Viral and bacterial infections are by far the most common causes of illness for most people. They cause things like colds, pneumonia, measles, mumps, malaria, AIDS and so on. The job of your immune system is to protect your body from these infections. The immune system protects you in three different ways:
- It creates a barrier that prevents bacteria and viruses from entering your body.
- If a bacteria or virus does get into the body, the immune system tries to detect and eliminate it before it can make itself at home and reproduce.
- If the virus or bacteria is able to reproduce and start causing problems, your immune system is in charge of eliminating it.
Sometimes your immune system is not able to activate itself quickly enough to outpace the reproductive rate of a certain bacteria, or the bacteria is producing a toxin so quickly that it will cause permanent damage before the immune system can eliminate the bacteria. In these cases it would be nice to help the immune system by killing the offending bacteria directly.
Antibiotics work on bacterial infections. Antibiotics are chemicals that kill the bacteria cells but do not affect the cells that make up your body. For example, many antibiotics interrupt the machinery inside bacterial cells that builds the cell wall. Human cells do not contain this machinery, so they are unaffected. Different antibiotics work on different parts of bacterial machinery, so each one is more or less effective on specific types of bacteria.
Bacteria aren’t particularly intelligent. However, it is possible, and unfortunately all too common, for bacteria to “learn” how to survive even with antibiotics around.
There are several ways that bacteria can become resistant. All of them involve changes in the bacteria’s genes.
- Bacterial genes mutate (change), just like the genes of larger organisms mutate. Some of these changes happen because of chemical or radiation exposure; some just happen randomly, and no one’s sure quite why. If bacteria with a changed gene is less susceptible to an antibiotic, and that antibiotic is around, the less susceptible (and more resistant) version of the bacteria is more likely to survive the antibiotic and continue to multiply. This is particularly likely to happen if the amount of antibiotic around isn’t quite enough to kill all of the bacteria quickly — as can happen if you don’t take enough of the antibiotic to keep its level in your body high, or if you stop taking the antibiotic too early. This is why when you are prescribed an antibiotic you MUST take it exactly as prescribed, and for as long as it was prescribed. It’s also why we don’t (or shouldn’t) give you an antibiotic for an illness like a cold that isn’t likely to be bacterial: the antibiotic will kill off the susceptible bacteria, leaving bacteria that are resistant to that antibiotic.
- Although there are many different species of bacteria, some bacteria can “trade” genes with other bacteria. If you have a relatively harmless bacteria in you — say, in your mouth or your intestines (both places are chock full of bacteria) — and you’ve used (or overused or misused) antibiotics some of those harmless bacteria will become resistant to the antibiotics you’ve used. They can then give the resistance genes they have developed to other, harmful bacteria.
- There are viruses around that attack bacteria rather than plants, animals, or people. Most of these viruses just kill the bacteria, but sometimes the viruses can copy genes, like the antibiotic resistance genes, from one kind of bacteria to another.
Kinds of Antibiotics
There are now so many different antibiotics on the market that it’s hard to keep track of them all.
Penicillins and Cephalosporins
In the early 20th century, Alexander Fleming discovered that a mold called Penicillium (the cells are pencil-shaped when you look at them under a microscope) produces chemicals which kills most of the bacteria nearby. (The mold is green when it grows in large amounts, and is often found on bread. This, however, does not mean that eating moldy bread will cure your ear ache… or anything else. There are other things produced by molds, too.) Sometime later, another mold was found which produced a bacteria-killing chemical; this chemical and its cousins were called “cephalosporins” after the mold it came from.
The vast majority of antibiotics are either penicillins or cephalosporins; chemical changes have been made to the molecules over the years to improve their bacteria-fighting abilities and to help them overcome breakdown and “immunity” of resistant bacteria. Most bacterial cells have double layers on their outside. The outermost layer, the “cell wall”, is similar to the outer layer of plant cells, but is missing in human and animal cells. This wall must grow along with the cell, or the growing cell will eventually become too big for the wall and burst and die. Penicillins and cephalosporins kill bacteria by messing up the wall-building system. Since we don’t have cell walls, and plants have a different wall-building system, neither we, nor animals, nor plants are affected by the medicine.
Penicillins and cephalosporins usually don’t cause many problems for a patient. Like all antibiotics, they can cause mild side effects like diarrhea. Less common side effects include rashes (which may or may not imply a true allergy) and hives (which usually means you’re allergic to the medicine). The rarest — and scariest — side effect is “anaphylactic” allergy, in which your airway swells up when you take a dose of the medicine, sometimes to the point where you can’t breathe.
Macrolides (Erythromycin, Klaricid, Azithromycin)
Erythromycin is another antibacterial produced by a mold. There are a couple of new relatives of erythromycin (azithromycin and clarithromycin) that work the same way, but kill more bugs and have slightly fewer side effects. The erythromycin-like antibiotics are also known as macrolides.
