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We describe a cell culture experiment utilizing Mars soil simulant and yeast as a study for extraterrestrial-microorganism detection in future. The following conditions were combined to create different sample configurations: solid medium (agar plate) or liquid medium, with the Mars soil simulant (100 mg) or without, and various yeast cell densities (30 μL of a suspension of yeast prepared to achieve dilution rates of 10−2, 10−3, 10−4, and 10−5 from the original suspension). For redundancy, two samples were created for each setting. On each agar plate, colonies of yeast cells were visible. In contrast, only samples created with a dilution rate of 10−2 and 10−3 were proven to show the presence of yeast proliferating in liquid media. The agar plates were observed via imaging using a digital camera. Colony counting was conducted using functions of the ImageJ software; the yeast density in the original suspension was estimated to be 2.5 × 107 colony forming units (CFU)/mL. Based on the results of cell culture on the agar plates, the detection limit that was experimentally demonstrated in this work amounted to 7.5 CFU/100 mg of soil, which corresponded to 75 CFU/g of soil. Thus, we confirmed that the Mars soil simulant reduced the number of colonies to one-third of that detected without the Mars soil simulant. In the discussion, we pointed out that the method that used the solid media has potential to provide high sensitivity using a simple, compact, and lightweight hardware. Microscope with no optics is also discussed. The lower limit of detection obtained in this study was compared with those obtained using other methods. We compared and discussed extraterrestrial life search based on (1) in situ experiments by unmanned missions, (2) analysis of samples returned to Earth, and (3) dedicated in situ analyses by manned bases located on extraterrestrial sites in the future. The perspectives discussed herein are applicable not only to the samples from Mars but also to the samples from other solar system bodies. Identifying cell culture conditions for these applications is a critical difficulty. Notably, it is a possible scenario that cell culture (although challenging) will become necessary to obtain clear evidence of extraterrestrial life. We also note that in cases where there are multiple species of microorganisms, it can happen that microorganisms that fit the culture conditions used in an experiment are selectively cultured and detected, even if the culture conditions for each species are not fully discovered. It is worthwhile to conduct exploration in advance designed with the intention of determining culture conditions with the plan to culture microorganisms in the future.
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