Question

If a bacteria has a diffusion barrier in its capsule, which prevents harmful substances from entering in it, then how do antibiotics affect bacteria?

Answer 1

upon diffusion barrier, bactericidal antibiotics passes through the porins present over the bacterial cell to inflict the damage.

Explanation:

lot of antibiotics do use the diffusion methods from the cell wall across and affect the cell wall formation, protein synthesis machinery and first causes DNA damage and hampers DNA damage. more details can be read here.

diffusion method is not the only method by which antibodies invade bacterial cells. quote from an article "some antibiotics, particularlyβ-lactams, pass across the outer membrane through the water-filled channels of a class of proteins called porins."
source

these porins become the alternative ways for antibiotics to enter the bacterial cells. Once they enter the cell they causes the damage either of the three ways mentioned above. there are other ways through which bacteria can even tackle the antibiotics to enter through porins, including reducing the density of the porins in the cell wall, making these porins less susceptible to the antibiotic using the porin.