As science gets wiser, so do the bugs

May 9 2005

As science gets wiser, so do the bugs. The rates of drug-resistant bacteria infecting patients in the community and in the hospital have been increasing steadily in recent years, according to two new studies in the June 15 issue of Clinical Infectious Diseases.

Drug resistance in microorganisms has become a problem due in part to inappropriate prescribing and overuse of antibiotics. These drug-resistant "superbugs" can infect people and cause health problems that are difficult to address with the standard antibiotic regimens. One of the culprits is methicillin-resistant Staphylococcus aureus, or MRSA, an organism that was associated mainly with hospital-acquired infections, but is becoming increasingly common in the general community, as has been reported recently in the medical literature. It can cause problems ranging from skin infections to severe bloodstream infections and even death.

Researchers at the Baylor College of Medicine and Texas Children's Hospital conducted a three-year study of S. aureus infections in children. They found that among S. aureus isolates acquired in the community, the proportion of isolates that were MRSA had reached 76 percent in 2003. Over the preceding three years, the number of MRSA infections acquired in the community had more than doubled. The MRSA isolates caused skin and soft tissue infections in most cases, and more than 60 percent of these children were admitted to the hospital.

The rapid rise in pediatric community-acquired MRSA infections in Texas should raise red flags for health care workers everywhere. "There have been deaths related to this organism, although the vast number are skin and soft tissue infections," said Sheldon Kaplan, MD, lead author of the Texas study. He added that because of this "very dramatic increase" in MRSA infections, physicians should learn what percentage of staphylococcal isolates are drug-resistant in their own communities so they can monitor for increases and adjust treatment accordingly.

Prevention of MRSA infections mainly involves common sense. "If you get a cut or an abrasion, try to keep it clean and dry" and don't share towels or washcloths with others, Dr. Kaplan said. Most importantly, he added, "Wash hands, wash hands, wash hands."

And if regular drug-resistant bacteria weren't bad enough, some bacteria have become multidrug resistant (MDR). Researchers at the Beth Israel Deaconess Medical Center and Harvard Medical School studied the prevalence of bacteria resistant to three or more drugs over a six-year period. From 1998 to 2003, there was a significant increase in the incidence of patients carrying MDR bacteria when they were admitted to the hospital. Of the four species of MDR bacteria that the researchers examined, three of them--including Escherichia coli, a familiar bug that can cause urinary tract infections--were involved in the upswing.

The spread of multidrug resistance in bacteria appears to have two main causes, according to Aurora Pop-Vicas, MD, lead author of the Massachusetts study. The first is the intrinsic ability of bacteria to mutate and acquire resistance under antibiotic pressure. The second is the spread of MDR bacteria from patient to patient, often in hospitals or long-term care facilities like nursing homes. In the study, living in a long-term care facility, being 65 or older or taking antibiotics for two or more weeks were all factors that increased people's likelihood of carrying MDR bacteria upon admission to the hospital.

Physicians need to be aware of the risk factors for MDR bacterial infections, Dr. Pop-Vicas said, and should be judicious in prescribing antibiotics. Treating infections caused by MDR bacteria is "a therapeutic challenge," Dr. Pop-Vicas said, because for severe infections, physicians may need to administer medicine before the bacterial culprit--and its potential resistance to antibiotics--is known. In such cases, combination antibiotic therapy (using more than one drug at a time) may be preferable to monotherapy (using one drug) because physicians want to "maximize the chance that the organism is sensitive to at least one of them," Dr. Pop-Vicas said. "Starting to treat an infection with inadequate therapy is associated with a worse outcome than using adequate therapy from the start," she added. Once the bacterium's drug resistance is known, the therapy can be adjusted accordingly.

"We need to learn more about ways to prevent the spread of multidrug resistance," said Dr. Pop-Vicas. "What everybody wants to avoid is having an infection with an MDR bacteria resistant to all the antibiotics currently available."