Elsevier

Microbial Pathogenesis

Volume 104, March 2017, Pages 97-109
Microbial Pathogenesis

Pathogenesis of microbial keratitis

https://doi.org/10.1016/j.micpath.2016.12.013Get rights and content

Highlights

  • Microbial keratitis is caused by bacteria, fungi, and protist pathogens.

  • Epithelial defects and injuries are key predisposing factors in microbial keratitis.

  • Factors from the host and the pathogen contribute to the disease.

  • Successful prognosis requires early diagnosis and aggressive treatment.

  • This is a timely review of our current understanding of microbial keratitis.

Abstract

Microbial keratitis is a sight-threatening ocular infection caused by bacteria, fungi, and protist pathogens. Epithelial defects and injuries are key predisposing factors making the eye susceptible to corneal pathogens. Among bacterial pathogens, the most common agents responsible for keratitis include Staphylococcus aureus, Pseudomonas aeruginosa, Streptococcus pneumonia and Serratia species. Fungal agents of corneal infections include both filamentous as well as yeast, including Fusarium, Aspergillus, Phaeohyphomycetes, Curvularia, Paecilomyces, Scedosporium and Candida species, while in protists, Acanthamoeba spp. are responsible for causing ocular disease. Clinical features include redness, pain, tearing, blur vision and inflammation but symptoms vary depending on the causative agent. The underlying molecular mechanisms associated with microbial pathogenesis include virulence factors as well as the host factors that aid in the progression of keratitis, resulting in damage to the ocular tissue. The treatment therefore should focus not only on the elimination of the culprit but also on the neutralization of virulence factors to minimize the damage, in addition to repairing the damaged tissue. A complete understanding of the pathogenesis of microbial keratitis will lead to the rational development of therapeutic interventions. This is a timely review of our current understanding of the advances made in this field in a comprehensible manner. Coupled with the recently available genome sequence information and high throughput genomics technology, and the availability of innovative approaches, this will stimulate interest in this field.

Introduction

The unique structure of the human eye along with its exposure to environment, renders it susceptible to a number of agents responsible for causing infection. Injuries and epithelial defects impair defense mechanisms and exposure to pathogenic microbes can lead to corneal inflammation or keratitis. The intact ocular surface thwarts most microorganisms but once anatomical barriers are breached, host defenses against pathogens are less than sufficient to prevent infection that can lead to eventual loss of vision. Microbial or infectious keratitis is a potentially sight-threatening ocular condition caused by bacteria, fungi, protists etc. It is the inflammation of the cornea caused by pathogenic microbes that eventually invades the corneal stroma causing inflammation, and ultimately destruction of these structures [7], [210]. The most common pre-disposing factors to develop infectious keratitis include the use of contact lenses, especially overnight or extended wear lenses, inadequate disinfecting solutions, trauma, ocular surgery especially corneal surgery, chronic ocular surface disease, systemic disease like diabetes mellitus and/or extended use of topical corticosteroids [180], [210]. Patients usually present with redness, tearing, rapid onset of pain and blur vision. Clinical presentation may vary depending on the causative agent responsible for causing keratitis. The condition should be treated as a medical emergency and adequate treatment should commence promptly. If appropriate antimicrobial treatment is delayed, only 50% of the eye gains good visual acuity [83]. Appropriate management and timely onset of treatment can reduce the incidence of severe visual loss restricting corneal damage. Here, we present a concise review of the bacterial, fungal and protist keratitis. Numerous microorganisms can infect the eye either by direct or indirect introduction into the eye. The most common clinically important microorganisms involved in eye infections are reviewed in this article with relation to the anatomical part of the eye involved in the disease, along with a discussion of the pathogenic mechanisms and management of the disease.

Section snippets

Acanthamoeba keratitis

Acanthamoeba keratitis is a rare but sight threatening corneal infection, caused by an opportunistic protist pathogen belonging to the genus Acanthamoeba. They are ubiquitous, free-living protists dispersed in a variety of environments including air, soil, freshwater, tap water, hospital equipments, surgical instruments, showers, ventilation ducts, air-conditioning units, chlorinated swimming pools, sewage etc. [37]. Phenotypic switching into a cyst form enables Acanthamoeba to withstand

Mycotic keratitis

Mycotic keratitis is the fungal infection of the cornea caused by either filamentous and/or yeast-like fungi. They may account for more than 50% of all culture-positive microbial keratitis especially in tropical and sub-tropical countries [213], [129], [196]. A strong geographical correlation has been reported to exist between the occurrences of different types of keratomycosis. For example, the proportion of keratitis due to yeast-like fungi show a tendency to increase towards temperate

Bacterial keratitis

The major bacterial agents of infectious keratitis include Staphylococcus aureus, Pseudomonas aeruginosa, Streptococcus pneumonia and Serratia species [210]. The community acquired cases of bacterial keratitis are usually resolved with an empirical treatment, however if left un-attempted, may result in perforation, endophthalmitis and loss of vision [23], [210]. Clinical presentation of bacterial keratitis include acute pain, redness, photophobia and corneal ulceration [179]. Pseudomonas ulcers

Concluding remarks

Microbial keratitis, is a complex entity with many considerations when it comes to its treatment. It is a major public health concern particularly in developing countries where access to care is limited and economic barriers are huge where it can become a leading cause of visual loss in a population that is young. As with all corneal infections, proper identification of the causative agent followed by appropriate targeted therapy can abate the risk of complications. A better understanding of

Conflict of interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by Aga Khan University, Pakistan and Sunway University (2016-05), Malaysia.

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