Which type of force(s) will cause a change in an object's motion?

Any force that's not cancelled by another force, with identical strength but pointed the opposite way, will cause a change in the object's speed or direction (or both).

When that happens, we say that the forces on the object are "unbalanced", and the object is experiencing "acceleration".

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 =