Ocular
Microbiology is the branch of science which deals with the eye and its
infections. The eye is a paired organ, located in the orbital cavity and its
function is to capture images that are sent to the cortical vision centre
(Castro et al., 1985). Due to the
constant exposure to the external medium, the eyes, along with associated
structures, are subjected to intense microbial contamination (Trindade et al., 2000).

The infections spread to adjacent
tissues, from the conjunctiva to the cornea, the inner eye, the orbit and to
the brain (Willimson and Sorsby, 1950). The common eye infections are
conjunctivitis, blepharitis, internal and external hordeolum, microbial
scleritis, canaliculitis, keratitis, dacryocystitis, preseptal cellulitis,
orbital cellulitis, endophthalmitis and panophthalmitis (Modarrres et al., 1998).

The
eye is a unique organ that is virtually impermeable to most environmental
agents. Continuous tear flow aided by the blink reflex mechanically washes
substances from the ocular surface and prevents the accumulation of
microorganisms. In addition to it lysozyme, lactoferrin, secretary immunoglobulins
and defences, which are present at high levels in tears, can specifically
reduce bacterial colonisation of the ocular surface (Haynes, Tighe and Dua,
1999; McClellan, 1997). However in some circumstances, infectious agents gain
access to the posterior segment of the eye following one of three routes namely
(i) as a consequence of intraocular surgery (Mistlberger et al., 1997),
(ii) following a penetrating injury of the globe                                 (Abu et al., 1999) and (iii) from
haematogenous spread of bacteria to the eye from a distant anatomical site
(Srinivasan et al., 1999).

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The eye is infected by bacteria, fungi, parasites,
or viruses. The external ocular surface acquires a microbial flora at birth and
some of the commensals may become resident flora in the conjunctiva and lids
and have the potential to turn into pathogens. Apart from the resident flora,
any microorganisms, from the environment, can form a transient flora in the
eye.

1.1 Ocular infections

1.1.1
Bacterial Ocular infections

Several microorganisms causes eye
infections by Streptococcus pneumonia,                      Escherichia coli, Haemophilus
influenza, Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas
aeruginosa, Chlamydia trachomatis, Bacillus cereus and                                Neisseria gonorrhoea
(Everett et al., 1995; Starr, 2000; Cuong and Michael, 2002 and               Hirotoshi et al., 2006).
External ocular infections are most commonly caused by                 Staphylococcus aureus
and Staphylococcus epidermidis (Everett et al., 1995; Starr,
2000).

1.1.2
Fungal Ocular
infections

The fungal species such as Fusarium oxysporum,
Fusarium solani, Aspergillus flavus, Candida albicans, Aspergillus niger and
Penicillium notatum causes eye infections                        (Denning, 1998; Gutleb et
al., 2002. These infections cause blindness if left untreated         (Fabiana et al., 2004). Symptoms
include blurring vision, redness and photophobia. These infections are usually
treated with Amphotericin B and Natamycin (Fabiana et al., 2004; Cesaroet
al., 2004 and Foulis et al., 1990).

1.1.3 Viral Ocular infections

The Viral infections are caused by Herpes
Simplex Virus-1, Coxsackie virus and Adenovirus (Foulis et al., 1990;
Liesegang, 2001; Kojaoghlanian et al., 2003). In developed countries 95%
of ocular herpes infections are caused by HSV-1 (Pavan, 2001).

1.2 Epidemiology
of Ocular Diseases

In
world approximately 40 million people are blind and another 100 million have
substantial visual problem. The principal causes of blindness and visual
disability are quite distinct in developed countries when compared with
developing countries. 

Conjunctivitis affects many people and
causes economic and social burdens. Six million people were annually affected
by acute conjunctivitis in the United States (Udeh et al., 2008).
Initially a majority of conjunctivitis patients are treated by primary care
physicians rather than eye care professionals. Approximately 70% of patients
with acute conjunctivitis were admitted in primary care and urgent care (Kaufman, 2011).
The prevalence of conjunctivitis varies according to the patient’s age, as well
as the season of the year.

The most common cause of infectious
conjunctivitis is the viral conjunctivitis in the adult population (Hørven, 1993;
Stenson et al., 1982; Rönnerstam et al., 1985; Harding et al.,
1987; Uchio et al., 2000; Woodland et al., 1992 and Fitch et
al., 1989) and is more prevalent in summer (Høvding, 2008).
The second most common cause is bacterial conjunctivitis which affects majority
(50%-75%) of cases in children it is observed from December through April. The
most frequent population of about 15% to 40% are affected by allergic
conjunctivitis which is observed more frequently in spring and summer (Høvding, 2008).

1.3
Treatment for Ocular infections

Antibiotics are generally used to treat, or to
prevent a bacterial eye infection. It is most effective against certain
bacteria, and sometimes an infection that cannot be cured with one medication
may be eliminated by another. The most common antibiotics used are
sulfacetamide, erythromycin, gentamicin, tobramycin, ciprofloxacin and
ofloxacin (Bartlett, 2003).

Treatment
with antibiotics, that has a broad spectrum of activity against gram-positive
and gram-negative organisms, such as ciprofloxacin and chloramphenicol, are the
best to control these bacterial infections. Gentamicin is aminoglycoside
antibiotic, and it is effective against Gram-negative bacteria. It can be toxic
to epithelia; and this antibiotic is used in conjunctivae infection.
Erythromycin is indicated for Gram-positive coverage and has been used to treat
pulmonary, corneal and conjunctival infections (Murray et al., 1998 and
Silverman., 2001). Chloramphenicol,
neomycin, gentamycin, framycetin, sulphacetamide, tobramycin, ciprofloxacin and
norfloxacin can be administered for bacterial infections.
For chronic non-specific conjunctivitis decongestants (antihistamines), phenylephrine (neosynephrine), naphazoline, antazoline and zinc sulphate are prescribed. For
dry eye infections, and artifical
tears, methylcellulose, polyvinyl alcohol and hypromellose are suggested as
treatment                        (Khaw et al., 2004).

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