found that blue (465–470 nm) and red (625–630 nm) LEDs were effective in increasing chlorophyll and β-carotene contents in leaves of pea seedlings (Wu

Conclusion

The degree of bacterial inactivation using the 461 nm LED was much higher than that achieved using the 521 nm LED. The illumination temperature played an important role in the photodynamic inactivation using the 461 nm LEDs in that only a bacteriostatic effect was observed at 20 °C. The red LED of 642 nm did not influence the bacterial behavior during illumination. These results indicate that the efficacy of the LEDs in inactivating these four foodborne pathogens was dependent not only on the

Acknowledgment

This research was funded by the A*STAR grant (SERC 112-177-0035).

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