Which type of bond forms when two or more atoms share electrons?

Covalent bond

Explanation:

the reason a covalent bond is formed is because of two atoms sharing one or more electron with each other like Na and CI becoming NaCI which would be also known commonly as "table salt"

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 =