C12E17: Antibacterials Part 2

The next few drugs target cell membranes.

Remember that the cell membrane is made of phospholipids, whereas the cell wall is peptidoglycan.

So instead of breaking down the cell wall, like the beta-lactam antibiotics and vancomycin that I talked about in the last episode, these drugs go straight for that plasma membrane.

And the first example is polymyxin B, which you've probably seen before in topical ointments like Neosporin.

Polymyxins acts almost like a detergent, disrupting the bacterial cell membrane and causing the cell to leak and die, so they're bactericidal.

Polymyxins are especially useful against Gram-negative bacteria, including Pseudomonas species, but they can be toxic to the kidneys, which is why they're limited to topical use or are very controlled in other forms.

Another drug in this category is daptomycin, which is more active against Gram-positive bacteria and is used for more serious infections.

This is a good time to point out that not all drugs work equally well on all bacteria, and a lot of that comes down to differences in cell envelope structure.

Gram-negative bacteria have an outer membrane that can act as both a target, like it is with the polymyxins, and a barrier, which can limit the ability of some drugs to even reach their site of action.

Gram-positive bacteria don't have this outer membrane, but they do have a much thicker peptidoglycan layer, which can also influence how drugs interact with the cell.

So whether a drug is targeting the cell wall like the beta-lactams, the cell membrane, or something inside like ribosomes or DNA, it still must physically reach that target to be effective.

That's why understanding the structural differences between Gram-positive and Gram-negative cells is so important.

It explains why some a- antibiotics work well against certain bacteria but not others.