Bacterial keratitis is a bacterial infection of the cornea, the clear front surface of your eye. It’s treated as an ocular emergency because certain bacteria can perforate the cornea in as little as 72 hours. About 80% of cases are caused by just three types of bacteria: Staphylococcus aureus, Streptococcus pneumoniae, and Pseudomonas species. With prompt treatment, most small, non-central ulcers resolve well, but delayed care can lead to permanent scarring and vision loss.
How Bacteria Infect the Cornea
Your cornea has a thin outer layer called the epithelium that acts as a barrier against infection. Under normal circumstances, most bacteria can’t get through it. The infection begins when that barrier is broken, whether by a scratch, a dry patch, or prolonged contact lens wear. Once the surface is compromised, bacteria invade the exposed tissue and trigger a rapid inflammatory response.
White blood cells flood into the area to fight the invading bacteria, causing the cornea to swell and become cloudy. As the infection progresses, the tissue begins to break down, forming an open sore called a corneal ulcer. The walls of the ulcer swell with fluid and immune cells, and pus can accumulate. In more severe cases, bacterial toxins irritate structures deeper in the eye, including the iris. This can cause a visible layer of white inflammatory cells to pool at the bottom of the front chamber of the eye, a sign called a hypopyon.
A handful of especially aggressive bacteria, including the one that causes gonorrhea, can actually penetrate an intact corneal surface without needing a scratch or break. These are the exception, not the rule.
Common Causes and Risk Factors
Contact lens wear is the single biggest risk factor, particularly for infections caused by Pseudomonas aeruginosa and other gram-negative bacteria. Sleeping in lenses, rinsing them with tap water, or wearing them past their replacement schedule all create conditions bacteria thrive in. Pseudomonas aeruginosa is the most frequently isolated and most destructive of the common culprits; it can cause full-thickness corneal perforation within 72 hours of symptom onset.
Staphylococcus aureus and Streptococcus pneumoniae are the other major players. These gram-positive bacteria tend to be associated with pre-existing eye surface problems like chronic dry eye, eyelid disease, or prior corneal injury. Any break in the corneal surface raises the risk: a fingernail scratch, a tree branch, a foreign body, or even aggressive eye rubbing. People with weakened immune systems or those using steroid eye drops long-term are also more vulnerable.
Symptoms to Recognize
Bacterial keratitis usually comes on fast and affects one eye. The hallmark symptoms are:
- Eye pain that can range from moderate to severe
- Redness concentrated around the cornea
- Blurred vision, especially if the ulcer is near the center of the cornea
- Sensitivity to light
- Excessive tearing
- Discharge that may be white, yellow, or greenish
Pseudomonas infections tend to produce a more dramatic presentation with diffuse corneal swelling and heavy discharge, while Staphylococcus or Streptococcus infections may appear more localized. If you wear contact lenses and wake up with a painful, red eye that’s sensitive to light, remove your lenses immediately and seek urgent eye care. Hours matter with this infection.
How It’s Diagnosed
An eye doctor can often suspect bacterial keratitis from the appearance of the cornea under a slit lamp microscope. Bacterial ulcers typically show a well-defined white infiltrate, sometimes with a ring-like (wreath) pattern or an overlying surface plaque. These features help distinguish it from fungal keratitis, which tends to have irregular, feathery borders, and from Acanthamoeba keratitis, which often presents with a ring-shaped infiltrate.
For moderate to severe infections, the doctor will take a corneal scraping. This involves gently collecting material from the ulcer’s surface and edges using a small needle or specialized spatula. The sample goes onto glass slides for staining (to quickly identify whether the bacteria are gram-positive or gram-negative) and onto culture plates to grow and identify the exact organism. Culture results take a day or two, but the initial stain can guide treatment within hours. The gold standard for confirming the diagnosis is finding the same organism growing on two or more culture plates, or growth on one plate that matches what the stain showed.
Treatment and What to Expect
Treatment starts immediately, before culture results come back, because waiting risks rapid tissue destruction. You’ll be prescribed antibiotic eye drops, typically either a fluoroquinolone alone or a combination of two fortified antibiotic drops that together cover both gram-positive and gram-negative bacteria.
The dosing schedule is intense. For most patients, drops are given every 30 minutes around the clock, including overnight. In severe ulcers, a loading dose of one drop every five minutes for the first half hour may be used. This aggressive schedule continues until the infection shows clear signs of improvement: less pain, reduced redness, a smaller infiltrate, and less discharge. At that point, the frequency is gradually tapered based on how your eye responds.
One important thing to know: with Pseudomonas and other gram-negative infections, the eye can actually look worse during the first 24 to 48 hours of treatment even when the antibiotics are working. This happens because dying bacteria release inflammatory compounds that temporarily increase swelling and redness. Your doctor will evaluate the overall trend rather than any single day’s appearance.
Steroid eye drops are sometimes added after the first 48 hours to reduce scarring, but only once the bacteria have been identified and the infection is clearly responding. Antibiotic resistance is a growing concern. MRSA and Pseudomonas have shown increasing resistance to fluoroquinolone drops over the past two decades, which is one reason culture results matter for adjusting treatment.
Possible Complications
The most common long-term consequence is corneal scarring. As the ulcer heals, the damaged tissue is replaced by opaque scar tissue rather than clear cornea. If the scar sits over the center of the cornea (the visual axis), it permanently reduces vision. Small peripheral ulcers that are caught early often heal with minimal scarring and little impact on sight.
More serious complications include deep tissue destruction that thins the cornea to the point of perforation, which is a surgical emergency. Toxins from the infection can also inflame the iris and the fluid-producing structures behind it, causing secondary inflammation inside the eye. In the worst cases, the infection can spread to the interior of the eye (endophthalmitis), though this is uncommon with proper treatment. Some patients with extensive scarring eventually need a corneal transplant to restore vision.
Protecting Yourself
For contact lens wearers, proper hygiene is the single most effective prevention strategy. That means washing your hands before handling lenses, never sleeping in lenses unless they’re specifically approved for overnight wear, replacing your lens case at least every three months, and using only sterile contact lens solution (never tap water or saliva). Remove your lenses before swimming or showering.
If you’re not a contact lens wearer, the main preventive step is protecting your eyes from trauma. Wear safety glasses during yard work, construction, or any activity that could send debris toward your face. If you do get a corneal scratch and you wear contacts or the injury involved contaminated material, preventive antibiotic drops are standard practice to keep bacteria from gaining a foothold in the damaged tissue.