Antibiotics are used to fight bacteria and fungus in and on the body. Bacteria are classified according to their ability to cause disease. Pathogenic bacteria are capable of producing a particular illness or infection. Nonpathogenic bacteria live on or within the host but do not cause illness under normal conditions. These bacteria are called normal flora. Some actually produce substances necessary to the well-being of the host. For example, bacteria in the bowel synthesize vitamin K, which is necessary for blood clotting. Rarely, nonpathogenic bacteria will overgrow and cause symptoms due to their sheer numbers.
Antibiotics fall into two categories: Bacteriostatic and fungistatic drugs inhibit the growth of microorganisms but don’t kill them; bactericidal and fungicidal drugs destroy the microorganisms outright.
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Antibiotics may not always be effective, for a number of reasons.
Inadequate Wound Care
Antibiotics enter the bloodstream and are carried to the source of the infection. Abscesses, wounds that contain devitalized tissue, and wounds with foreign bodies (dirt or splinters, for example) are resistant areas. Under such circumstances, antibiotics can’t penetrate the wound completely. Accordingly, it is essential to drain abscesses, clean dirty wounds, and remove foreign bodies.
An antibiotic chosen to treat an infection must be effective against the specific type of bacteria that is infecting the body. The best way to determine susceptibility is to sample the organism, grow it on a culture plate, and identify it by the way its colony appears and by microscopic characteristics. Antibiotic discs are then applied to the culture plate to see which discs inhibit the growth of bacteria colonies. The results are graded according to whether the bacteria are sensitive, indifferent, or insensitive to the effects of the antibiotic. Laboratory findings, however, do not always coincide with results in the patient. Nonetheless, sensitivity testing is the best way to select the most effective antibiotic.
Route of Administration
An important medical decision is choosing the best route for administration. In a dog with severe infections, antibiotics may be given intravenously or by intramuscular or subcutaneous injection. Some oral antibiotics should be given on an empty stomach; others with a meal. Incomplete absorption is one cause of inadequate levels of antibiotics in the blood. If the dog is vomiting, oral medications may not be absorbed.