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If someone asked you what antibiotics do, you’d probably say they kill bacteria. But that’s not always the case. In this lesson, learn about the general types of antibiotics and the differences between them.

General Types of Antibiotics

We all know what antibiotics are, right? They’re drugs that kill bacteria, and we take them when we have an infection that our body can’t quite handle on its own. But did you know that only some antibiotics actually kill bacteria, and others just stop them from growing? Bactericidal antibiotics kill bacteria directly, and bacteriostatic antibiotics stop bacteria from growing.Another important thing to remember about antibiotics is that they don’t all work against all types of bacteria.

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Narrow-spectrum antibiotics are only effective against a narrow range of bacteria, whereas broad-spectrum antibiotics are effective against a broad range of bacteria. In this lesson, we’ll look more closely at these general types of antibiotics, and we’ll see what makes a given antibiotic fit into each category.

Bactericidal Antibiotics

Like I mentioned before, bactericidal antibiotics kill bacteria directly. You can remember this word because the suffix ‘cidal’ means kill, like in the words homicide or suicide. How do bactericidal antibiotics actually kill bacteria? Well, there are many different antibiotics that all have different mechanisms, and you can learn more about them in other lessons.

But here are a couple examples.The antibiotic polymyxin B injures the plasma membrane of bacteria, allowing their contents to leak out. Under normal circumstances, bacteria and other cells have to keep a perfect balance of ions on both sides of the plasma membrane because of osmosis. Polymyxin B disrupts this balance, and also lets other important molecules, like DNA and RNA, leak out, so the bacterium is a goner.

Bacteriostatic Antibiotics

In contrast to bactericidal antibiotics, bacteriostatic antibiotics stop bacteria from growing.

This word is also easy to remember: the suffix ‘static’ means staying stable. The bacteria don’t die, but they can’t grow or replicate either.So, how do bacteriostatic antibiotics help clear up an infection, if they don’t actually kill bacteria? Well, bacteria normally divide really quickly in our bodies, and their numbers can get totally out of control. But if an antibiotic stops them from growing and dividing, the host’s immune system will be able to get rid of the bacteria.Tetracycline is an example of a bacteriostatic antibiotic.

It inhibits the bacterial ribosome, so that no new proteins can be made. This doesn’t kill the bacteria; they already have the proteins they need to survive for a while. However, they can’t replicate, because they would need to make tons of new proteins in order to make a whole new bacterial cell.Another class of bacteriostatic antibiotics is the sulfa drugs. They prevent the production of important metabolites that the bacterium needs in order to make new DNA, RNA and proteins. Again, this doesn’t kill the bacteria, but there’s no way they can replicate and make new bacteria without new DNA, RNA and proteins. Okay, now we understand the difference between bactericidal and bacteriostatic antibiotics.

Let’s look at the difference between broad-spectrum and narrow-spectrum antibiotics.

Narrow-Spectrum Antibiotics

Narrow-spectrum antibiotics are only effective against a narrow range of bacteria. For example, penicillin G is very effective at killing gram-positive bacteria, but not very effective against gram-negative bacteria.Why is that? What causes an antibiotic to have a narrow spectrum of antimicrobial activity? Often, it has to do with the ability of the antibiotic to penetrate inside of the bacterium. Gram-positive bacteria have a relatively loose outer wall that many antibiotics can diffuse through. However, gram-negative bacteria have a complex outer layer that prevents the passage of many larger or fat-soluble molecules.

Another reason that antibiotics can have a narrow spectrum of activity can be their target molecules.If an antibiotic targets a molecule that a bacterium doesn’t even have, of course it won’t be effective against that bacterium. For example, isoniazid specifically targets mycobacteria, such as the bacterium that causes tuberculosis. It’s specific because it prevents the synthesis of mycolic acids, which are found in the cell walls of mycobacteria, but not most other types of bacteria.

It’s good to treat patients with antibiotics that have a narrow spectrum of activity, because then the ‘good’ bacteria that normally live inside of us won’t all get killed off along with the pathogen that caused the infection. However, when a patient comes into a clinic with an infection, it’s often not clear exactly which microbe is causing it. So in the case of severe infections, when it’s really important that an antibiotic work quickly so that the patient can survive, narrow-spectrum antibiotics would not be the best choice.

Broad-Spectrum Antibiotics

In the case of a severe infection with an unknown microbe, it would be better to use a broad-spectrum antibiotic. Broad-spectrum antibiotics are effective against a broad range of bacteria, so there’s a good chance that the antibiotic will clear the infection and possibly save the patient’s life.An example of a broad-spectrum antibiotic is tetracycline. As you remember from earlier in the lesson, tetracycline inhibits the bacterial ribosome and prevents protein synthesis. Tetracycline works against gram-positive, gram-negative and even obligate intracellular bacteria, which are bacteria that can only live inside their host’s cells, so they’re even more difficult to target with antibiotics.

There are two main reasons that tetracycline has a broad spectrum of activity:

  1. The bacterial ribosome is highly conserved among different types of bacteria and
  2. Tetracycline can enter most bacterial cells

One problem with broad-spectrum antibiotics is that they often kill a patient’s ‘good’ bacteria too. If our ‘good’ bacteria are killed, we can get opportunistic infections, which happen when a microbe that’s usually non-pathogenic takes advantage of a situation and causes disease. When a host’s ‘good’ bacteria are killed, it provides extra living space and nutrients that an opportunistic pathogen can take advantage of.But again, in the case of a severe infection, killing the patient’s ‘good’ bacteria is obviously better than the infection killing the patient.

Any opportunistic infections can be dealt with later, when it’s not an emergency.

Lesson Summary

In this lesson, we’ve learned some basic types of antibiotics. We’ve seen that bactericidal antibiotics kill bacteria directly, and bacteriostatic antibiotics stop bacteria from growing. Whether an antibiotic is bactericidal or bacteriostatic depends on what exactly the drug does to the bacterium.We’ve also learned about the spectrum of activity of antibiotics.

Narrow-spectrum antibiotics are only effective against a narrow range of bacteria, but broad-spectrum antibiotics are effective against a broad range of bacteria. It’s a good idea to use narrow-spectrum antibiotics when possible, because they won’t kill the patient’s ‘good’ bacteria and cause opportunistic infections. However, in the case of a serious infection with an unknown microbe, it’s best to use broad-spectrum antibiotics, because there’s a better chance of curing the infection quickly.

Learning Outcomes

After seeing this lesson, you should be able to:

  • Understand the differences between bactericidal (kills bacteria) and bacteriostatic (stops but does not kill) antibiotics
  • Define narrow-spectrum (treats a select few) and broad-spectrum (treats a broad amount) antibiotics

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