Right so then couldn't it follow that human survivors may have no impact on their gut bacteria? If there are only two people and their microbiomes, and the snap kills 1 person and their entire microbiome, then the surviving person would have no or microscopically small impact on their bacteria assuming an even distribution of bacteria across the two people. Basically the OOP is assuming that of the people that died, half of their bacteria would survive, impacting survivors' microbiomes, rather than assuming 100% of bacteria would die with their hosts, leaving the surviving population's bacteria intact.