Effects of Low-Temperature Plasma-Sterilization on Mars Analog Soil Samples Mixed with Deinococcus radiodurans

Abstract

We used Ar plasma-sterilization at a temperature below 80 °C to examine its effects on the viability of microorganisms when intermixed with tested soil. Due to a relatively low temperature, this method is not thought to affect the properties of a soil, particularly its organic component, to a significant degree. The method has previously been shown to work well on spacecraft parts. The selected microorganism for this test was Deinococcus radiodurans R1, which is known for its remarkable resistance to radiation effects. Our results showed a reduction in microbial counts after applying a low temperature plasma, but not to a degree suitable for a sterilization of the soil. Even an increase of the treatment duration from 1.5 to 45 min did not achieve satisfying results, but only resulted in in a mean cell reduction rate of 75% compared to the untreated control samples.

Keywords: Deinococcus radiodurans; Mars; plasma sterilization; regolith.

Figure 1

Desiccated regolith analog samples: (a) 1.25 g regolith mixed with cells of D. radiodurans in 100 mL glass bottles with wide opening and screw caps; Close ups showing (b) S-MRA; (c) P-MRA; (d) JSC Mars-1A.

Figure 2

Results of the first experimental setup: estimated via plate count; Only for JSC Mars-1A a significant reduction in cell numbers could be observed before (Start) and after the treatment (Plasma). Error bars indicate the standard deviation, n = 3. The plasma sterilization parameters for this test were: Argon-Nitrogen Plasma (100:5) at p = 10 Pa, P = 500 W, T < 80 °C; duration: t = 1.5 min.

Figure 3

Results of the second experimental setup: estimated via plate count (here only JSC Mars-1A was used). Cell numbers of the plasma sample were reduced compared to the starting sample. No significant difference in counts between the plasma sample and the vacuum control sample could be observed. Error bars indicate standard deviation, n = 3. The plasma sterilization parameters of this test were: Argon-Oxygen Plasma (100:5) at p = 10 Pa, P = 1500 W (1 kHz, 10% duty cycle), T < 80 °C; duration: t = 45 min.