Proper Selection of Antibiotics in Ophthalmology

Abstract

An antibiotic is a chemical substance produced by a microorganism which has the capacity, in dilute solutions, to inhibit the growth of or to kill other organisms. They include antibacterials and antifungals. Strictly speaking, only the natural products of microorganisms should be termed antibiotics.In practice, drugs that are obtained by chemical modification of the natural substances in this category and those which are entirely synthetic, are also included under the name antibiotics. A balance of the microbial and human defense mechanisms dictates the normal microbial flora of the eye.Without adequate host defenses, contaminating microbes would colonize, colonizing microbes would invade, and invading microbes would infect. Cell division and protein synthesis in both bacteria and fungi is a process that is targeted by many antibiotics.According to their mechanism of action, both antibacterial and antifungals are classified into different groups.Antibacterials include cell wall synthesis inhibitors, protein synthesis inhibitors, nucleic acid inhibitors and folate antagonists. Meanwhile, antifungals include; Polyenes, Imidazoles, Triazoles, Allylamines and Ecchino- candins. Each of these groups has its own spectrum of activity.It may be broad spectrum, narrow spectrum or may have a limited spectrum. Some agents kill the microbes (cidal) and others only inhibit their growth (static). Tables (12) and (13) list some common commercially available ophthalmic antibacterial and antifungal agents. Ophthalmic antibiotics may be administered via different routes. Topical administration is most efficient in treatment of superficial ocular infections. The periocular route delivers a higher intraocular concentration of antibiotics than both topical and systemic routes. It can be used in patients non-compliant to frequent topical administration and in geriatric patients with comorbidities, to whom high doses of systemic antibiotics can be relatively toxic. Intravitreal injection proves to be the most efficient route for antibiotic delivery to the posterior segment of the eye. Systemic antibiotics are used alone for adnexal infections.They are only an adjunctive treatment in cases of intraocular infection as the blood ocular barrier, limits their intraocular penetration. Some serious ocular infections such as necrotizing fasciitis, orbital cellulitis, gonococcal conjunctivitis, meningococcal conjunctivitis and endogenous endophthalmitis necessitate prompt administration of parentral antibiotics to prevent vision threatening complications as well as to treat other features of systemic illness.