Multi-drug resistant bacteria that cause typhoid fever are on the move around the world and replacing strains that once were susceptible to antibiotics, researchers have found in a new analysis.

To prevent further expansion of the multi-drug resistant bugs, an international consortium of scientists is calling on the global health community to initiate surveillance for the highly problematic typhoid strains.

Bacterial species of all kinds -- not just the strains that cause typhoid fever -- are developing fierce mechanisms to resist antibiotics, leaving medicine with an ever smaller cache of drugs to counter them, scientists say.

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Reporting in the journal Nature Genetics, an international scientific team analyzed nearly 2,000 specimens of Salmonella typhi, which causes typhoid fever, and found evidence of extreme drug resistance in virtually all.

Dr. Pascal Imperato, founding dean of the School of Public Health at SUNY Downstate in Brooklyn, said resistance has been mounting in Salmonella typhi for decades. Only now, he said, has the problem reached a global tipping point.

"Resistance doesn't develop overnight -- it takes time," said Imperato, a former New York City health commissioner and 33-year resident of Manhasset. "Multi-drug resistance [in Salmonella typhi] has been a problem going back to the 1980s," he said. "Even in the 1970s we noticed that Salmonella typhi had developed resistance to the principal drug, chloramphenicol, which had been highly effective. But with the passage of time, it was found that the bacterium developed resistance to the second line of drugs, which included ampicillin and co-trimoxazole."

In more recent years, the bugs learned to thwart the fluoroquinolones, drugs in the third line of defense, Imperato said.

The newly reported research uncovered extreme drug resistance in specimens from Asia and Africa where sanitation is poor and powerful antibiotics can be purchased over the counter. When bacteria are overexposed to antibiotics they develop the genetic capacity to repel the drugs, scientists say.

Worse, added Imperato, is the bacterial tendency to package resistance genes on mobile units called plasmids that can be easily transferred from one bacterium to another. Also important, he said, is Salmonella typhi's potential to be ferried from one country to another by healthy carriers. Poor hygiene, Imperato noted, underlies infection.

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Mary Mallon -- Typhoid Mary -- was a carrier and cook who infected about 50 people in the metro area, killing three, during the pre-antibiotic era. She was confined for life by health authorities in 1915 to an East River institution.

Dr. Stephen Baker, an author of the new typhoid investigation said these days, asymptomatic carriers make drug-resistant typhoid an urgent threat. "Bacteria do not obey international borders and any efforts to contain the spread of antimicrobial resistance must be globally coordinated."