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Gain an Understanding & Appreciation for Antibiotics

If you’ve ever had strep throat or a urinary tract infection, you’ve likely been prescribed antibiotics to help clear up the infection. How do these drugs work? Brandon Gill, a student pharmacist at Samaritan Lebanon Community Hospital, explains what’s happening behind the scenes when you take an antibiotic.

What Do Antibiotics Do?

Antibiotics work in many unique ways to help your immune system fight off bacteria. Some stop bacteria from being able to grow and multiply, while others can kill bacteria directly. The main objective for antibiotics is to disrupt the growth and spread of bacteria long enough for your body’s immune system to identify and eliminate the infection. The human body has a truly remarkable defense system, but sometimes it just needs a little help from antibiotics to overpower a bacterial invader.

Selecting the Right Antibiotic

There are many different factors your health care provider takes into consideration to help guide their decision when choosing antibiotics. The first, and most important, is to determine whether bacteria are the likely cause of the problem. If you have an infection caused by a virus (like the common cold or the flu) or a fungus, antibiotics won’t be very useful.

When a provider concludes that bacteria are the cause of infection, the next factor to consider is the location of infection. Different types of bacteria are more likely to be found around different parts of your body, such as Staphylococcus (a “staph” infection) in a skin wound or E. coli causing problems in the urinary tract. Providers can select an initial antibiotic treatment based on what is the most likely bacteria to be causing these issues.

For more complicated or severe infections, providers will collect a sample of the bacteria from the infected site and have it identified by the medical lab. Once the bacteria sample is identified, the best antibiotic for that specific organism can be used if the initial antibiotic choice wasn’t adequate.

The final step is to make sure the antibiotic is safe for the patient. Like all medications, each antibiotic has unique risks that need to be weighed against a patient’s other medical conditions. Some common considerations include allergies, kidney or liver issues, heart conditions, or other medications a patient takes.

Rise of Antibiotic Resistance

Bacteria have evolved over time to develop their own ways to fight off antibiotics, which is called antibiotic resistance. Resistance to normal antibiotics can certainly complicate treatment, but providers have another tool in their kit to guide their decisions called a sensitivity analysis.

The medical lab can use the sample of bacteria they have to test out different antibiotics to see which ones will still be effective for these specific bacteria. The lab can even measure the concentration of antibiotic needed to find the right dose needed to be most effective.

What Are the Different Classes of Antibiotics?

Antibiotics can be grouped together in multiple different ways, including how they work, what infections they typically treat, or how many different types of bacteria they can treat.

Antibiotics that only treat a few types of bacteria are referred to as narrow spectrum, whereas antibiotics that can treat many different types of bacteria are called broad spectrum. This can be further divided into antibiotics that treat more dangerous and resistant bacteria, such as MRSA or Pseudomonas, a type of bacteria that can cause serious infections in people who have weakened immune systems.

These different groupings of antibiotics come into play when providers want to treat a patient right away because they are very sick, don’t have a clear picture of what kind of infection may be present, and don’t have time to wait for a sample of bacteria to be cultured and identified in the lab. In these cases, providers will start treatment by giving a broad-spectrum antibiotic to cover as many of the likely bacteria as possible. Then, when the lab has had time to analyze and identify any samples taken, the provider can change the treatment to a more narrow-spectrum antibiotic that will work just as well, if not better, against that specific bacteria.

Effectively treating an infection with the narrowest-spectrum antibiotic is an important goal of therapy. Since bacteria are becoming more and more resistant to different types of antibiotics, we don’t want to give those bacteria the chance to encounter the broadest-spectrum antibiotics for long enough to create new resistance mechanisms. 

Is It Better to Use a Variety of Antibiotics or the Same One Every Time?

This is a great question to discuss with your provider. If you have some antibiotics left over from a previous infection, they may not be the right choice to treat a new infection. This is especially true if the new infection is the same kind as the previous one and happens shortly after the old infection seemed to resolve or keeps coming back. This reoccurrence may be a sign that the bacteria responsible for the old infection were not eliminated and may have even created a resistance mechanism for the antibiotic you were prescribed previously.

