Why has penicillin been less effective?

Answer 1

Penicillin less effective in bacteria that have slowed down their multiplication rate because antibiotics are generally active against multiplying bacteria, but are much less effective against non-replicating (latent) bacteria.

Also some bacteria change the shape of their cell walls so the penicillin can't stick or produce a substance that breaks down penicillin.

Answer 2

Penicillins inhibit the cross-linking of peptidoglycan strands, an important component in the cell wall of bacteria. This cross-linking (linking strands to adjacent strands) gives the cell wall mechanical strength and stops the cell from exploding open when it grows (and when water moves into the cell making it larger).

When exposed to penicillin, peptidoglycan strands are still being made by the growing cell but they're no longer being cross-linked so all you're left with are floppy, incoherent peptidoglycan layers. The cell keeps on growing but it's cell wall is becoming weaker until the cell wall can't hold the cell together and the cell basically explodes outwards.

Note that osmosis is another cause of exploding - water moves into the cell, expanding it.

If the cell isn't actively growing then the cell isn't getting bigger and so having a compromised cell wall isn't as bigger of a problem.

Answer 3

Because alot of bacteria have become immune to it over the years.