Antibiotic Resistance: How A Global Health Problem Develops

The public sphere has been pumped full of information about how unnecessary use of antibiotics contributes to the development of resistant bacterial strains. Just take a look at this news article suggesting that more than 25 million pounds of antibiotics are given to livestock every year. However, what is less often explained is how this works at the molecular level. How does bacteria develop antibiotic resistance?

The World Health Organization has called antibiotic resistance one of the greatest global health concerns to date.

Before answering that question it is important to understand how bacterial cells work. Bacterial cells look and work differently than say a cell from our body. They have a genetic code (within DNA) but some of that code floats freely within the cell in circular structures called plasmids. One of the particularities of bacterial cells is their ability to pass plasmids amongst each other (plasmid transfer), allowing them to share traits on an extremely rapid scale. Furthermore, one bacterium can divide into two cells without the need for sexual reproduction between two parent cells.

Like us, bacteria survive on chemical based processes, which allow them to grow and replicate. Protein molecules are essential to these processes. They allow for three things:

  • Destroy/change other molecules
  • Form physical structures and barriers
  • Help build new molecules

Interference with any one protein can mean death for the bacteria. In fact, that is the principle that antibiotics are based upon. For example, penicillin works by creating a strong cell wall that interferes with a specific protein.

During cell replication, mistakes can cause the formation of abnormal proteins or mutations. These mutations can be the cause of cancers or other diseases but are also the basis for evolution over time. As mentioned above, bacterial cells can replicate and divide very quickly (in hours rather than years). Therefore, mutations occurring within bacterial cells can also develop and spread more rapidly. In some cases, mutations result in a protein that is altered just enough to keep its functions but no longer recognizable to an antibiotic molecule. Many have speculated that this is why a regular Staph infection can be treated with a course of antibiotics while MRSA can be life threatening.

There are four ways in which this new resistant bacterium will deal with antibiotic treatments:

  1. Can produce enzymes (a type of protein molecule) that destroy the antibiotic molecule before it can have an effect.
  2. The protein might be so changed that the antibiotic molecule can no longer associate with it.
  3. Bacterium can use different metabolic pathways and abandon the one targeted by the antibiotic.
  4. Can produce protein channels and pumps that help rid the molecule of antibiotic molecules.

If it were only a question of one mutated bacterium giving resistance to a specific antibiotic, there would be no problem. However, if this bacterium has all the room and nutrients it needs to grow and divide, a whole new population of bacteria can grow from this one parent cell in a matter of hours. As the genetic code is passed on as the cell divides, the new population of cells will have the same antibiotic resistance. Furthermore, as plasmids can be transferred from bacterium to bacterium, mutation can be transferred to non-descendant bacteria, creating totally different resistant strains. It is therefore easy to see why a single small genetic mutation can cause a global health concern.

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