What Are Antibiotics? - Definition, Types & Side Effects

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  • 0:02 What Are Antibiotics?
  • 0:23 Discovering…
  • 2:38 Types of Antibiotics
  • 4:59 Side Effects of Antibiotics
  • 6:01 Antibiotic-Resistant Bacteria
  • 6:42 Lesson Summary
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Lesson Transcript
Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Antibiotics are medicines created to fight infections caused by bacteria. They accomplish this by destroying the bacteria in a variety of ways. Learn more about how antibiotics work and take a short quiz at the end.

What Are Antibiotics?

Nobody likes getting sick. A sore throat, throbbing head, and aching body are all signs of impending doom. Desperate to feel better, many of us trek to the doctor in hopes of some medicinal relief. In other words, we want antibiotics! These are medicines designed to fight bacterial infections, and they can cure many ailments.

Discovering Antibiotics

The first antibiotic used by humans was penicillin, and it was discovered quite by accident. A bacteriologist named Alexander Fleming had been studying bacteria in Petri dishes. He left for vacation and did what many of us do in our own homes: left his used dishes sitting in the sink. Most of the Petri dishes were stacked in a tub of Lysol disinfectant to be cleaned later, but a few remained above the liquid.

When Fleming returned from vacation, the Petri dishes of bacteria were, of course, still piled in the sink. As he trudged over to begin the task of cleaning, he noticed something unusual. Mold had grown in some of the Petri dishes that had not been submerged in Lysol. In those dishes with mold, the bacteria was dying. It was clear that this mold, which was found to be the fungus Penicillium, inhibited and killed the bacteria. This would prove to be one of the most crucial breakthroughs of modern medicine.

How Do Antibiotics Work?

As we now know, antibiotics are medicines created to fight infection caused by bacteria. The word antibiotic literally means 'against life.' The goal is to kill or stop growth of these tiny infection-causing organisms, known as pathogens. But, how exactly does an antibiotic make this happen?

We've already established that certain fungi are active in fighting infection. But there is another type of microbe that is commonly utilized in antibiotics, and it just so happens that often times, the component in an antibiotic that straps on its boxing gloves to fight off infection is actually another type of bacteria. This type of bacteria is usually found in soil, and examples include Streptomyces and Bacillus.

Now let's look at how the antibiotics actually fight off the infection. Antibiotics are designed to target the pathogens that are making you sick, killing them or stopping their growth. It is microbe against microbe, a battle to the death. But, how do the active components of antibiotics know which cells in your body to attack? In other words, how do they tell bacterial cells and human cells apart? Well, there are many different types of antibiotics, and they attack different kinds of cells in different ways.

Types of Antibiotics

A great deal of scientific research has brought about many different types of antibiotics. They often stem from just a few different drugs and are classified based on how they act on pathogens.

One class of antibiotics called beta-lactam zeroes in on a structure unique to bacterial cells. Their main tactic is to target the cell wall, which holds the cell together. Just like a water balloon would certainly break apart if part of the balloon were missing, the same goes for the bacterial cell. As the bacteria builds its cell wall, the components in the antibiotic step in to disrupt this process. Unable to stay intact, the infectious bacteria bursts. Penicillin is an example of an antibiotic that works in this fashion.

Another class of antibiotics called macrolides work by stopping the synthesis of protein by the bacteria. Every cell needs proteins to do work. In this case, the macrolides make it impossible for the proteins in the bacteria to be created. It's like disabling part of the machinery that helps the bacteria work. Without the protein, the bacteria dies.

Still another type of antibiotics called quinolones work by messing up the process of DNA replication in bacteria. Bacteria colonies reproduce and grow by first replicating their genetic information, or DNA. When the bacterial cells attempt to replicate and make a new strand of DNA, quinolones work by breaking the DNA strands. It's like someone knitting the first strand of a scarf, and someone else snipping it before it's finished. Without intact DNA, the bacteria cannot successfully reproduce, and thus, the colony is extinguished.

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