Chemical disinfectants are not only toxic for the user but are also awkward to use and dispose of. Cryostats with UVC disinfection enhance safety and efficiency when cutting cryosections. The method is simple to use and requires no warming of the cryo-chamber. To prove the efficiency of UVC radiation, Dr. Ingo Maier, ecoscope, Laboratory for Microbiology and Ecotoxicology (Amtzell, Germany) carried out test series with cryostats. The results confirm the germicidal effect of UVC disinfection.
Disinfection after 30 to 40 minutes
As there is no worldwide standardised testing method, germs commonly used to prove the efficiency of chemical disinfectants were used as “biodosimeters“. In the first test series, ecoscope tested the effect of UVC on the standard test germ Staphylococcus aureus ATCC 6538. Thirty drops of in total 0.5 ml test suspension were symmetrically applied to stainless steel plates and surface-dried. These plates were placed in four different positions (Fig. 1) inside the cryochamber and irradiated at a temperature of –20 °C. The bacteria suspension used was adjusted to a density of about 109 colony forming units per millilitre. A retrieval rate of 70% was established without UV irradiation after preparation, freezing and retrieval. According to the European DIN EN 1040 standard for chemical disinfectant suspension testing, a germicidal reduction by at least 99.999% is required. This value was achieved in the Leica CM1850 UV after 30 minutes of radiation, and in the Leica CM1900 UV after 40 minutes.
|Bacterial Spores||Bacillus ssp||36–371|
|Simian Virus 40||440|
UVC-resistant viruses inactivated
In the second test series, the extremely UVC-resistant Simian Virus 40 (SV 40) was chosen as a surrogate. To adapt to practical conditions, the virus suspension was charged with cell culture medium and the addition of 2% bovine foetal serum. The original virus titre had an average value of around 10–6.8/ml. The retrieval rate determined in order to calculate the virus titre reduction afterwards was practically 100%. After an irradiation time of 95 minutes in Position B (Fig. 1), the titre decreased by 99.99%.
Taking other test positions as well as a safety factor into consideration, ecoscope recommends an irradiation time of 180 minutes for the Leica CM1850 UV and 240 minutes for the Leica CM1900 UV in order to achieve high-level disinfection at which all microorganisms are destroyed. A comparison with UVC doses for germ reduction from scientific literature (Tab. 1) indicates the disinfection efficiency of the UVC lamp in Leica cryostats. In the case of the Hepatitis B Virus (HBV), for which no measured data is currently available, size or structure of genome allow conclusions to be drawn on the UVC sensitivity. Like SV 40, HBV is an extremely small dsDNA virus that is likely to be destroyed by similar UVC doses.