Indian Journal of Medical Microbiology Medknow Publications on behalf of Indian Association of Medical Microbiology ISSN: 0255-0857 EISSN: 1998-3646 Vol. 26, Num. 3, 2008, pp. 241-242

Indian Journal of Medical Microbiology, Vol. 26, No. 3, July-September, 2008, pp. 241-242

Brief Communication

Surface disinfection by exposure to germicidal UV light

Katara G, Hemvani N, Chitnis S, Chitnis V, Chitnis DS

Department of Microbiology and Immunology, Choithram Hospital and Research Centre, Manik Bagh Road, Indore - 452 014, MP
Correspondence Address:Department of Microbiology and Immunology,Choithram Hospital and Research Centre, Manik Bagh Road, Indore - 452 014,MP
ds_chitnis@rediffmail.com

Date of Submission: 23-Jul-2007
Date of Acceptance: 02-Sep-2008

Code Number: mb08074

Abstract

Keywords: Germicidal, micro-organism, UV light

Introduction

The medical profession was the first to endorse germicidal effect of UV lamps ^[1] and it has been used traditionally to disinfect operation theaters. ^[2],[3] Absence of any residual effect is the greatest advantage of UV disinfection. In spite of its usage for over 50 years, the guidelines for the position of the fixtures, the effective distance, exposure time required etc. are not well-documented in hospital practice. Our preliminary study indicated that efficient and uniform microbial inactivation is achieved when the UV tubes are hung from the ceiling at seven feet height rather than fitted over the side wall (unpublished data). The aim of the present study was to design a simple experimental model for checking surface disinfection using bacterial challenge and to standardise the position, distance and time for UV light exposure. It was also aimed to find out efficacy of UV light against wide range of medically important bacteria, the bacterial spores and fungi under the defined experimental conditions.

Materials and Methods

The organisms used in the study are listed in the figure. The bacteria were subcultured on nutrient agar (Hi-Media, India). Candida albicans and Aspergillus niger were subcultured on Sabouraud dextrose agar (Hi-Media, India). Aspergillus niger was allowed to grow for seven days and the spores were scraped and suspended in sterile phosphate buffered saline (PBS). Bacillus subtilis was sporulated and the spore suspension heated at 70°C for 30 minutes to inactivate vegetative forms. Clostridium perfringens grown in Robertson′s cooked meat medium for seven days and the spores were heated at 70°C for 30 minutes to inactivate vegetative forms. The wild isolate of Clostridium perfringens was selected because of its ability to form large number of spores. Mycobacterium fortuitum was sub cultured on Lowenstein Jensen medium and incubated at 37°C for 48hours.

Experiment to determine the optimal distance from UV light
Overnight growth of E.coli on nutrient agar slant was harvested in normal saline and the opacity was adjusted to 0.5 McFarland standard. Ten microliter suspension was spread over the nutrient agar plates. UV light tube was placed as a horizontal up assembly on a table and the plates were exposed for 20 minutes at varied distance of two feet to 16 feet at increment of two feet each. The E. coli suspension (McFarland standard 0.5) was subjected to viable count using ten log dilutions to confirm the count. Reduction in viable count was expressed as ratio of initial viable count to viable count after exposure to UV light.

Experiment to standardize the exposure time
The E.coli inoculated plates were exposed to UV light at a fixed distance of eight feet and time of exposure varied from 5 minutes to 40 minutes at an increment of 5 minutes each.

Effect of UV light exposure on different organisms
The distance from the UV source was kept at eight feet and time of exposure was kept at 30 minutes. The suspensions of various organisms were subjected to viable count. Clostridium perfringens inoculated blood agar plates were incubated at 37°C for 48 hours in anaerobic condition using Difco - Oxoid Gas Pack System. M. fortuitum inoculated nutrient agar plates were incubated for 48 hours at 37°C. Aspergillus niger and Candida inoculated Sabouraud dextrose agar plates were incubated at room temperature for 48 hours. For the rest of the organisms nutrient agar plates inoculated with bacteria were incubated at 37°C for 24 hours. All the experiments were repeated thrice and observations and interpretations were as in experiment one.

Results

In the second experiment, the distance was fixed at eight feet and time of exposure varied. After UV exposure for 10 minutes viable count reduced by 0.2 x 10 ⁴ , at 15 minutes by 2.6 x 10⁴ and after 20 minutes reduced by 3.5 x 10 ⁴ . Further time increment showed only marginal increase in bacterial inactivation. Thus, 20-25 minutes exposure to UV light seemed adequate for disinfection and 30 minute exposure time is recommended for UV disinfection.

The efficacy of inactivation of various organisms at the distance of eight feet from UV source and exposure time of 30 minutes is shown in the figure. Four-log reduction was seen for all the organisms including the bacterial spores and excluding Candida, Aspergillus spores and M. fortuitum. Inactivation of Candida and M. fortuitum was around three logs while inactivation of Aspergillus spores was even lower than three logs. The overall results however, suggest satisfactory inactivation of approx 4 log reduction when the UV source is at ≤8 feet and the exposure time is 30 minutes.

Discussion

The inactivation was almost one log less efficient for Candida and Mycobacterium fortuitum while f or B. subtilis and Aspergillus spores a reduction of approx 3 and 2.5 logs was observed.

The maximum advantage of UV light disinfection is for areas like operation theater, and biosafety hoods. ^[4] While recommending the use of UV light at appropriate distance and for appropriate time we wish to emphasize that standard safety guidelines^[5] need to be observed during usage of UV light.

Acknowledgement

References

Copyright 2008 - Indian Journal of Medical Microbiology