Millions of Americans use eye drops for conditions such as dry eye, glaucoma and itchiness caused by allergies or air pollution. So, they may be worried by a health alert from the Centers for Disease Control and Prevention about serious bacterial infections linked to at least one product contaminated by Pseudomonas aeruginosa, a dangerous organism that is difficult to treat.

The CDC identified 55 cases of infection in 12 states, with at least five of them resulting in permanent vision loss and at least one death. The agency urged consumers to stop using EzriCare Artificial Tears, an over-the-counter preservative-free product and the prime suspect in the outbreak.

"Eye infections are always an obvious concern as severe complications including permanent vision loss can result," said Paul Volberding, professor emeritus of epidemiology and biostatistics at the University of California San Francisco School of Medicine. "The current report of infections linked to eye drops is quite alarming."

How concerned should eye drops users be about their risk of infection? We asked eye experts and infectious-disease specialists. Here's what they advised.

What is the difference between eye drops and artificial tears?

Eye drops is an all-encompassing term that refers to all eye medications, including artificial tears and prescription products that treat specific diseases, such as glaucoma.

Artificial tears are a "subset" of eye drops, experts said. Some artificial tears contain preservatives, others do not, and artificial tears often are chemically different from each other.

Should consumers avoid preservative-free products?

People who use eye drops infrequently are the least at risk, said Dave Patel, an ophthalmologist at the Mayo Clinic in Phoenix. Casual users tend to use bottled preserved tears that use benzalkonium chloride, or BAK, as a preservative. "Non-preserved products don't have that protection and are in danger of being contaminated," he said.

Those with severe or poorly controlled dry-eye symptoms, or who have had surgery or other illnesses, however, require chronic moisture and must use preservative-free products to avoid the cumulative toxic effects of BAK, Patel said.

"Using it once or twice a day, is not detrimental to the eye," he said. "But if you use it six to eight times a day for years, it builds toxicity and irritates the eyes."

How can you protect yourself if you must use a preservative-free solution?

Preservative-free artificial tears come in both multiuse bottles and single-use vials. The safest approach to prevent infection is to stick to the latter, experts said.

"Consumers do not need to fear or avoid preservative-free drops," said Jeffrey H. Ma, an ocular surface disease specialist at the University of California at Davis Eye Center in Sacramento and assistant professor of ophthalmology at the University of California at Davis S school of Medicine. "They are a great option for dry eyes, as they tend to be more gentle on the eyes and can be used more frequently throughout the day."

Infection risk from preservative-free drops is "extremely low when used correctly," he said. People should not touch the tip of the vial to their eyes or keep a vial for more than a day, referring to those who try to get more than one dose from the small containers.

Wash your hands before using any eye products, said Stuart H. Cohen, chief of the infectious diseases division of UC Davis Health. And if there is a dropper, do not touch your eye directly with it.

Cohen also urged consumers to not make their own artificial tears solutions.

Have any other brands of artificial tears been associated with this outbreak?

The CDC said some patients who suffered infections reported using other brands, although not as often as EzriCare, and the agency did not identify them.

Other brands of artificial tears were reported, but none have been reported as frequently as EzriCare or are currently suspected to be a source of the outbreak strain, said Martha Sharan, a CDC spokeswoman. "As part of the ongoing investigation, we continue to work with health departments to investigate each case and products used, in order to assess whether there might be additional products of concern," she said.

How dangerous is Pseudomonas aeruginosa?

Pseudomonas is a common and aggressive bacterium, especially in immune-compromised people, and is resistant to most antibiotics used against bacterial infections, experts said. In the immune-compromised, the organism can cause pneumonia, and infections of the skin, bloodstream and urinary tract, they said. When eye drops are contaminated, the disease is called keratitis, an inflammation of the cornea.

"Pseudomonas aeruginosa is very destructive," said Elizabeth Connick, professor of medicine and immunobiology and chief of the division of infectious diseases at the University of Arizona. "It secretes proteins that can destroy the clear tissue at the front of the eye — the cornea — and allows it to invade the eye. It can impair vision or even blind someone."

How does Pseudomonas aeruginosa affect the body?

The bacterium is everywhere in the environment — water, soil and human waste — and is especially problematic in health-care settings and for those with weakened immune systems.

"It is extremely good at acquiring antibiotic resistance from direct exposure to antibiotics or horizontally by gene transfer from other bacteria," said Robert T. Schooley, distinguished professor of medicine in the division of infectious diseases and global public health at the University of California at San Diego. "It tends to cause problems in places in which immune responses are blunted."

When medical products such as eye drops become contaminated, "the concentration of bacteria — or viruses, or fungi — in the product can be extremely high and overwhelm local immune responses, even when people are not overtly immune compromised," Schooley added.

It also can target other body parts such as the lungs when people breathe it in. When it gets into the bloodstream, it could cause death, Cohen said.

Since the bacterium loves water, other potential reservoirs can include hot tubs and humidifiers, he said.

Were any of the patients involved in the outbreak immune-compromised?

Some but not all. "This is an ongoing investigation," Sharan said, and the CDC is "looking more at underlying conditions among patients."

What are the symptoms of an eye infection?

It may be difficult for people to distinguish between a potentially serious eye infection and other less threatening conditions, since the symptoms often are similar, experts said.

Signs of infection may include a yellow, green or clear discharge from the eye; eye pain or discomfort; redness; the feeling that something — such as a foreign body — is in the eye; increased sensitivity to light; and blurry vision.

Other conditions, however, can also cause these same symptoms, "including dry eye disease and various autoimmune or inflammatory diseases of the eye," Ma said. "In general, if symptoms do not improve with artificial tears or are getting worse, one should seek medical care."

It's important to seek professional help since people cannot determine on their own whether it's an infection, Patel said. "If it is truly bacterial, a direct eye exam will determine it, since there are certain clinical features we can see that would suggest bacteria and the degree of infection," he said.

How are eye infections treated?

Nonresistant bacterial infections will respond to antibiotic drops, experts said. They should never be used against viral eye infections such as conjunctivitis, also known as pinkeye. Viral infections do not respond to antibiotics and using them against viruses contributes to the development of resistance.

Resistant bacterial infections, such as Pseudomonas, are difficult — sometimes impossible — to treat. Eye-care professionals may try stronger, "off-label" medications, in some urgent cases, although toxicity is a risk. "There also are 'salvage' surgeries to eradicate infection, but they are rare," Patel said.

Some infectious-disease experts see phage therapy as a promising strategy against the growing problem of resistant bacteria, including Pseudomonas, said Schooley, co-director of UC San Diego's Center for Innovative Phage Applications and Therapeutics. The approach involves using viruses — which don't make people sick — to target and destroy specific resistant bacteria.

The Food and Drug Administration has not yet formally approved phage therapy, but allows phages to be used in research and in individual life-threatening cases. "Patients with highly resistant organisms like the one in these eye drops are ones for whom a phage intervention would be easy to justify,"