Introduction

Bacterial keratitis (BK) is a corneal ulcer and a common sight-threatening infection of the eye caused by various bacteria. It is a progressive condition that can easily spread into the corneal microvasculature.¹ It may present as acute, transient, or chronic. 

BK occurs when bacteria breach all the anatomical barriers of the eye, such as the bony corneal rim, eyelid, tears, and lacrimal gland. This condition is most commonly observed in individuals who wear contact lenses. Geographic location, environmental factors, and climatic conditions influence the incidence and occurrence of BK.¹

Timely and appropriate treatment is essential, as this ulcer can lead to corneal penetration and tissue destruction, potentially extending the infection to adjacent ocular areas.² While mild and moderate BK can be treated effectively, if left untreated for an extended period, it may result in permanent vision loss.¹

Aetiology and risk factors

BK can be caused by various pathogens, including bacteria, viruses, and fungi. These microorganisms may be present on the contact lens surface or in the storage container where contact lenses are kept. Prolonged contact lens wear, combined with poor hygiene, can lead to contamination of the cornea, resulting in BK.³

Common pathogens causing BK include: 

• Staphylococcus aureus: commonly found on the human body, particularly on the surface linings of the eyes, nose, and mouth
• Pseudomonas aeruginosa: typically found in soil and water

Risk factors for developing BK include:

• Contact lens use:
  • Prolonged wear, especially overnight
  • Inadequate disinfection of contact lenses
  • Corneal reshaping from rigid lens use
  • Sharing contact lenses
  • Improper handling or failure to change the storage solution after each use
• Recent cornea injury:
  • Trauma from metallic or non-metallic tools
  • Chemical exposure
  • Foreign body invasion
• Eye conditions:
  • Dry eye syndrome
  • Eyelid inflammation or tearing
• Other factors:
  • Weakened immune system
  • Use of visibly contaminated lens solution
  • Prolonged corticosteroid exposure

Clinical presentation

Patients with BK may experience the following symptoms:⁴

• Eye pain and inflammation
• Excessive tearing (watering of the eye)
• Visible eye discharge
• Redness or bloodshot appearance
• Blurred vision
• Photophobia (light sensitivity)
• Sensation of heaviness in the eye
• Difficulty opening the eyelids
• Visible corneal ulcer, which may penetrate surrounding tissues

Diagnosis

Effective treatment of BK depends on identifying the particular causative organism through appropriate sampling techniques. Standard diagnostic methods include:

1. Sample collection for culture:  Samples are taken from both the eyelid and conjunctiva (via corneal scrapings), even when the ulcer appears in only one eye. Corneal scraping is particularly indicated when the ulcer is large, centrally located, and extends into the mid-to-deep stroma, especially if accompanied by pain and visual blurring.  The culture positivity rate for BK is approximately 73%, compared to 52% for Gram staining, and about 32% of samples may contain multiple bacterial species. For accurate pathogen identification, it is important to obtain scrapings from the edge of the ulcer, where bacterial activity is highest, rather than the centre. Scrapings can be performed using a blade, microsurgical blade, or trephine. Samples are then transferred to glass slides and cultured using appropriate media. Additionally, contact lenses, along with their solution and storage case, should be submitted for culture to identify potential sources of infection⁵
2. Slit lamp examination: This is the most common ophthalmological method for diagnosing ocular infections. It offers magnified views of the anterior and posterior segments of the eye and provides information about relevant parameters such as corneal endothelial cell count, corneal thickness, anterior chamber depth, and pupil size⁶ 
3. Penlight examination: A dye (e.g., fluorescein) is applied to the eye. Using a penlight, the stained areas of corneal damage become visible. It also allows assessment of pupil size and reactivity

Active treatments

Antibiotic eye drops (topical therapy)

Topical antibiotics are the first-line treatment for BK. The selection depends on factors such as broad-spectrum activity, toxicity, availability, cost, resistance patterns, and regional epidemiology of pathogens.⁷

First-line treatment options include:

• Broad-spectrum antibiotics: moxifloxacin 0.5% or gatifloxacin 0.5%
• Fortified antibiotics (traditional therapy for BK): tobramycin 14 mg/mL, vancomycin 50 mg/mL, or cefazolin 50 mg/mL (one drop hourly)

As fortified antibiotics are toxic to the corneal epithelium and inhibit healing, it is recommended to stop or reduce the dose once the patient is recovering from the infection.

