Why do some antibiotics work better against gram positive then gram negative bacteria?

Answer 1

Part of describing the function of an antibiotic is stating whether it is effective against Gram-positive bacteria, Gram-negative bacteria or both. (There are other aspects such as whether there is effect on aerobic versus anaerobic bacteria (those that can versus cannot survive in the presence of oxygen), but I'll address that some other time).

Some antibiotics have broad spectrum activity, being able to kill Gram-positive and Gram-negative bacteria. At first that might seem like a good thing, and indeed it is when you are treating an infection with an unknown bacterium. However, broad spectrum activity is not desirable when you know exactly what bacterium you are trying to eliminate. If possible, it's much better to target treatment more specifically against the offending bacterium for various reasons, such as to reduce the risk of resistance emerging in other bacterial groups.

Some antibiotics are narrow spectrum. Some are most effective against Gram-positive or Gram-negative bacteria, with little activity against the other. In general, we want to use an antibiotic of the most narrow spectrum possible.

Answer 2

The difference between susceptibility between the gram positive and gram negative bacteria is in the cell wall of both types. Gram positive cell wall is much thicker than gram negative cell wall so what will kill gram + will not kill the gram- bacteria.

Answer 3

I believe it is because Gram positive bacteria have a cell wall and some antibiotics, such as penicillin effect the cell wall. Gram negative bacteria do not have cell walls, so penicillin has no effect on these organisms.

Answer 4

Bacteria are classified as gram negative and gram positive based on the nature of the cell wall. Gram negative bacteria have a thin layer of peptidoglycan surrounding the cell membrane which in turn is enveloped by thick lipid rich layer also refered as the outer membrane. Gram positive bacteria lack this lipid rich outer membrane, instead they have a thick peptidoglycan layer alone.

The antibiotics affecting the protein synthesis, nucleic acid synthesis and any other metabolic processes within the bacterial cell has persumably the same effect on both the gram negative and gram positive bacteria. Only those antibiotics which affect the cell wall synthesis or any other feature of the cell wall shall exhibit a difference in their action in these two groups of bacteria.

Answer 5

The only thing that Gram- and Gram+ have to do with antibiotics is that each stain can give a clue as to which antibiotic might work against the microbe being stained.

The outer membrane of Gram-negative microbes prevents antibiotics from entering. Gram-negative bacteria include: E. coli, Salmonella, Shigella, Yersinia. These resistant to penicillin, streptomycin and ampicillin.

Most infections due to Gram-positive organisms can be treated with quite a small number of antibiotics. Penicillin, cloxacillin, and erythromycin should be enough to cover 90 per cent of Gram-positive infections.