Many genes for resistance to antimicrobial agents are carried by transmissible extrachromosomal circles of DNA called plasmids. Until 1998 everyone thought that plasmids didn’t carry resistance to quinolones, but then a plasmid gene called qnrA was discovered. A second gene called qnrS followed in 2005.

“The hope is that by understanding more about how bacteria acquire and express resistance, we can use antibiotics more effectively and also discover new agents that are not subject to existing resistance mechanisms”

This paper describes a newly discovered third gene, qnrB. People are interested because the discovery of plasmid-mediated quinolone resistance was unexpected, the mechanism for resistance is novel, how much qnr genes contribute to the rising frequency of quinolone resistance remains to be determined, and the qnr family challenges our understanding of where resistance genes originate and how they become established in bacterial pathogens.

Quinolones, such as ciprofloxacin and levofloxacin, have been very useful antimicrobial agents because they are highly potent, active against a wide range of bacteria, and relatively non-toxic. Their broad use, however, has been followed by rising rates of resistance.

Quinolone resistance has traditionally been understood to arise either by mutations that altered DNA gyrase and topoisomerase IV, enzymes that are the targets for quinolone action or by mutations that increased expression of efflux pumps that actively eliminate the agents from the cell. Neither type of resistance is transmissible since both are due to mutations on the bacterial chromosome.

The qnrA1 gene was found on a transmissible plasmid in a clinical isolate of Klebsiella pneumoniae from Alabama. qnrB1 was discovered on a plasmid from K. pneumoniae isolated in India and related genes, qnrB2, qnrB3, qnrB4, and qnrB5 have been found on plasmids from other gram-negative pathogens in the United States and elsewhere. The qnr mechanism is thus widespread.

The qnr genes promote only low-level quinolone resistance, but they facilitate selection of higher level, clinically significant resistance, and so contribute to the problem of rising quinolone resistance. Their location on transmissible plasmids enhances the ability of quinolone resistance to spread to other organisms and to be linked to other antibiotic resistance genes.

Cloning and sequencing the qnrB gene revealed that, like qnrA, it encodes a protein belonging to the unusual pentapeptide repeat family in which amino acids repeat at regular intervals. QnrB protein was purified by linking it to a polyhistidine tag. When added to bacterial DNA gyrase, QnrB blocked inhibition by ciprofloxacin thus demonstrating that it directly protects the quinolone target from inhibition.

I’m trained as an internist and infectious disease specialist and have a long-standing interest in how bacteria acquire resistance to antibiotics because this has a profound effect in treating patients. Much of the qnr story developed after I retired from clinical practice and could focus on research. It’s being done in close collaboration with David Hooper at the Massachusetts General Hospital.

We plan to look for qnr genes in clinical samples from other parts of the world, in environmental bacteria, in gram-positive as well as gram-negative bacteria, and in samples stored before wide-spread quinolone use. We hope to explore the details of Qnr structure and how Qnr interacts with DNA gyrase and topoisomerase IV using techniques of protein-protein interaction and x-ray crystallography. We also intend to investigate how qnr genes were captured by plasmids and whether other genes on plasmids contribute to quinolone resistance.

The hope is that by understanding more about how bacteria acquire and express resistance, we can use antibiotics more effectively and also discover new agents that are not subject to existing resistance mechanisms.