When the cause of an infection is presumed or proven to be bacterial, antibacterial drugs are used to stop it. Antibacterial drugs can be either bactericidal or bacteriostatic. A bactericidal drug directly kills the bacteria. In contrast, a bacteriostatic drug keeps the bacteria from multiplying, holding it in check until our own defenses can eliminate it. Bactericidal drugs are usually preferred over bacteriostatic ones. Antibacterial drugs can work in a variety of ways to accomplish their goal. They can disrupt the wall of the cell, alter cellular membranes or protein production, disrupt synthesis of vital components, or alter cellular DNA.
Not all bacteria are susceptible to every antibacterial drug. Some drugs may be bacteriocidal to some organisms and bacteriostatic to others; other bacteria may not be affected at all. The range of bacteria that a drug is effective in eliminating is known as that drug's spectrum of action. A drug that has a broad spectrum is effective against a wide range of bacteria. A drug with a narrow spectrum affects only a few species of bacteria.
Bacteria can be classified according to the structure of their cell walls. A common way to do this classification is to use the gram stain test. In this test, a slide is smeared with bacteria and
flooded with gram stain. Based on the structure of the cell wall, the bacteria stains a certain color and is then classified as either gram positive or gram negative. Certain antibiotics affect mainly gram-positive organisms while others affect mainly gram-negative organisms. Some drugs affect both. However, a drug can be effective against most bacteria in a certain class and be ineffective against a few others in that same category. It is always wise to obtain a culture to determine which bacteria is causing the infection. Once this is known, an antibiotic effective against that organism is selected.
Certain bacteria are more common than others in ophthalmic practice. Examples of gram-positive organisms important in ocular infections include Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus pneumoniae. Examples of gram-negative bacteria are Neisseria gonorrhea, Haemophilus influenzae, and Pseudomonas aeruginosa. It should be noted here that the treatment of certain other infections involving acanthamoeba (a protozoan) and parasites (eg, Toxoplasmosis gondii and Chlamydia trachomatis) are often treated with antibacterial drugs. These infectious organisms may share common physiologic properties with bacteria; thus, antibacterial drugs are sometimes used in the management of nonbacterial infections.
A commonly occurring problem is that bacteria that were once susceptible to the actions of a certain drug can develop resistance to this agent over time. After exposure to a drug, bacteria can become resistant through mutation, selection, and adaptation. This resistance is becoming widespread, making some other drugs, once very effective, now of much less value. This is one reason the production of new pharmaceuticals is necessary.
Like other drugs, administration of antibiotics may be by way of injection (intravenous or local), orally, or topically depending on the location, duration, severity, and type of infection. Most ocular infections are superficial and involve the conjunctiva and cornea. Topical administration by drop or ointment is preferred—usually 1 drop 2 to 4 times a day but up to 1 drop every
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