Macrolides works by blocking the bacterial cell’s machinery for making new proteins. Since proteins both make up much of the cell’s structure and make the enzymes that direct all the cell’s chemical reactions, blocking protein manufacturing makes the cell unable to function. Macrolides in low doses will stop bacteria from growing and multiplying, but you need a higher concentration to kill the bacteria. However, if you can stop growth until your immune system kicks in, that will help you get rid of the infection.
Since all protein making is affected, erythromycin can slow down or kill any bacteria, even those without cell walls. Because of this, we use the erythromycins for several diseases, including bacterial bronchitis, chlamydia, and whooping cough, that penicillins and cephalosporins can’t touch.
The biggest problem with these medicines is that they can irritate the stomach. Always take erythromycin with food or milk. The same goes for clarithromycin. Azithromycin doesn’t irritate the stomach nearly as much as the others and should be taken on an empty stomach.
The sulfas (more properly “sulfanilamides” or “sulfonamides”) were the first man-made antibiotics to be developed. They interfere with certain “manufacturing” systems in the bacterial cell, including ones that bacteria use to produce new DNA for new bacteria. Sulfas can stop bacteria from growing, but they cannot actually kill the bacteria.
Sulfas also have a tendency to produce allergic reactions. We use sulfas nowadays mainly in combination with another drug which attacks a different part of the bacteria. The drugs we usually combine with sulfas are either erythromycin or trimethoprim
Trimethoprim-Sulfamethoxazole (Septra, Bactrim)
Trimethoprim (TMP) is another man-made antibiotic. Like the sulfas, trimethoprim blocks an important step in the bacteria’s system for making new DNA — but it’s a different step. By itself, TMP can kill bacteria, but very slowly. Usually, though, we use TMP in combination with sulfamethoxazole (SMX), and the combination of TMP and a sulfa kills bugs better. In fact, bacteria that are partly resistant to either TMP or SMX can still be killed by the combination of the two. The combination is widely used for urinary tract infections, airway and skin infections.
Nitrofurantoin is another synthetic antibiotic, used mainly for urinary tract infections.(Since it is excreted in the urine, it concentrates in the bladder very nicely.) Nitrofurantoin stops bacteria from growing, and can kill bacteria with a high enough level, by blocking the bacteria’s ability to use energy it makes by “digesting” nutrients like sugar, and by blocking other chemical reactions that use the same system. It should be taken with food ie yoghurt to prevent upset stomach. Resistance is limited and it can be taken safely during pregnancy
Aminoglycosides (Gentamycin, Tobramycin)
The aminoglycosides are drugs which stop bacteria from making proteins; they work by attaching permanently to the protein machinery. Since they attach permanently, the bacterial cell will die if it gets enough of the drug. They can be used by themselves, or along with penicillins or cephalosporins to give a two-pronged attack on the bacteria.
Since aminoglycosides are broken down easily in the stomach, they can’t be given by mouth and must be injected or given IV When injected, their side effects include possible damage (temporary or permanent) to the ears and to the kidneys; this can be minimized by checking the amount of the drug in the blood and adjusting the dose so that there is enough drug to kill bacteria but not too much of it. Generally, aminoglycosides are given for short time periods, and in hospital settings.
The chinolones or quinolones, of which the best known is ciprofloxacin (Cipro®:), interfere with an enzyme called DNA gyrase that is essential for duplication of bacterial DNA. (Bacteria have only one long chromosome (DNA molecule); the chromosome gets twisted during replication, like a telephone cord, and, again like the telephone cord, the chromosome can become so twisted that nothing more can be done with it. DNA gyrase is the “untwisting” enzyme.) This interference is completely different from the interference of other antibiotics with bacterial “machinery”, and so bacteria that are resistant to other antibiotics may be sensitive to the chinolones.
However, bacteria can develop resistance to the chinolones, too.
Chinolones should not be taken together with calcium or antacids since it reduce the absorption.
Tetracycline kills bacteria and protozoa by inhibiting the manufacture of specific proteins needed by the organisms to survive.
Tetracycline Antibiotics is a group of antibiotics produced by certain sepcies of the fungus Streptomyces. Tetracycline drugs (also known as broad-spectrum antibiotics) are effective against many different types of bacteria.
Doxycycline is used in the treatment of infections of the skin, bone, stomach, respiratory tract, sinus, ear, and urinary tract. Lyme disease and certain sexually transmitted diseases (gonorrhea and chlamydia) can also be treated with Doxycycline. Doxycycline is also recommended for the treatment of Anthrax.
Tetracyclines enhance sensitivity for sunlight and are preferably taken on empty stomach.