Question: What is the most common bacteria to be found?

Marialuisa Crosatti answered on 5 Mar 2019:

The short answer is it depends on when and where. As far as I know nobody tried to quantify the absolute amount of bacteria present on Earth mainly because it would be in continuous flux. There have being lots of publication establishing the microbiome (the bacterial component) of many type of environments, from land to lakes to glacier to human bodies just to name a few. If we consider the bacterial component of our gut, it changes before and after a meal, it changes based on what we eat, our age and is different in different people. To estimate what is the most abundant type of bacteria in the human gut, we would need to sample enough people (not everybody but still a pretty high number) at different times of the day, at different ages and then extrapolate the overall amount of each type of bacteria. This is a task that we are not equipped to do yet but maybe it could be done in your generation.