There are a few things one has to understand (and appreciate) about bacteria. They are living organisms and similar to us, can put on or shed weight. But unlike us, they can do so very rapidly and dynamically, depending on the environmental conditions. When we say bacterial mass, we generally refer to dry mass of the bacteria, which (of course) is less than the mass of bacterium in a solution. And, similar to human beings, the mass of a bacterium will differ from species to species (and there are thousands of them).
With that knowledge in mind, there is no fixed number for mass of bacterium. Kindly check the link provided for several scientific references on the matter. The mass of bacterium has been reported to be anything from `3 xx 10^(-19) mg` to `6.25 xx 10^(-13) mg`.
Hope this helps.