yeast
See: http://mychrobialromance.blogspot.com/2012/06/what-is-bacterial-population.html
"What about dramatic lifestyle changes and population bottlenecks such as in Vibrio fischeri. V. fischeri can be found living freely as saprophytes in the open ocean (see link within here), but all it takes is one single bacterium to infect a juvenile squid and be amplified by the billions. Bottlenecking from millions of cells to 1 dramatically altars Ne since calculations of this parameter place extra weight on low numbers."