Careful ZB, sounding cautiously reasonable. Some commentators to this site seriously resent any expression of rational empiricism.
Previously, I have expressed my concerns that contamination by Earth life would be misconstrued as 'proof' of alien life. And the resulting confusions threatening public support for interplanetary research.
Also, to date there is no evidence that Earth life can survive for sustained periods, outside the Earth's protective magnetosphere, in low to zero gravity.
A way to test this? How about the data collected by Mars probes doing life research. If Earth life is detected after the long voyage in the extreme environment of space? I would grudgingly accept that as a probability for the hardiest of micro-life.
To understand the consequences of being trapped for lengthy duration in a tin can? I suggest involving the info from the US Navy's submarine program. Specifically the boomers as they spend long periods skulking in isolated deep ocean.