Fifty-two patients at Karolinska University Hospital here have been infected with an antibiotic-resistant bacteria and four of the contaminated patients have died.

The hospital believes the bacteria could be the cause of death in at least one of the four cases.

"It's not easy to determine since these are patients who were severely ill for other reasons, but we strongly suspect it in at least one case," chief physician Elda Sparrelid told Swedish news agency TT.

The bacteria, called Klebsiella pneumoniae, is found in the normal flora of the mouth, skin and intestines, but can cause destructive changes to human and animal lungs if inhaled. Klebsiella organisms are often resistant to multiple antibiotics.

The outbreak started last spring but the actual spread of the infection likely started in the fall and has been restricted to the hospital's thoracic clinic. All contaminated patients had undergone open-heart surgery.

"We have taken a host of measures and now we're hoping the spread has stopped since we haven't seen any new cases in the past two to three weeks," said Sparrelid.

Karolinska University Hospital has monitored the spread of the bacteria and tightened hygiene procedures in order to stop the spread of the infection after finding that hygiene rules were not always followed. Sparrelid said several new routines were now in place.

The new routines "concern everything from caution among staff members to routines around clothes, equipment, keyboards and instruments...All in all, we have taken around 100 measures," said Sparrelid.