Exploring Our Fluid Earth

Modern physicians frequently prescribe antibiotic medications to help people fight infections. One of the first antibiotics discovered was penicillin. Penicillin was first used to treat bacterial infections in 1942 and is derived from the fungus Penicillium sp. When used as an antibiotic treatment, penicillin operates by a very specific mechanism. Penicillin interferes with the production of a molecule called peptidoglycan. Peptidoglycan molecules form strong links that give the bacterial cell strength as well as preventing leakage from the cytoplasm. Nearly every bacterium has a peptidoglycan cell wall.

The composition of the cell wall differs depending on the type of organism, so penicillin does not affect other organisms. The cell walls of plants, for example, are made from cellulose. The cell walls of algae are highly variable. Algae cell walls can be made of cellulose, xylan, silica, carrageenan or a variety of other materials. The cell walls of most fungi are made from chitin. Composition of the cell wall in the archaea is more diverse.

Within bacteria, there are two types of bacterial cell walls. Gram-positive bacteria have a peptidoglycan layer on the outside of the cell wall. Gram-negative bacteria have peptidoglycan between membranes. Penicillin works best on gram-positive bacteria by inhibiting peptidoglycan production, making the cells leaky and fragile. The cells burst open and are much easier for the immune system to break down, which helps the sick person heal more quickly. Human cells do not contain peptidoglycan, so penicillin specifically targets bacterial cells.

SF Fig. 2.1. (A) Prokaryote bacterium with a cell wall made of peptidoglycan

Image courtesy of Ali Zifan, Wikimedia Commons

SF Fig. 2.1. (B) Diagram comparing gram-negative and gram-positive bacteria

Image courtesy of Graevemoore, Wikimedia Commons

Other antibiotics target different molecules that inhibit bacterial growth while leaving human cells undamaged. Sulfa antibiotics target a specific enzyme that inhibits bacterial growth. Tetracycline antibiotics bind to bacterial ribosomes that are responsible for protein production and inhibit bacterial protein synthesis. Ciprofloxacin, one of the strongest antibiotics, attacks bacterial DNA replication while leaving human cellular DNA unaffected.

Antibiotics are highly specific to a certain bacterial function, and are not helpful for treating non-bacterial illnesses. Viruses are unaffected by antibiotics because they do not have peptidoglycan cell walls or ribosomes, and they do not replicate their own DNA.

Bacteria can become resistant to antibiotics through the process of selection and evolution. Penicillin kills most of the bacterial cells, but it does not kill them all. Bacteria resistant to the effects of the antibiotic remain, but in small numbers they can be eliminated from the body by the immune system. It is important to finish all the antibiotics that are prescribed, so that the immune system doesn’t have to work as hard to fight the infection. Both unfinished antibiotic courses and overuse of antibiotics have also led to increased instances of antibiotic resistant bacteria.