Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.
What Is an Infection?
Have you ever been to the doctor, maybe sick with the flu in the winter, and they tell you there is nothing they can do? You might wonder, why did I even come here and why can't I have antibiotics? Well, if you don't get antibiotics, the doctor has probably found you have a viral infection, not a bacterial infection. Doctors won't prescribe antibiotics for viral infections because they won't work! Although it seems like a pain when you're the sick patient, this practice is for a good reason for, and that's what we're going to learn today.
Virus Versus Bacteria
To get started, let's review the difference between a virus and a bacterium. Viruses are non-living, infectious particles. They require a host cell to survive as they are not alive on their own. Viruses are beyond microscopic and require a special type of microscope to be seen. Bacteria are much larger comparatively and are living, single cells. Some bacteria are good for us, and we need them to survive. Other pathogenic bacteria infect our bodies and make us sick. The diagram compares the two types of pathogens.
So how do viral infections happen? Well, first a virus must enter the body through the airways, digestive or reproductive systems, or cuts in the skin. Once the virus has entered the body, it will make its way to a specific tissue. Both viruses and bacteria infect specific types of tissue. This is why you have specific symptoms for each infection. Once the virus has reached its destination, it binds, or attaches, to the host cell. The virus either enters the cell, or directly injects its genetic material, or instructions, into the cell. Once the virus has access to the cell, it hijacks the host cell's machinery to make more viruses. The host cell is a slave to the virus, creating more and more viruses that burst out of the host cell to invade other tissues. Sometimes, this is a rapid process and the host cells die quickly. Other times, the virus may wait or exit the cell slowly, prolonging the infection. The image shows a general example of viral replication. The specific method depends on the virus, but all viruses work similarly.
An example of a viral infection is human immunodeficiency virus (HIV), the virus that causes acquired immunodeficiency, or AIDS. HIV is transmitted through blood, semen, or vaginal secretions. It quickly invades the blood stream and attacks the host's immune cells. With a damaged immune system, the patient can no longer fight other diseases and often dies of secondary infections.
Since bacteria are alive, they don't need to invade host cells specifically to replicate. Bacteria find the tissue they like to live in, similar to the virus. The bacteria may take up nutrients from the host tissue, secrete toxins that kill the host cells, or replicate inside the host cells, killing them as the bacteria break out. However, bacteria replicate on their own, unlike viruses that need the host cell to make more of them. In the diagram, we see the two ways bacteria can kill a host cell.
An example of a bacterial infection is E.coli O157:H7, a particular strain of E.coli that causes food poisoning. Not all E.coli are bad for us, but this strain secretes a powerful toxin causing vomiting and diarrhea and can be fatal if not treated. The E.coli in the image is colored pink and the background is purple.
Implications for Medicine
You have probably heard of or been treated with antibiotics. 'Anti' means against and 'biotic' means life. So, antibiotics only work against living organisms, like bacteria. Antibiotics do not work on viruses, since they are not alive. It is very important to use antibiotics sparingly and only for bacterial infections. When antibiotics are used too much, they cause antibiotic resistance. This is a condition in which bacteria have adapted to live in the presence of an antibiotic and it no longer affects them. This is good for the bacteria, but bad for us. As bacteria become resistant to more and more antibiotics, fewer choices are available to treat serious infections. Currently, the most dangerous antibiotic resistant bacteria is Methicillin-resistant Staphylococcus aureus (MRSA). This strain of bacteria is extremely prevalent in hospitals and is resistant to many antibiotics. Only a few antibiotics remain that can effectively kill MRSA. The image shows the human immune system (green) trying to fight off a MRSA infection (purple).
So, when you go to the doctor and they refuse to give you antibiotics for your flu, go home, rest, and don't press the issue! Tight regulation of antibiotics is important for global health and safety, despite how annoying it is to walk away empty handed.
In summary, viruses are non-living, infectious particles that can cause illness in humans and bacteria are much larger, living cells that sometimes cause illness in humans. Viruses replicate by finding the correct tissue, invading host cells, and hijacking their machinery to make more viruses. Bacteria find the correct tissue, secrete toxins that damage the host, and sometimes may replicate inside the host cell, bursting it as the bacteria escape. Antibiotics are a treatment only for bacteria because they only kill living things. Using antibiotics for viral infections or not taking them as directed can cause antibiotic resistance, which is when bacteria fail to die from antibiotic treatment. Antibiotic resistance is a huge problem in hospitals, where cases of MRSA are increasing with limited treatment options currently available.
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