Antibiotics: How Do They Work?

by Dr. Beverly A. Dixon

Used incorrectly, antibiotics can do more harm than good
Sooner or later, every hobbyist is confronted with a bacterial infection in a prized fish. This problem usually presents itself after some stressful event, such as breeding, the introduction of other fish into the tank or, more likely, a newly acquired fish that is sick. The problem is how to treat the fish. Surely the pet store will have a cure. Yes, the pet store has a cure. In fact, most self-respecting pet stores will have at least a dozen or more antibiotics, antibacterials and antimicrobials either alone or in various combinations. So you pick one or, better yet, two, and begin a regime of chemical warfare on your unsuspecting fish. For their own good, of course. But, how do you know that your treatment will work? Is it the right antibiotic? Will it work in saltwater or in the soft water in your discus tank? What about the fry that your African cichlid is carrying? Stop! Before you drop in that one tablet per 10 gallons, what about the good guys? You don't want to kill off the good bacteria, the nitrifiers. You know, the ones that convert ammonia to nitrite and then to nitrate so that your fish don't live in a toxic sewer.
Just how do these tablets or capsules work? You've heard their names before: tetracycline, erythromycin and sulfa drugs. How do these treatments effectively kill bacteria, and how do they know which bacteria to kill? Maybe it's time for a little lesson in micro biology, the study of microscopic creatures including bacteria.
By technical definition, an antibiotic is a chemical produced by a microorganism, such as a bacterium or fungus, that it then uses to kill another microorganism. It's their way of waging chemical warfare on the nearest competition. This becomes particularly important in areas where the nutrient supply is low, such as in some types of soil.
In fact, most of our antibiotics are produced by bacteria, and especially fungi, that live in the soil. For example, the bacteria in the group Streptomyces produce more antibiotics than all other groups of microorganisms combined. And that blue-green mold on your orange, that's Penicillium, the Oscar winner of all molds from which Alexander Flemming originally isolated penicillin back in 1939.
So you get the idea: An antibiotic is a compound naturally produced by a microorganism to kill an other microorganism. Not to be confused with an antibacterial, which is a compound that was designed in some chemist's lab. These are also familiar: sulfa drugs, furans and quinolones (like nalidixic acid) the so-called designer antibacterials. Chemists are always whipping up new antibacterials, many of which don't work or are too toxic for the host. Now, let's talk about the bacteria. We microbiologists like to classify bacteria as either gram positive or gram negative. (You may have heard these terms before, particularly if you've had the occasion to treat a sick fish.) These states of "gramness" actually refer to the way that the bacteria stain or hold color by a particular method. When exposed to this method, the gram positive bacteria will stain violet and the gram negative bacteria will stain pink.
The difference is due to the structure and composition of their cell walls. The way that the cell wall is put together makes a big difference in the way that the bacteria respond to antibiotics. The old rule of thumb was that gram positives respond better to antibiotics that affect their cell wall, and gram negatives respond better to antibiotics that interfere with their protein synthesis. Of course, there are exceptions on both sides. To give an example, the bacterium that causes strep throat, Streptococcus pyogenes, is a gram positive that is best treated with penicillin. There have been more than two dozen different bacteria reported from infections in fish. Except for about half a dozen of these, the overwhelming number of warmwater bacterial fish pathogens are gram negative. Of these, the most common infection in freshwater fish is caused by the rod-shaped bacteria Aeromonas, whereas in saltwater fish it is the slightly curved, rod shaped bacteria Vibrio. Because of their cell wall composition, neither of these is susceptible to the action of penicillin or its semi-synthetic derivatives, such as ampicillin.
Penicillin and its related antibiotics work by preventing the bacterial cell walls from being built and repaired by the bacteria. If the cell walls break down, the bacteria can not grow or reproduce and thus die quickly. The penicillins work better on gram positive bacteria because they contain more of the chemical components that the drug works on than do gram negative bacteria Now we will look at how different groups of antibiotics work what are called mechanisms of action. We'll start with the familiar penicillin. Penicillin is a true antibiotic. Over the years, chemists have altered the shape and structure of penicillin by adding small molecular side chains to the original core. These new structures, such as ampicillin, are called semi-synthetic because a portion of the original is retained. The reason for this chemical designing was to outwit the bacteria that very quickly became resistant to the action of penicillin. Penicillin works best against gram positive bacteria. In tropical fish, these bacteria, such as Streptococcus, are rarely reported. Penicillin works by preventing the cross-linking of components in the cell wall necessary to hold the wall together. Without an intact wall, the cell contents leak out and the cell dies.
Penicillin has a narrow spectrum of action and only works well against a few types of bacteria, most of which do not infect aquarium fish. The newer semi-synthetics have a broader spectrum against even some gram negative bacteria, but these medications are still not the drugs of choice for most fish pathogens. Even though they are big sellers, antibiotics like penicillin and ampicillin are not effective against Aeromonas infections.
While we are discussing gram positives, lets talk about erythromycin. This drug is used in human medicine to treat gram positive infections in people who are allergic to penicillin. Erythromycin works by binding with ribosomes, thus interfering with the metabolic process of protein synthesis in bacteria. In other words, the bacteria cannot manufacture the components that they need to live.
