MICRO-ORGANISMS (Bacteria, Viruses, Fungi, ...)



1.1 Inactivation

Microorgansim (cells, viruses,...) can be inactivated by UV radiation, that means that they lose the ability of reproduction. Inactivation is a strochastic process and can be desribed by a dose-survival curve. Dose-survival curves are rarly linear, just in the case that when a single hit is sufficient to inactivate the organism. Often, however, inactivation of a single cell requires more than one hit and mathematical treatment of this condition leads to shouldered survival curves (Harm 1980) that are very often observed experimentally. The UV light effective for inactivating microorganisms is in the UV-B and UV-C ranges of the spectrum (200–320 nm), with maximum effectiveness around 265 nm. The range of highest effectivity is also called germicidal range.

UV inactivation is thought to occur as a result of the direct absorption by the microorganism of the UV radiation, bringing about an intracellular photochemical reaction that changes the biochemical structure of the molecules (probably of the nucleic acids) that are essential to the microorganism’s survival.

Thymine bases on DNA and ribonucleic acid (RNA) are particularly reactive to UV light and form dimers (thymine–thymine double bonds) that inhibit transcription and replication of nucleic acids, thus rendering the organism sterile.

The wavelength range where UV is most effective in inactivation is quite similar to that where DNA has the highest absorbance.

1.2 Dose Respone

The inactivation of micro-organisms by UV could be described with first-order kinetics using fluence inactivation. For some micro-organsims no inactivation at low fluences (Offset) and/or a slow increase (Shoulder) and/or no further increase of response (inactivation) at higher fluences (Tailing) was observed.

Fig.1: Schematics of a dose response curve showing offset, linear range and tailing.

The parameters that can be used to describe inactivation are the inactivation rate constant k (cm2/mJ, m2/J), the maximum inactivation demonstrated and (e.g. for bacterial spores and Acanthamoeba) the offset value. These parameters were the basis for the calculation of the microbial inactivation credit (MIC=”log-credits”) that can be assigned to a certain UV fluence. The most UV-resistant organisms are viruses, specifically Adenoviruses, and bacterial spores. The protozoon Acanthamoeba is also highly UV resistant. Bacteria and (oo)cysts of Cryptosporidium and Giardia are more susceptible with a fluence requirement of <20 mJ/cm2 for an MIC of 3 log.

Several studies have reported an increased UV resistance of environmental bacteria and bacterial spores, compared to lab-grown strains. This means that higher UV fluences are required to obtain the same level of inactivation. Hence, for bacteria and spores, a correction factor of 2 and 4 could be applied to the MIC calculation, respectively, whereas some wastewater studies suggest that a correction of a factor of 7 is needed under these conditions. For phages and viruses this phenomenon appears to be of little significance and for protozoan (oo)cysts this aspect needs further investigation. Correction of the required fluence for DNA repair must be considered.

Additionally inactivation is decrease by magnitudes if a (pathogenic/parasitic) micro-organsim has entered a host.

1.2 Photoreactivation

Thymine dimmers can be repaired in a process termed ‘photoreactivation’ in the presence of light, or ‘dark repair’ in the absence of light (Jagger, 1967). As a result, the strategy in UV disinfection has been to provide a sufficiently high dosage to ensure that nucleic acid is damaged beyond repair. xxx xxx