UV light for transfer disinfection: Is there any point?

Jul 95 min read

Updated: Jul 16

Opinion Review Article | Open Access | Published 16th July 2024

UV light for transfer disinfection: Is there any point?

Tim Sandle, Ph.D., CBiol, FIScT | EJPPS | 292 (2024) | Click to download pdf

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One of the choices to be made when purchasing a biosafety cabinet or transfer hatch is whether to add the additional feature of an ultraviolet light as an additional step to aid the process of microbial kill, in addition to liquid chemical disinfection. Does this add any value?

Ultraviolet light is typically provided by mercury-vapour based lamps and it needs to be within the UV-C range (a window of 220 to 280 nm, with 262 nm being the peak germicidal wavelength) and with sufficient energy, in order to be effective at killing vegetative cells¹. There are, however, other factors that determine whether UV-C is likely to be effective at achieving microbial kill. Before we look at that, let’s consider how UV-C kills bacteria.

UV-C effects

UV light at appropriate wavelengths causes adjacent thymine molecules (pyrimidine bases) on DNA to dimerize; if a sufficient number of these defects accumulate within a microorganism's DNA, its replication is inhibited which means that the organism’s enzyme-mediated repair processes no longer function, thereby rendering microorganisms harmless, even though the organism may not be killed outright.

Challenge of bacterial spores

Killing vegetative cells is one thing; killing spores is more complex. Dormant spores of Bacillus species are 5 to 50 times more resistant to UV-C radiation than are corresponding vegetative cells. This is because the DNA in the dormant spores has a different UV photochemistry from the DNA in the growing cell. The elevated spore UV-C resistance is due to:

 1.    The photochemistry of DNA within spores, based on the relative lethality of a photoproduct called spore photoproduct (SP; 5-thyminyl-5,6-dihydrothymine).

2.     The efficiency of photoproduct formation.

3.     The efficiency of DNA repair in relation to the photoproducts during spore germination.

What influences UV-C effectiveness?

Whether UV-C is effect within a biosafety cabinet or transfer hatch is dependent upon a series of factors. These include:

The length of time a microorganism is exposed to UV.

The homogeneity of the UV light matters.

Power fluctuations of the UV source that impact the wavelength.

The low penetration capacity of UV for materials with a high absorption coefficient.

The presence of particles or shaded areas that can protect the micro-organisms from UV.

The specific microorganism’s ability to withstand UV during exposure.

The efficacy of UV surface treatment is further influenced by surface topography. Crevices, and similar features, of dimensions comparable to the size of microorganisms (that is of a few microns) may shield microorganisms from potentially lethal UV rays and enable them to survive.

Kill can be enhanced with:

Increasing the treatment temperature from 25 to 60 °C.

Subjecting spores to prior UV exposure in order to sensitize them to subsequent heat / UV treatments.

Experimental research

How effective is UV surface disinfection as an application for pharmaceuticals and healthcare? The literature is decidedly mixed.

1.     One study considered the contamination rates on working surfaces disinfected with UV light irradiation for 15 minutes and subsequently with 70% ethanol and compared with those disinfected using only 70% ethanol. The results showed that the numbers of contaminated plates with bacterial and fungal colonies after disinfection with 70% ethanol alone were not significantly different from those after disinfection with UV light plus 70% ethano²l.

2.     Another study challenged actively growing Bacillus cereus and B. anthracis vegetative cells or spores across exposure times ranging from 15 minutes to 2 hours with varying inoculums ranging from 10^3 to 10^9 colony-forming unit per sample. This showed that ceiling-mounted UV lamps were effective in reducing the viability of both B. cereus and B. anthracis vegetative cells and spores after an exposure time of 1 hour at an intensity of 8 uW/cm2³.

3.     The efficacy was supported by a study that showed UV from germicidal lamps in biosafety cabinets and pass-boxes is sporicidal for fully exposed spores on hard surfaces (15 minutes at ≥ 3.8 W/m2). However, the effect was readily nullified by the slightest obstacle to direct exposure, such as nitrocellulose, paper or the plastic of a Petri-dish⁴.

4.     Research has shown the distance of the UV light and the position to be important. An increase in distance results in a considerable loss in the irradiance value. Here every 5° increment in the tilt angle caused a decrement in the irradiance value by about 14% compared to the original value (0°)⁵.

5.     Levels of kill achieved are in the region of 3 to 4-logs. This, plus the issue with spores, meets criteria for decontamination but does not fall within the generally accepted definition of sterilization.

There is one standard that exists to carry out an evaluation - ASTM E3135-18 “Standard Practice for Determining Antimicrobial Efficacy of Ultraviolet Germicidal Irradiation Against Microorganisms on Carriers with Simulated Soil.” The standard does not capture occluded surfaces or shadowing.

The above research strands relate to standard mercury-vapour lamps. Alternatives include vacuum-UV and pulsed xenon UV-light source technology emitting a broad spectrum (200-300 nm) of UV light⁶.

Assessment

What can we make of this? The majority of class II biosafety cabinets and transfer hatches are equipped with UV lights. UV-C can destroy microorganisms and spores, provided the wavelength is correct, the imparted energy is sufficient, and the time is sufficiently long. However, there are weaknesses, not least because places not in the direct line of sight of the UV-C source do not obtain adequate disinfection.

For a clean, empty biosafety cabinet, left with the UV light on for a prolonged period of time, a UV light provides an option as a decontamination step. However, for a transfer hatch, as an attempt to decontaminate items, the combination of occluded surfaces and short transfer time is unlikely to offer any advantage.

References

01. Chang, J.C.; Ossoff, S.F.; Lobe, D.C.; Dorfman, M.H.; Dumais, C.M.; Qualls, R.G.; Johnson, J.D. UV inactivation of pathogenic and indicator microorganisms. Appl. Environ. Microbiol. 1985, 49, 1361–1365

02. Lapamnouysup, A., Ganjanasiripong, P., Kaewpan, A., & Chuen-im, T. (2022). Comparative study of product contamination rates in class II biological safety cabinets with and without ultraviolet light disinfection. Science, Engineering and Health Studies, 16, 22030004. https://doi.org/10.14456/sehs.2022.13

03. Sambol, A. and Iwen, P. Biological Monitoring of Ultraviolet Germicidal Irradiation in a Biosafety Level 3 Laboratory, Applied Biosafety 2006 11:2, 81-87

04. Turnbull, P., Reyes, A., Chute, M., and Mateczun, A. Effectiveness of UV Exposure of Items Contaminated with Anthrax Spores in a Class 2 Biosafety Cabinet and a Biosafety Level 3 Laboratory Pass-Box, Applied Biosafety 2008 13:3, 164-168

05. Faramawy, S. Estimation of the error factors in irradiance measurements of electromagnetic sources inside disinfection cabinets, Rev Sci Instrum 93, 125104 (2022) https://doi.org/10.1063/5.0125317

06. Wang T, MacGregor S, Anderson J, Woolsey G. Pulsed ultra-violet inactivation spectrum of Escherichia coli. Water Res. 2005;39(13):2921–5