The bacteria behind pink eye depend largely on the patient’s age. In children, the three most common culprits are Haemophilus influenzae, Streptococcus pneumoniae, and Moraxella catarrhalis. In adults, Staphylococcus aureus takes the lead, with Streptococcus pneumoniae and Haemophilus influenzae also playing a role. A few less common bacteria, including Chlamydia trachomatis and Neisseria gonorrhoeae, cause more severe forms that need prompt treatment.
The Main Bacteria in Children
Bacterial conjunctivitis is actually more common than viral conjunctivitis in kids. A study of 428 children with clinically diagnosed conjunctivitis found that 55% of cases were caused by Streptococcus pneumoniae, Haemophilus influenzae, or Moraxella catarrhalis. Of these three, nontypeable Haemophilus influenzae (a strain that doesn’t have a protective capsule) dominates, responsible for roughly 44% to 68% of acute bacterial conjunctivitis cases in children. One analysis found it in nearly 62% of eye discharge samples from pediatric patients.
Streptococcus pneumoniae, the same bacterium behind many ear infections, is the second most frequent cause in children. Moraxella catarrhalis rounds out the top three. All three bacteria spread easily in daycares and schools through direct contact with eye discharge or contaminated hands and surfaces.
The Main Bacteria in Adults
In adults, the picture flips: viral conjunctivitis is more common overall, though bacterial cases still make up a significant share. When bacteria are the cause, Staphylococcus aureus is the most frequent organism. This is a skin bacterium that can easily transfer to the eye from your hands or contaminated cosmetics. Streptococcus pneumoniae and Haemophilus influenzae still appear in adults but less often than in children.
Adults who wear contact lenses face a distinct set of risks. Wearing lenses too long, sleeping in them, or not cleaning them properly gives bacteria easier access to the eye’s surface. While contact lens problems more often lead to corneal infections (keratitis) than simple pink eye, the two conditions can overlap and look similar early on.
Bacteria That Cause Severe Forms
Two organisms deserve special attention because they cause more aggressive infections.
Neisseria gonorrhoeae, the bacterium responsible for gonorrhea, can cause hyperacute conjunctivitis. This form comes on fast and produces massive amounts of pus, significant eyelid swelling, eye pain, and sometimes decreased vision. It is a medical emergency because the infection can damage the cornea rapidly. In newborns, it can be transmitted during delivery if the mother has an active gonorrheal infection.
Chlamydia trachomatis causes a different pattern depending on the patient. In sexually active adults, it typically affects one eye with redness and thick discharge. In newborns exposed during birth, symptoms include watery or bloody discharge and severe swelling that develops 5 to 12 days after delivery, a longer incubation period than most other bacterial causes. Worldwide, repeated chlamydial eye infections (trachoma) remain a leading cause of preventable blindness.
How Bacterial Pink Eye Feels Different
Bacterial conjunctivitis typically produces thick, yellow or green discharge that can crust your eyelids shut overnight. This is the hallmark that sets it apart from viral pink eye, which tends to produce thinner, watery discharge and often accompanies a cold or upper respiratory infection. That said, the CDC notes that symptoms overlap enough between bacterial and viral forms that diagnosis based on appearance alone can be difficult.
The chronic form of bacterial conjunctivitis behaves differently. It tends to linger and often occurs alongside blepharitis, an inflammation of the eyelid margin that causes flaky debris and warmth along the lid. Staphylococcus aureus is a common cause of this chronic pattern.
Timeline From Exposure to Recovery
Most bacterial conjunctivitis has an incubation period of 24 to 72 hours, meaning symptoms appear one to three days after the bacteria reach your eye. The exception is chlamydial conjunctivitis, which takes 5 to 12 days to develop.
Without treatment, a typical case lasts anywhere from 2 days to 2 to 3 weeks. Antibiotic eye drops or ointment can shorten that window and, importantly, reduce how long you’re contagious. You can spread the infection from the moment symptoms start until about 48 hours after beginning antibiotic treatment. After that 48-hour mark, most people are no longer contagious.
When Treatment Is Needed
Mild bacterial pink eye sometimes clears on its own, but antibiotics are typically prescribed when there’s noticeable pus, when certain aggressive bacteria like Neisseria gonorrhoeae are suspected, or when the patient has a weakened immune system. Antibiotics shorten the infection, reduce the risk of complications, and help prevent spread to others. They’re usually given as eye drops or ointment.
Signs the Infection May Be Spreading
A corneal ulcer can initially look like ordinary pink eye but progresses to something more serious. Watch for blurry or hazy vision, increasing pain (not just irritation), sensitivity to light, very watery eyes alongside the pain, or a white patch visible on the surface of the eye. Severe or worsening pain, vision changes, swelling of the eyelids or surrounding skin, or a headache alongside eye symptoms all signal that the infection may have moved beyond the conjunctiva and needs prompt evaluation.