**1. Introduction**

Infectious keratitis (IK) is a condition that can occur as a consequence of pathogen invasion into the tissue or as an autoimmune disease accompanying systemic diseases. IK is a corneal infection also known as corneal opacity or corneal ulcer. IK represents the fifth leading cause of blindness globally, accounting for ~3.2% of all cases [1]. It is estimated to be responsible for 1.5–2.0 million cases of unilateral blindness annually [2]. According to WHO, 1.9 million people have corneal blindness due to the opacification of the cornea, which accounts for about 5% of the total patients who have blindness [3]. Corneal Opacity accounts for 3.46% of global blindness and 1.65% of global blindness and visual impairment. Infectious keratitis can be divided into microbial keratitis, including bacteria, fungi, or parasites and viral keratitis, including herpes viruses [4]. Microbial keratitis is an infectious disease of the eye, in which the cornea is inflamed. Bacteria are most concerning due to rapidly progressive vision-threatening keratitis with irreversible visual sequelae. The localization of corneal inflammation is important, and acute inflammations usually affect the central part of the tissue,

while peripherally located forms of corneal inflammation are more often of prolonged inflammation with an etiology that is difficult to clearly determine.

Bacterial keratitis (BK) is the most common type among all types of infectious keratitis. BK accounts for approximately 65–90% of all microbial keratitis [5]. BK rarely occurs in the healthy eye because of the human cornea's natural anatomical barrier to infection. BK is caused by varied bacterial species, and it can be an acute, chronic, or transient infectious process of the cornea. BK is one of the most common causes of visual impairment in working-age adults. BK is one of the most serious ocular infections, and it can progress rapidly and may lead to serious complications including vision-threatening keratitis. Acute keratitis may progress with tissue necrosis and its perforation within even several dozen hours. When analyzing the causes of bacterial keratitis, a number of external factors should be taken into consideration such as climate, geographical zone, level of hygiene, patient's workplace, use of contact lenses, and the endemic occurrence of various eye diseases. Whereas local factors include medical history, especially dry eye syndrome, other local disorders of the eye surface, especially those affecting the epithelium and the human margin, surgical procedures, or the presence of sutures. The diagnosis of BK is based on clinical and microbiological evaluation. Thus, to avoid a serious complication early and immediate medical treatment is needed. Recently, in the past few decades have seen increasing contact lens users, resulting in proportionately increased of bacterial keratitis and corneal ulcers [6].

## **2. Etiology**

The surface of the human eye has not only excellent and efficient defense mechanisms protecting against the invasion of pathogens but also against bacteria existing on the surface of the conjunctiva and skin. The main barriers protect to microbial infection are anatomical barriers (eyelids, intact conjunctiva, corneal epithelium, and tear film) and antimicrobial barriers (tear film constituents IgA, complement components, lactoferrin, lysozymes, and conjunctiva-associated lymphoid tissue (CALT)) [7]. These barriers could be disrupted and predispose to infection. Every break in the continuity of the epithelium may predispose to pathogen invasion into the cornea. Every minor injury, foreign body, or wound can be a trigger factor of inflammation.

The bacterial spectrum from different areas or periods is widely reported in the literature, and those differences could be associated with weather, rural vs. urban area, and etiology of keratitis. The most common pathogens that are associated with bacterial keratitis include *Staphylococcus aureus*, Coagulase-negative staphylococci, *Staphylococcus epidermidis*, *Streptococcus pneumoniae*, *Pseudomonas aeruginosa*, and species of the *Enterobacteriaceae* family [8]. This group of bacteria is characterized a good adherence to the epithelium and to the surface of contact lenses. *Staphylococcus epidermidis* and *Staphylococcus fusarium* species are the most commonly implicated in polymicrobial keratitis with trauma being the most common inciting factor [9]. The bacterial species that can penetrate the intact corneal epithelium are *Neisseria gonorrhoeae*, *Corynebacterium diphtheriae*, *Hemophilus aegyptius*, and *Listeria monocytogenes* [10].

Contact lens use is one of the major causes of bacterial keratitis in developed countries, whereas trauma is the main risk factor in developing countries [11, 12]. The etiology of CL-related keratitis is most commonly associated with *Pseudomonas aeruginosa* and *Acanthamoeba* species. These two types of bacteria are free-living microorganisms that are omnipresent in the environment, including water and CL solutions [13]. The risk factors of CL-related IK include: tear recession under

CL, reduction of tear exchange during blinking, and reduced corneal epithelial cell desquamation. These result in accumulation and adherence of microbes to the cornea and provide to increase risk of IK. Other local predisposing risk factors for BK are ocular surface disease (OSD), including dry eyes, corneal suture-related infection, abnormalities of eyelid anatomy and function, trichiasis, blepharitis, chronic dacryocystitis, ectropion, entropion conjunctivitis, lagophthalmos neurotrophic keratopathy, recurrent corneal erosions, epithelial defect, secondary bacterial keratitis after viral keratitis, bullous keratopathy, corneal disease, previous keratitis, xerophthalmia, blepharoconjunctivitis, fifth and seventh cranial nerve palsy. Other risk factors include mechanical or thermal injury, ocular trauma, foreign body injury, previous ocular or eyelid surgery, immunosuppression, previous corticosteroids and NSAIDs [14, 15]. Risk factors predisposing to BK are the systemic conditions such as diabetes mellitus, atopic dermatitis, connective tissue or autoimmune pathologies, Steven-Johnson syndrome (SJS), ocular mucous membrane pemphigoid (OMMP), compromised immune systems, graft-versus-host disease, immunosuppression (AIDS), chronic alcoholism, and malnourishment [16].
