A pair of stylish sneakers could be considered a
because it is not a necessity

want

Explanation:

because it is not actually needed for the existence of life at that particular time

It's a want

Explanation:

Needs are something that is required for you to survive.

Wants are something that you may want, but you wouldn't die if you don't have them.

the body's situation won't change the forces are equal opposit and on the same body

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 =