Oral antibiotic medications

Oral antibiotics are not typically the primary treatment for BK, which is usually treated with topical antibiotics. However, in cases of severe keratitis, especially in resource-limited settings, oral fluoroquinolones are used. Fluoroquinolones are broad-spectrum synthetic antibiotics. Agents such as moxifloxacin and gatifloxacin offer enhanced efficacy and broad antimicrobial coverage. Moxifloxacin is preferred due to its superior penetration into ocular tissues and its ability to reduce the risk of resistance development. These antibiotics work by inhibiting bacterial enzymes essential for DNA replication: in Gram-positive bacteria, the primary target is topoisomerase IV, whereas in Gram-negative bacteria, the main target is DNA gyrase.

Second-generation fluoroquinolones, such as ciprofloxacin and ofloxacin, are also used to treat BK. Besifloxacin ophthalmic suspension is another fluoroquinolone used for BK, typically applied every hour. It is active against anaerobes and Gram-positive organisms.⁸ Vancomycin (50 mg/mL) is used for methicillin-resistant Staphylococcus. The antibiotic dosage should be tapered based on clinical improvements, such as:

• Blunted edges of the stromal infiltrate
• Decreased infiltrate density
• Reduction in endothelial plaque and oedema
• Re-epithelialization of the corneal defect

Combined therapy approach

This approach involves the use of fortified antibiotics in combination with aminoglycosides. These combinations are effective against Gram-negative bacteria, as well as Streptococcus and Staphylococcus species.

Dosage is based on the severity of the clinical presentation. In cases of severe corneal ulcers, the recommended regimen is one drop every 5 minutes for the first 30 minutes.⁹

Fortification is typically achieved by adding 80 mg of antibiotic (from an injectable formulation) to 2 mL of a standard ophthalmic solution, which usually has a volume of 5 mL at a concentration of 1.35%.

Adjunctive and supportive treatments

Adjunctive therapies aim to reduce inflammation, limit corneal damage, and improve healing. These include:¹⁰

• Povidone iodine (5%)  – preoperative antiseptic with microbicidal properties
• Hyperbaric oxygen therapy – involves breathing 100% oxygen under pressure. It may enhance healing and reduce systemic toxicity
• Cyanoacrylate glue – seals corneal perforations and offers bacteriostatic action (especially against Gram-positive bacteria)
• Amniotic membrane transplantation – promotes re-epithelialization and wound healing. It also has anti-inflammatory and antibacterial properties
• Matrix metalloproteinase inhibitors – such as tetracycline, acetylcysteine, and ascorbate- prevent stromal damage
• Mitomycin C – used postoperatively to reduce corneal haze and scarring
• Autologous serum eye drops – derived from the patient’s serum; they promote healing and treat dry eye
• Cryotherapy – useful in Pseudomonas-related keratitis or when the infection reaches the sclera
• Cycloplegics – relieve pain and prevent posterior synechiae in iritis-related BK

Follow-up and monitoring

For good post-operative care, counselling and regular follow-up are essential. Based on the clinical response, the dose of the drug should be tapered, and antibiotic eye drops should be administered every 1 to 2 hours for at least 48 hours to 1 week.

Follow-up depends on the disease severity. If the corneal ulcer is large and affecting vision, two antibiotics should be administered every hour. The application of adjuvants in conjunction with antibiotics can enhance the clinical response.¹¹

After being free from infection, topical steroids are initiated, administered four times daily for 3 months, accompanied by regular and timely follow-up. Additionally, 0.5% timolol should be prescribed to prevent secondary cataracts and glaucoma.

Prognosis and complications

Complications of BK can include increased intracranial pressure, glaucoma, recurrent infection, corneal perforation, corneal scarring and blindness, and, ultimately, permanent vision loss.

Several factors influence prognosis and healing outcomes in BK. These include:

• Patient age
• Degree of corneal infiltration
• Type and location of the ulcer
• Corrected-distance visual acuity (CDVA) at presentation

Poor visual outcomes are more commonly observed in:

• Older patients
• Centrally located ulcers
• Ulcers with deep stromal infiltration
• Patients presenting with low baseline CDVA

In elderly patients, the presence of large, central ulcers is often associated with worse prognosis and reduced visual recovery. Corneal healing is further compromised in individuals with:

• Advanced age
• Deep corneal ulcers
• Co-existing ocular surface diseases, such as dry eye syndrome, neurotrophic keratopathy, or immunosuppression

These underlying conditions contribute to delayed epithelialization and poor overall healing in older adults affected by BK.¹²

Conclusion

BK, due to its aggressive nature, remains a sight-threatening condition which has the potential for corneal scarring and vascularisation. Early diagnosis, prompt management, and a proactive approach to eliminating the predisposing factor can prevent possible complications.

BK can be effectively managed with appropriate antibiotic therapy; however, responsible contact lens hygiene plays a critical role in its prevention.