Senior residential homes are breeding grounds for antibiotic-resistant bacteria, due to overprescription of antibiotics, a new study says.

Stool samples provided by Australian senior home residents contained bacteria with a wide range of genes linked to antibiotic resistance, researchers report in the Journal of Infection.

“Commonly used tablet antibiotics in the elderly increase many types of resistance bacteria carried in the gut and these so called ‘superbugs’ can increase resistance to other important life-saving antibiotic drugs,” lead researcher Sophie Miller, a doctoral student at Flinders University in Australia, said in a news release.

“High rates of antibiotic prescriptions in aged care settings are likely to be contributing to the proliferation of these bugs,” she added.

The World Health Organization considers antibiotic resistance one of the biggest threats to global health, researchers noted. A growing number of infections – pneumonia, tuberculosis, gonorrhea, salmonella – are becoming harder to treat because antibiotics are less and less effective against them.

For the study, researchers analyzed stool samples from 164 people living at one of five long-term senior care facilities in South Australia.

Three of five (61%) residents had been prescribed antibiotics at least once in the prior 12 months, researchers said.

The stool analysis revealed more than 1,100 unique antibiotic resistance genes conferring resistance against 38 different classes of antibiotics. These included 20 antibiotic resistance genes of high clinical concern.

Overuse of the antibiotic doxycycline appeared to be the biggest contributor to antibiotic resistance. Prescription of doxycycline increased the odds of finding high levels of antibiotic resistance genes by nearly 15-fold, researchers report.

“We discovered that an antibiotic commonly prescribed to aged care residents was strongly associated with an increase in resistance to other antibiotics the resident had not been prescribed,” Miller said.

Alarmingly, nearly all the seniors carried these resistant genes without displaying any symptoms of infection, researchers noted.

“Our findings suggest that even antibiotics that are not typically associated with major modifications in gut bacteria can significantly escalate the presence of resistance genes,” Miller said.

The results indicate that senior care facilities need to be more prudent in their use of antibiotics, said senior researcher Geraint Rogers, a fellow of medicine and public health at Flinders University.

“As the population ages and life expectancy extends, the implications of our findings emphasize the importance of a holistic approach to antibiotic management in long-term aged care settings,” Rogers said.

“There are concerns that practitioners may be over prescribing antibiotics, potentially increasing the risk of resistant bacterial infections, and this study’s findings suggest a need for extra caution when prescribing them for older patients,” he added.