MDRO stands for multidrug-resistant organism, a bacterium that has developed resistance to one or more classes of antibiotics that would normally kill it. These infections are a serious and growing threat: in the U.S. alone, more than 2.8 million antibiotic-resistant infections occur each year, leading to over 35,000 deaths. Globally, antimicrobial resistance directly killed at least 1.27 million people in 2019 and played a role in nearly 5 million deaths.
Which Bacteria Are Considered MDROs
Several well-known bacteria fall under the MDRO umbrella. The most commonly referenced include MRSA (methicillin-resistant Staphylococcus aureus), VRE (vancomycin-resistant Enterococci), and CRE (carbapenem-resistant Enterobacterales). Though the names suggest resistance to a single drug, these organisms are typically resistant to most available antibiotics, making them far harder to treat than their names imply.
Beyond those three, other MDROs of concern include resistant strains of E. coli, Klebsiella pneumoniae, Acinetobacter baumannii (which can be resistant to nearly every antibiotic available), and Pseudomonas aeruginosa. In long-term care facilities, multidrug-resistant Streptococcus pneumoniae is also a particular problem. The list continues to grow as bacteria evolve new ways to survive the drugs designed to destroy them.
How Bacteria Become Resistant
Bacteria develop resistance through genetic changes that let them neutralize or dodge antibiotics. Some produce enzymes that break down antibiotic molecules before they can work. Others develop molecular pumps that push the drug back out of the cell before it reaches its target. Still others alter the very structures an antibiotic is designed to attack, so the drug no longer recognizes its target.
What makes this especially dangerous is that bacteria can share resistance genes with each other, even across different species. A resistant strain of E. coli, for example, can pass its resistance genes to a completely different type of bacterium. This means resistance can spread quickly through a hospital or community, jumping between species like a shared instruction manual for surviving antibiotics. Overuse and misuse of antibiotics accelerates this process by killing off susceptible bacteria and leaving resistant ones with more room to thrive.
Who Is Most at Risk
MDRO infections disproportionately affect people who spend significant time in healthcare settings. The strongest individual risk factors, based on clinical research, are recent antibiotic use and the presence of invasive medical devices. Using antibiotics within the past 12 months more than tripled the odds of developing an MDRO infection in one study. Urinary catheter use carried a similarly elevated risk, with more than a three-fold increase in odds.
Other factors that raise your risk include:
- Long or frequent hospital stays. The longer you’re in the hospital, the more exposure you have to resistant organisms circulating in that environment.
- Invasive devices. Feeding tubes, central venous lines, breathing tubes, and urinary catheters all create pathways for bacteria to enter the body.
- Chronic or severe illness. Conditions that weaken the immune system make it harder for your body to fight off infections on its own.
- Advanced age. Older adults, particularly those living in nursing homes or long-term care facilities, face higher exposure and often have weaker immune defenses.
- Family members who have been hospitalized. Resistant bacteria can travel between household members, so a family member’s recent hospitalization is also a risk factor.
Symptoms and How MDROs Are Diagnosed
An MDRO infection doesn’t look or feel different from any other bacterial infection. You might have a urinary tract infection, a wound infection, pneumonia, or a bloodstream infection with all the usual symptoms: fever, pain, redness, swelling, or fatigue. The difference only becomes clear in the lab, when testing shows the bacteria causing your infection don’t respond to standard antibiotics.
Diagnosis typically starts with a standard bacterial culture. A sample from the infection site (urine, blood, wound, sputum) is grown in a lab, and the resulting bacteria are then exposed to a panel of antibiotics to see which ones can still kill them. This process, called susceptibility testing, tells doctors exactly which drugs will and won’t work. Molecular tests that detect specific resistance genes can speed up this process, identifying resistance markers in hours rather than the days a traditional culture requires. Faster identification means doctors can switch to an effective antibiotic sooner, which improves outcomes.
How MDRO Infections Are Treated
Treatment depends entirely on which organism is involved and what it’s resistant to. There is no single “MDRO antibiotic.” Instead, doctors rely on susceptibility test results to choose from a shrinking menu of options. For many MDRO infections, treatment requires newer combination antibiotics that pair a traditional drug with a compound designed to overcome the bacteria’s resistance mechanism.
For the most dangerous resistant infections, like those caused by carbapenem-resistant bacteria, treatment often involves IV antibiotics given in a hospital setting. These regimens are more complex, more expensive, and carry more side effects than the standard antibiotics that would have worked against a non-resistant version of the same bacteria. Some MDRO infections require weeks of IV therapy, and the treatment course may need to be adjusted if the first choice doesn’t produce improvement.
The practical reality for patients is that MDRO infections take longer to get under control. Hospital stays are measurably longer: an MDRO infection adds roughly one to two and a half extra days in the hospital depending on the organism. Costs rise accordingly, with MDRO-associated infections adding anywhere from about $1,700 to $4,600 in additional hospital charges per stay. When a patient has more than one MDRO, the added cost averages around $3,570.
How MDROs Spread in Healthcare Settings
MDROs primarily spread through direct contact, most often on the hands of healthcare workers moving between patients. They also survive on surfaces like bed rails, doorknobs, and medical equipment, sometimes for days or weeks. This is why hospital-acquired MDRO infections remain a persistent challenge even in facilities with strong hygiene practices.
The cornerstone of prevention is hand hygiene, both for healthcare workers and visitors. Beyond that, hospitals use a system called contact precautions for patients known to carry an MDRO. This means the patient is placed in a private room whenever possible, and anyone entering the room wears a gown and gloves, which are removed and discarded before leaving. These precautions contain the organism within a defined space and reduce the chance of carrying it to the next patient.
Contact precautions typically stay in place until repeated testing shows the MDRO is no longer detectable, usually three or more negative cultures taken over one to two weeks in a patient who hasn’t been on antibiotics recently. In outbreak situations, hospitals may go further by assigning dedicated staff to MDRO patients, closing units to new admissions, or grouping colonized patients together.
Colonization Versus Infection
An important distinction that often causes confusion: you can carry an MDRO on your body without being sick. This is called colonization. The bacteria live on your skin, in your nose, or in your gut without causing symptoms. Colonization doesn’t require antibiotic treatment, but it does mean you can spread the organism to others, which is why hospitals screen for MDROs and may place colonized patients on contact precautions even if they feel perfectly fine.
Colonization can persist for months or even years. It becomes an infection only when the bacteria invade tissue or the bloodstream and trigger an immune response, producing symptoms like fever, pain, or inflammation. People who are colonized and then undergo surgery, receive a catheter, or experience immune suppression are at higher risk of that colonization progressing to an active infection.
Reducing Your Personal Risk
The most effective thing you can do is avoid unnecessary antibiotics. Every course of antibiotics you take creates selective pressure that favors resistant bacteria in your body. If your doctor prescribes antibiotics, take the full course as directed, but don’t pressure a provider to prescribe them for viral infections like colds or the flu, where they won’t help.
If you or a family member is hospitalized, don’t hesitate to ask healthcare workers whether they’ve washed their hands before providing care. Clean your own hands frequently, especially after touching shared surfaces. If you’re told a patient near you is on contact precautions, follow the facility’s instructions about gowns, gloves, and movement restrictions. These measures exist because MDROs are far easier to prevent than to treat once they take hold.