Saturday, August 8, 2009

How UV Kills Microorganisms

The lethal effects of UV towards microorganisms were discovered at the end of the nineteenth century and the first practical application of UV was in the disinfection of water. This remains the use to which UV is most commonly associated today and it is true to say that the technology for treating water can be thought of as relatively well accepted in the food and other industries.

Outside of the field of water treatment, UV is often referred to as a surface treatment. This view is only partially correct, insofar as it describes only one particular aspect of UV treatment. Specifically with reference to surface treatment, it is important to realize that UV is strongly absorbed by most materials and cannot penetrate beyond the surface layers of solid objects.

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

Another important factor determining survival is the intrinsic resistance of the microorganism to the effects of UV. This will be influenced to some extent by the physiological state of the cell, and is therefore not a fixed quantity.

Early planet Earth, lacking a protective ozone layer, was bathed in UV and while UV was an important agent of evolution in generating variation in early organisms, there ultimately was value to organisms in being able to protect themselves from its effects. Evolution appears to have conferred on microorganisms at least two independent strategies for specifically surviving UV exposure.

The first was to produce pigments that absorb UV strongly, and the protective effects of such pigments have been demonstrated by isolating non-pigmented mutants of the same species and comparing their UV resistance. The second has to do with the efficiency of DNA repair following UV exposure. While UV has the ability to chemically modify the structures of many of the chemical entities found in cells, it is its effects on DNA that will ultimately determine whether or not the organism will survive.

UV is known to cause a number of different lesions in DNA but the most common is the dimerization of adjacent pyrimidine bases on the same strand of DNA. This effectively interferes with DNA replication and to counter this, enzyme-mediated repair processes have evolved that essentially restore the DNA to its original state. Given the fundamental importance of DNA replication, it is not surprising that all living organisms possess such repair mechanisms. This is a factor that must be taken into account in commercial UV food treatment, that is, that the treated food is shielded from the relevant wavelengths for a sufficient period of time.

Yet another protective strategy that has been adopted by some microorganisms is growth in the form of “biofilms.” A biofilm may be thought of as a structured microbial community associated with solid surfaces. Attachment to surfaces occurs because certain members of the community are able to produce polysac-charides that serve as adhesives. Biofilms pose a very real threat in the food industry and contact of foods with biofilms invariably results in contamination as cells are shed from the biofilm to the food.

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