Bacterial pink eye is caused when certain types of bacteria infect the conjunctiva, the thin transparent membrane that lines the inside of your eyelids and covers the white of your eye. The specific bacteria responsible differ depending on your age, but the infection typically spreads through direct contact with contaminated hands, surfaces, or respiratory droplets.
The Bacteria Behind It
Several common bacterial species cause the majority of pink eye infections, and the most likely culprit depends partly on whether the patient is a child or an adult. In children, Haemophilus influenzae is the most common cause, followed by Streptococcus pneumoniae, Staphylococcus aureus, and Moraxella catarrhalis. In adults, Staphylococcus aureus takes the top spot, with H. influenzae, S. pneumoniae, and Moraxella species also playing significant roles.
A few less common bacteria deserve attention because they cause more serious infections. Neisseria gonorrhoeae can cause a hyperacute form of bacterial pink eye with severe swelling, heavy discharge, pain, and even vision loss. Chlamydia trachomatis causes pink eye in two distinct populations: newborns who contract it during delivery, and sexually active adults. In newborns, it produces watery or bloody discharge with significant swelling. In adults, it typically appears in one eye with redness and pus-like discharge. These aggressive bacteria produce enzymes that can damage the deeper tissue of the conjunctiva if left untreated.
How Bacteria Get Into Your Eye
Your eyes have natural defenses against infection: tears contain antimicrobial proteins, and the act of blinking physically sweeps debris away. Bacterial pink eye happens when bacteria overcome those defenses, usually by being introduced in large enough numbers or when the eye’s protective barriers are compromised.
The most common route is straightforward. You touch a contaminated surface, a doorknob, a shared towel, someone else’s hand, and then touch your eye. Bacteria can also reach your eyes through respiratory droplets when an infected person coughs or sneezes nearby. Once bacteria land on the conjunctiva, they adhere to the surface cells and begin multiplying. Your immune system responds with inflammation, which is what produces the redness, swelling, and discharge that define pink eye.
Risk Factors That Raise Your Chances
Certain conditions make bacterial pink eye more likely. Contact lens wear is a well-established risk factor. Wearing lenses too long, sleeping in them, or not cleaning and storing them properly creates an environment where bacteria thrive. Contact lenses can trap germs against the surface of the eye and create tiny abrasions that give bacteria an entry point. While contact lens infections more commonly affect the cornea (a condition called microbial keratitis), the conjunctiva is vulnerable too.
Other risk factors include a weakened immune system, existing eye conditions like chronic eyelid inflammation (blepharitis), and close contact with someone who already has an active infection. Children in daycare and school settings are particularly prone because of frequent hand-to-face contact and shared objects. People who have recently had a respiratory infection are also at higher risk, since the same bacteria that cause sinus and ear infections can spread to the eyes.
How to Tell It’s Bacterial
Distinguishing bacterial pink eye from viral or allergic pink eye can be tricky because the symptoms overlap. All three cause redness, irritation, and tearing. But bacterial pink eye has a few hallmarks that set it apart.
The most telling sign is the discharge. Bacterial pink eye produces thick, yellow-green pus that often causes the eyelids to stick together after sleep. Viral pink eye, by contrast, produces a thinner, watery discharge. Allergic pink eye usually causes intense itching in both eyes, something that’s less prominent in bacterial cases. Bacterial infections also tend to start in one eye and may spread to the other, while allergic pink eye almost always affects both eyes simultaneously.
The chronic form of bacterial pink eye looks different. It often accompanies blepharitis and produces flaky debris and warmth along the eyelid margin rather than the dramatic pus of an acute infection. This type tends to linger and recur rather than presenting as a sudden, obvious infection.
How It Resolves
Mild bacterial pink eye often clears up on its own in 2 to 5 days, though it can take up to 2 weeks to fully resolve without treatment. Antibiotic eye drops or ointment can shorten the duration of the infection, reduce the risk of complications, and limit how long you’re contagious.
Antibiotics are more important in certain situations: when there’s significant pus-like discharge, when the patient has a weakened immune system, or when a more aggressive bacterium like N. gonorrhoeae or Chlamydia is suspected. In those cases, untreated infection can lead to lasting damage.
Preventing Spread
Because bacterial pink eye spreads so easily through touch, prevention comes down to basic hygiene. Wash your hands frequently, especially after touching your face or eyes. Avoid sharing towels, pillowcases, or eye makeup. If you wear contact lenses, follow the recommended cleaning routine and replace your lens case regularly. If you already have an infection in one eye, be careful not to touch your unaffected eye with the same hand or use the same eye drop bottle for both eyes without guidance from your doctor.