However, using many different types of antibiotics, especially close together, can also cause problems in the normal balance of beneficial bacteria that work together with your body. For example, one course of an antibiotic may briefly disrupt some normal cohabitating bacteria, such as those in your digestive tract. This disruption may end up causing some mild indigestion and diarrhea for a few days at most. Now, multiple treatments with different types of antibiotics can amplify this problem, disrupting and killing many types of normal gut bacteria. This can lead to more complicated problems in the GI tract, such as an overgrowth of potentially harmful organisms like C. diff that are normally kept in check by those beneficial bacteria. These risks are carefully weighed by your provider when they are choosing the best treatments for you.

Five Common Antibiotics & Their Uses

Amoxicillin or Amoxicillin/Clavulanate (Augmentin) – This medication is the most commonly prescribed antibiotic and is one of the penicillin-type antibiotics. It is used to treat some of the more common infections like an infected wound, ear infection or strep throat. This medication has little to no side effects with the exception of allergic reactions. Always be sure let your provider know if you have an allergy to penicillins. Additionally, providers may ask which specific penicillin you had the reaction to, how long ago the reaction was, and what type of reaction it caused. Oftentimes, reported allergies to penicillins are not true allergies and don’t preclude you from receiving a penicillin again. Allergies can change over time, especially from childhood to adulthood, so a provider may wish to order a test to see if you are still allergic to a particular antibiotic that might otherwise be the best treatment option for you.


This medication is in a class of antibiotics called macrolides and is an excellent choice for respiratory infections like pneumonia. Azithromycin has a unique range of activity that can work against nearly all the bacteria that commonly cause lung infections. This antibiotic is usually very well tolerated and is easy to take – usually just once a day for five days.

One concern with this class of antibiotics is being watchful of patients with a specific heart rhythm issue called QT prolongation or “long QT.” Usually there is little to worry about since patients only take this medication for such a short period, but if you are taking other medications regularly that also affect your heart rhythm, your provider may be more vigilant or choose another option.


This antibiotic is in the class of tetracycline antibiotics, which all end with “cycline” in their names. Doxycycline can be used for many different types of infections, such as urinary tract infections, pneumonia, skin infections and some sexually-transmitted infections.  Tetracycline antibiotics like doxycycline work a little differently in the body and can cause some odd side effects. One important side effect to remember is light sensitivity. While taking doxycycline, your skin and eyes can be more easily damaged by the sun. Even on cloudy days, you may want to wear protective clothing to prevent sunburn.

Additionally, this is one of the classes of antibiotics that bind very well with calcium in your stomach and will stop it from being absorbed into your body. When taking this drug, we recommend spacing out any dairy products, multivitamins, or antacids like Tums by at least two hours before or after taking your doxycycline.  


This antibiotic is one of the fluoroquinolone antibiotics, which all end with “floxacin” in their names. Fluoroquinolones like ciprofloxacin are a versatile antibiotic class with many unique uses. Ciprofloxacin is very effective for treating more severe urinary tract infections and kidney infections, as it likes to hang out in the urine. Additionally, ciprofloxacin works well to treat or prevent some abdominal infections for patients with chronic diseases like Crohn’s disease or liver cirrhosis. You might even be prescribed this drug for an inner ear infection.

Unfortunately, the fluoroquinolone class of antibiotics does have its downsides. Just like doxycycline, this class of medications also likes to stick to calcium which can prevent it from being absorbed well. Again, spacing these medications out from dairy products, multivitamins and antacids will help achieve its full effects.

This antibiotic class can also cause not only many of the problems seen with other types of antibiotics, such as affecting heart rhythm (QT prolongation) and light sensitivity, but also other rare, serious problems like damage to the liver or tendons. For these reasons, providers tend to select other options if they can, but always carefully weigh the risks versus benefits if fluoroquinolone antibiotics may be needed.


This antibiotic combination is more commonly called Bactrim or Septra. This unique antibiotic doesn’t really belong in any other classes of antibiotics due to its unique properties. Bactrim can be used for a wide variety of infections, ranging from common uses like skin infections, urinary tract infections or abdominal infections to more rare, serious infections like meningitis, bone infections or MRSA infection.

Bactrim has its own unique set of side effects as well. This antibiotic is one of the “sulfa” drugs that patients can often be allergic to. Additionally, Bactrim can interact with many other medications – namely the blood thinner, warfarin. Your provider or anticoagulation clinic may have you temporarily change your dose of warfarin while you are taking Bactrim.

Above all, remember to take your medication as directed by your provider, even if you start to feel better.

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