الفهرس 
Nature of the target pathogens
Cell divisionOnly 14 pages are availabe for public view
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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 Ecchinocandins. 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.
Antibiotic prescription for each infection depends mainly
on the most common pathogen encountered. However, serious
infections as severe conjunctivitis, keratitis and endophthalmitis
necessitate microscopy, culture and antibiotic sensitivity tests.The
sensitivity tests done via Kirby method, depend mainly for their
classification of the susceptibility of the organisms to antibiotics
on antibiotic concentration in the serum.Though the results
provide a guide for therapy, they must be interpreted with caution
as the test does not generally take into account situations in which
extremely high concentrations of drug can be applied directly to a
relatively superficial lesion. An antibiotic to which the organism
is claimed to be resistant via the routine test, can be highly
effective when administered in high concentrations via topical,
periocular or intravitreal routes. Hence, if the initial empirical therapy, commenced before the sensitivity results are obtained
proves to be successful, it need not to be changed, even if the
sensitivity results show discrepancy.
To maximize the benefit-to-risk ratio of antibiotic therapy,
it is vital to ensure that the appropriate patients are treated with
the most suitable drugs.Patient factors as age, pregnancy,
presence of a critical illness as renal problem, hepatic problem or
a poor immune status should all be put into consideration on
selecting the antibiotic.This is more crucial when prescribing
systemic antibiotics.Topical antibiotics, through systemic
absorption have been associated in rare cases with severe allergic
reaction up to anaphylaxis. Hence, a history of previous allergic
reaction to a known antibiotic may also affect the choice.
Antibiotic use in surgical prophylaxis is also a known
indication. Though no topical medication is FDA approved for
prophylaxis before intraocular surgeries, many studies suggest
that the preoperative use of topical antibiotics, greatly reduce the
concentration of conjunctival flora, presumed to be the primary
source of bacteria for the sight-threatening complication
(endophthalmitis). Other studies reported a reduction in endophthalmitis rate following intracameral antibiotic prophylaxis
with but none is yet approved.
The four common errors that should be avoided in
ophthalmological practice to limit resistance to antibiotics are:
using a strong antibiotic for a trivial infection, non-indicated
combination therapy, incorrect dosing and full dependence on in
vitro susceptibility tests in antibiotic selection.