Penicillin Resistance in Bacteria: Before 1960

 
The Nobel Prize for the discovery and analysis of penicillin was awarded in 1945 [Nobel Laureates: Sir Alexander Fleming, Ernst Boris Chain, Sir Howard Walter Florey]. It was about this time that penicillin became widely available in Europe and North America.

By 1946 6% of Staphylococcus aureus strains were resistant to penicillin. Resistance in other species of bacteria was also detected in the 1940's. By 1960 up to 60% of Staphylococcus aureus strains were resistant with similar levels of resistance reported in other clinically relevant strains causing a wide variety of diseases (Livermore, 2000).

Penicillins are a class of antibiotics with a core structure called a β-lactam. The different types of penicillin have different R groups on one end of the core structure. A typical examples of a penicillin is penicillin G [Monday's Molecule #30]. Others common derivatives are ampicillin and amoxicillin.

The original resistance to this entire class of drugs was caused mostly by the evolution of bacterial enzymes that could degrade them before they could block cell wall synthesis. (Recall that bacteria have cell walls and penicillin blocks cell wall synthesis [How Penicillin Works to Kill Bacteria].)
It seems strange that the evolution of penicillin resistance would require a totally new enzyme for degrading the drug. Where did this enzyme come from? And how did it arise so quickly in so many different species?

The degrading enzyme is called penicillinase, β-lactamase, or oxacillinase. They all refer to the same class of enzyme that binds penicillins and then cleaves the β-lactam unit releasing fragments that are inactive. The enzymes are related to the cell wall transpeptidase that is the target of the drug. The inhibition of the transpeptidase is effective because penicillin resembles the natural substrate of the reaction: the dipeptide, D-alanine-D-alanine.

In the normal reaction, D-Ala-D-Ala binds to the enzyme and the peptide bond is cleaved causing release one of the D-Ala residues. The other one, which is part of the cell wall peptidoglycan, remains bound to the enzyme. In the second part of the reaction, the peptidoglycan product is transferred from the enzyme to a cell wall crosslinking molecule. This frees the enzyme for further reactions (see How Penicillin Works to Kill Bacteria for more information).

Penicillin binds to the peptidase as well and the β-lactam bond is cleaved resulting in the covalent attachment of the drug to the enzyme. However, unlike the normal substrate, the drug moiety cannot be released from the transpeptidase so the enzyme is permanently inactivated. This leads to disruption of cell wall synthesis and death.

Resistant strains have acquired mutations in the transpeptidase gene that allow the release of the cleaved drug. Thus, the mutant enzyme acts like a β-lactamase by binding penicillins, cleaving them, and releasing the products. Although the β-lactamases evolved from the transpeptidase target enzymes, the sequence similarity between them is often quite low in any given species. This is one of the cases where structural similarity reveals the common ancestry [see the SCOP Family beta-Lactamase/D-ala carboxypeptidase]. It's clear that several different β-lactamases have evolved independently but, in many cases, a particular species of bacteria seems to have licked picked up a β-lactamase gene by horizontal transfer from another species. The transfer can be mediated by bacteriophage or plasmids.

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Livermore, D.M. (2000) Antibiotic resistance in staphylococci. Int. J. Antimicrob. Agents 16:s3-s10.