Erythromycin has a broader range than does penicillin, and it may have a very limited effect against specific gram negative bacteria. A recent study in our laboratory (funded by the Western World Pet Supply Association, and reported in the July/August 1990 issue of AFM) showed erythromycin could be somewhat effective against a few strains of Aeromonas, particularly in domestic fish. Resistance to erythromycin was present in bacteria isolated from imported fish. Erythromycin works best when the pH of the water is neutral or slightly alkaline. Another group of antibiotics commonly used with aquarium fish is the aminoglycosides, although you probably won't recognize them by this name. They are more familiar by the names of kanamycin, neomycin, streptomycin and gentamicin. Of these, neomycin and kanamycin are the most commonly used. Both of these are soluble in water at slightly alkaline pH levels, and both provide a broad spectrum of activity against gram negative bacteria.
Aminoglycosides work to produce a bactericidal (killing) affect by inhibiting protein synthesis in bacteria. Traditionally, neomycin is used to treat superficial skin infections of freshwater fish, while kanamycin is used to treat the same types of infections in marine fish. Both can be used to treat fish from the African rift lakes, livebearers, brackish water fish and marine fish. Caution should be used when treating with these antibiotics. They should never be used in combination or in sequential treatment.
Another old stand-by for aquarium fish is the sulfa drugs. Sulfa drugs are used in combination (triple sulfa) or individually. Sulfonamide drugs are not true antibiotics but rather are antibacterials. These drugs produce a bacteriostatic effect, meaning they inhibit the growth of the bacteria but do not kill them. The killing is left to the white blood cells known as phagocytes (cell eaters) that engulf and destroy the debilitated bacterial cells.
Sulfa drugs arrest cell growth by inhibiting the synthesis of folic acid, a component required for growth by bacteria. Folic acid is a large molecule and is unable to enter bacterial cells, so the bacteria must synthesize the compound intracellularly. Animal cells are unable to synthesize folic acid and it must be provided in the diet. For this reason sulfa drugs are not toxic to animal cells.
Triple sulfa is a favorite because the combination produces a higher total concentration than a single sulfonamide. The sulfa drugs work bet ter at a slightly alkaline pH, and can be used on marine fish. However, bacterial resistance to these drugs has been reported in some imported fish. Like the sulfa drugs, the nitrofurans are also antibacterials. Drugs such as nitrofurazone and furazolidone have been used to treat fish for years. The bactericidal effect is produced by the ability of these drugs to inhibit several bacterial enzyme systems. The nitrofurans are most effective at a slightly acid pH. Their efficacy will decrease with increasing pH, so they are not the drugs of choice for brackish and marine fish infections. The furans are probably best used to treat infections of the softer water South American fish.
Probably the most familiar, and I say this based on my experience, the least effective of all the antibiotics used on aquarium fish is tetracycline. The tetracycline group consists of true antibiotics that bind to bacterial ribosomes, inhibiting protein synthesis. Tetracyclines readily bind to calcium and magnesium, forming an insoluble complex that reduces their effectiveness in hard or marine waters.
This group of antibiotics has provided an ongoing controversy regarding their usefulness in the treatment of fish diseases. Tetracycline is one of two antibiotics approved by the Food and Drug Administration for use on foodfish in this country (the other is a potentiated sulfonamide). In foodfish, this antibiotic is used as a medicated feed.
The controversy lies in the usefulness of the drug as a bath treatment. As the name implies, this is a large compound consisting of four ring structures. Because of its large size, the ability of tetracycline to cross the gill tissue to effect treatment for internal infection is in question. Studies in our laboratory have shown that depending on the geographical origin of fish, 60 to 90 percent of the Aeromonas isolated from infections are resistant to tetracycline. Tetracycline may still be useful in treating external infections or abrasions of wild caught Asian and South American fish, where prior antibiotic exposure is minimal.
The last group of drugs used to treat bacterial infections is the quinolone antibacterials. Nalidixic acid and its newer chemical analog provide a bactericidal effect against gram negative bacteria. This group of drugs works by selectively antagonizing DNA synthesis in bacterial cells. The action is directed against bacterial DNA gyrase, an enzyme necessary for replication. Interference with this enzyme causes the bacterial DNA gyrase to become lethally entangled, preventing DNA synthesis. This process does not affect DNA in animal cells.
Nalidixic acid works over a wide pH range and can be used to treat a variety of fish. Oxolinic acid is approved for use on foodfish in Europe but is not widely used in this country.
So there you have it. The next time you have occasion to drop in one tablet per 10 gallons of water, you'll know exactly what kind of chemical warfare you are about to wage.
WHEN USING ANTIBIOTICS
A word of caution about using antibiotics. These are potent drugs and can be easily overused and abused. Whenever using antibiotics, or any other treatment procedure for that matter, always maintain aeration during the treatment period.
Follow the manufacturer's directions. If one tablet per 10 gallons is required, two are not necessarily better. Be sure to change the water as directed on the package following treatment. If activated carbon is present in the filter, remove it during treatment. Carbon may adsorb the drug, decreasing its effectiveness.
Most importantly, don't use more than one antibiotic at a time. While some combinations, such as triple sulfa, are synergistic and provide an enhanced effect, others are antagonistic and may actually cancel one another or create a toxic effect on the fish. Combinations that should not be used: nalidixic acid and nitrofurans, tetracyclines with aminoglycosides, and aminoglycosides in combination or sequentially. Erythromycin can kill the nitrfying bacteria in the filter bed. These bacteria are essential for maintaining water quality and a healthy tank. Finally, remember that sensitivity to drugs can vary with individual fish species. It is always better to treat the individual sick fish separately in a hospital tank.
Beverly Dixon is a microbiologist and teaches biology at a California university. She has done extensive research on the immunology, diseases and treatment of fish.