“Doc, he’s just not getting well like he should. I think he needs a stronger antibiotic.”
This is a sentence one hears a lot in the medical professions. You may have even said it yourself. Today we will explore the “strengths” of antibiotics, and learn how “strength” differs from “spectrum.”.
First, let’s talk about what antibiotics are used for. Antibiotics are medications which are produced by one microorganism that have the ability to kill other microorganisms. Think of penicillin, which originally was discovered to be produced by a certain species of mold.
In modern times, many antibiotics are made by man, in a laboratory through gene manipulation, but the basic building blocks were first discovered in nature. Man has used his chemical expertise to modify the chemical structure of “natural” antibiotics to make them more effective.
Back in penicillin’s early days it would kill almost any “bad” microorganism, meaning those able to cause infection, and, thus, harm to the body. Over time, and with misuse, penicillin began to become less effective against some infections. We have been able to lessen that problem as we learned more about bacterial resistance to antibiotics.
To a great extent, such resistance came about because early doctors gave penicillin for a specified amount of time, and it wiped out most infections, but left some infections lingering. Think of it like a war: If you fight a battle for 5 days, and don’t totally defeat your enemy, who is left? Not the weaklings, they died in the first skirmish! Rather, the enemy that remains is the toughest of the tough.
Now switch back to penicillin’s battle of infection. Fight the battle for 5 days, then stop. If all of the infection isn’t gone, the bacteria that remain are the toughest, and they may also be the bacteria who can change their genes just enough that the penicillin no longer kills them. That’s called bacterial resistance.
Yes, the problem of not “taking your medicine until all is gone” started with the first antibiotic.
Now the doctor has two possible choices, and this is where a “stronger” antibiotic comes in.
The “strength” of an antibiotic is measured by how much one dose weighs. If one penicillin pill contains 250 milligrams (a milligram is one one-thousandth of a gram), then its strength is 250 milligrams (mg). A 500 mg tablet is twice as “strong” as a 250 mg tablet. If the doctor feels, or knows by testing, that the infection will still be susceptible to penicillin if the dose is higher, and the dose for the patient can be increased while still keeping it in what the doctor determines to be a safe range (called therapeutic index), another 5 or 10 or 15 days of penicillin at the higher dose may knock out any remaining infection.
However, sometimes infections have a nature that is totally different from those infections that are susceptible to penicillin. Sometimes infections are complicated, and more than one kind of bacterium is present in the infection. In that case, the doctor may want to change to an antibiotic that has a “broader or different spectrum.” An antibiotic with a broader spectrum might kill all of the bacteria that penicillin kills, plus kill bacteria that have become resistant to penicillin.
An antibiotic with a “different spectrum” might kill certain very resistant bacteria, but not be much help with bacteria that good old penicillin kills quite well.
Usually, those antibiotics with a broad or specialized spectrum cost more than older antibiotics such as penicillin.
Switching antibiotics can sometimes be good for a patient’s response to treatment. Oftentimes antibiotics are chosen based on a two-part test called “culture and sensitivity“. The first part of the test, culture, grows that organism(s) causing the infection in the laboratory and identifies them. The second part, sensitivity, subjects those bacteria to a variety of antibiotics to determine which ones will kill the infection and which ones won’t. Choosing antibiotics based on the results of culture and sensitivity often saves time and money, without having to change antibiotics.
