Opening the door to antibiotic resistance

Antibiotic resistance is a major threat to health. About two million people in the US contract an antibiotic-resistant infection every year, according to the Center for Disease Control and Prevention (CDC). Gram-negative bacteria, including types such as E. coli and Salmonella are often more difficult to kill because of their two-part defenses – they have two membranes rather than one, and also have many toxin pumps built into the membranes to expel any antibiotic that might have passed. Jefferson researchers have now discovered how to target these two defenses in one go, which could help make antibiotics more effective.

"We have shown that by interfering with a transfer RNA molecule (tRNA), in a bacteria-specific way, the ability of the bacterial cell to produce the membrane proteins needed for the bacterial cell is paralyzed. # 39; drug barrier activity and efflux, "says the main author Ya-Ming Hou. Ph.D., professor of biochemistry at Sidney Kimmel Medical College Jefferson (University of Philadelphia + Thomas Jefferson University). The work was published in the journal Cellular systems.

TRNA molecules are not a typical antibiotic target. These molecules are part of the mechanism of construction of proteins essential to the daily functioning of the cells of each living being. However, Dr. Hou's team has been looking at some kind of chemical "decorating" process in bacterial tRNAs, absent from human cells. This difference between bacteria and humans makes this process a better target for drugs because it affects less human cells.

The tRNAs are decorated with chemical groups that are added after the synthesis of tRNAs in a cell. Dr. Hou's group examined one of these decorations, adding a methyl group to a particular location in the spine of several tRNAs. In previous work, Dr. Hou's lab had shown that when these tRNAs lacked this methylation, they were more likely to create protein formation errors. But not just any protein, deficient tRNAs were particularly prone to errors in the construction of proteins located in the cell membrane.

This result led Dr. Hou to think that a defect in tRNA methylation might affect not only the toxin pump of the bacteria, but also a host of other types of proteins that help to maintain the membrane stable and cohesive.

In this article, in collaboration with postdoctoral researcher and lead author Isao Masuda and others, Dr. Hou tested whether these defective tRNAs could make bacteria more susceptible to antibiotics by creating genetically deficient bacteria for decoration of the methyl group.

Dr. Hou's team demonstrated, through an elegant series of experiments, that the membranes of these bacteria were less cohesive and more permeable than normal. Bacteria with defective tRNAs were less effective at extracting chemicals than normal bacteria, suggesting that their toxin pumps were affected. Finally, the team showed that when bacteria containing defective tRNAs were exposed to various antibiotics, they died faster and were also less able to develop drug resistance.

"The speed of death is important for antibiotics," says Dr. Hou. "The longer bacteria die for antibiotics, the more likely they are to develop resistance."

While pharmaceutical companies such as AstraZeneca and GSK have discovered compounds that can prevent the enzyme from producing critical methylation on tRNAs, progress is stalled. The main reason is that inhibitors are unable to penetrate through the structure of the bacterial membrane, which resonates with the major challenge facing the entire field of antibiotic discovery.

Dr. Hou recognizes the challenge. "First, we need to formulate inhibitors so that we can penetrate the cell more effectively," says Dr. Hou. "Then, combining these inhibitors with traditional antibiotics to kill bacteria faster and reduce the risk of antibiotic resistance."

For the moment, there is no medicine that can effectively attack this pathway. Dr. Hou's lab is currently developing better inhibitors.