This pulley system has a MA (mechanical
advantage) of 8 meaning the output
forces is 8 times the input force (the
force he pulls with). Since work doesn't
change and his input force is less, what
must increase?

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =