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New Delhi: Chinese researchers have discovered a new superbug gene called mcr-1 that enables bacteria to be highly resistant to polymyxins, the last line of antibiotic defence the world has left.

This gene was found to be widespread in Enterobacteriaceae (a bacterial strain) taken from pigs and patients in south China. The researchers also found strains with epidemic potential, according to the research published in The Lancet Infectious Diseases journal.

“These are extremely worrying results. The polymyxins (colistin and polymyxin B) were the last class of antibiotics in which resistance was incapable of spreading from cell to cell. Until now, colistin resistance resulted from chromosomal mutations, making the resistance mechanism unstable and incapable of spreading to other bacteria," Jian-Hua Liu of the South China Agricultural University in Guangzhou said in a statement.

While this is one of the most worrying developments in the world’s fight against superbugs threatening to usher in a post-antibiotic era, here are some of the other antibiotic resistant genes that have been discovered over the years:

MRSA and Penicillin Resistance: Methicillin-resistant Staphylococcus aureus (MRSA) resists most β-lactam antibiotics using a protein that modifies the cell wall. Methicillin was introduced in 1960, and strains of MRSA were observed around two years later. Since methicillin was mostly immune to the enzymes that could destroy penicillins, MRSA was resistant to the drug. MRSA had spread to hospitals across the world within a few years and the impact was especially intense in the 70s and 80s.

NDM-1: New Delhi metallo-beta-lactamase-1 (NDM-1) was observed when a Swedish man was hospitalized in 2007 with an infection that resisted standard antibiotic treatments. The gene was responsible for new superbugs multiplying successfully, wreaking havoc in hospitals in several countries including the US and UK. The name of the gene was the subject of controversy as Indian government officials fretted that it could hurt medical tourism.

Multi-drug resistant Salmonella Typhi: Since 1989, strains of Salmonella Typhi resistant to chloramphenicol, ampicillin, and trimethoprim have caused outbreaks of Typhoid in the Indian subcontinent, Southeast Asia, and Africa. MDR strains were also isolated from immigrant workers in countries in the Arabian Gulf, as well as in developed countries from returning travellers. In the Indian subcontinent and Southeast Asia, the multidrug-resistant (MDR) Salmonella Typhi H58 clone, is now endemic.

CRE, the nightmare bacteria: The Carbapenem-resistant Enterobacteriaceae is a deadly superbug that has been spreading in hospitals in Australia this year and is reportedly killing half the people it affects. Carbapenems are a group of last resort antibiotics that usually work against these bacteria, but some bacteria have become resistant to these antibiotics across the world.

Shigella Dysentry: In the mid-1940s during the world war, Salfonamides were used in Japan during an epidemic of Shigella Dysentry. However, by 1952 most of the Shigella isolates started showing resistance to the drugs. By 1955, researchers were reporting several strains of Shigella dysenteriae resistant to the same four antibiotics at once. Even worse, the resistance itself was contagious. Since then, multi-drug resistant Shigella dysenteriae has spread to several countries including India where Shigella dysenteriae type 1 caused an extensive epidemic of shigellosis in eastern India in 1984.

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