University of Minnesota researchers develop antimicrobial peptide that defeats drug-resistant bacteria

At a time when bacteria are increasingly developing an ability to stand up to antibiotics, U-M School of Dentistry scientists have developed an antimicrobial peptide that kills drug resistant bacteria without causing new resistance behaviors.  The new peptide is even effective against bacterial biofilms, a sticky mass of bacteria that are difficult to kill with traditional antibiotics. Bacterial biofilms are responsible for dental plaque and have been estimated to affect 80% of bacterial infections in the body. The results of their research were reported in March 2018 in the peer-reviewed journal PLOS ONE. 

The research team, consisting of faculty and students from the School of Dentistry, found that bacteria that can defend themselves against one version of the new peptide (termed left-handed) are unable to defend themselves against the mirror image of the new peptide (termed right-handed). The new peptide also killed vancomycin-resistant Enterococci, drug-resistant bacteria that are associated with infections of blood, urinary tract, heart valves and brain in susceptible individuals.

“The ability of many bacteria to defend themselves against the benefits of antibiotics is a global health threat,” says Sven-Ulrik Gorr, PhD, principle investigator and co-author of the article. Gorr is a professor in the dental school’s Division of Basic Sciences. “Our new antimicrobial peptides might represent an alternative treatment for killing bacteria, and one that may be less susceptible to bacterial resistance.”

Gorr’s new antimicrobial peptide is based on the sequence of the human salivary protein BPIFA2. “It’s long been recognized that human saliva contains a host of antimicrobial proteins that are thought to control the growth of bacteria invading the oral cavity,” says Gorr.  “However, the chemical functions that allow that to happen are not fully understood.”

Gorr’s team first sequenced the BPIFA2 gene in 2001 and has been studying it ever since.  In 2008, he was successful in decoding the BPIFA2 structure to produce the antimicrobial peptide GL13K.  He and colleague Conrado Aparicio, PhD, MScEng.,  were able to incorporate GL13K into a novel coating used for dental implants and are now testing its effectiveness in preventing infections, the most common cause for implant failure.  It’s an application with the potential for use in a broader category of medical devices and implants, such as those used for hip and knee replacement surgery. Aparacio is an associate professor in the dental school’s Division of Biomaterials and Biomechanics.

“Many organisms produce antimicrobial peptides. They are part of their defense against infections,” says Gorr.  Through careful analysis of the structures of BPIFA2 and similar peptides from other species, and a fair amount of serendipity, they were able to design the new antimicrobial peptides.

In the new work , they used the left-handed and right-handed versions (scientists call these L- and D-enantiomers) of the new  peptide and tested them against the Gram-positive bacteria Enterococcus faecalis and Streptococcus gordonii to understand the role of bacterial enzymes and cell wall modifications in bacterial resistance. 

One of the significant challenges in killing bacteria is that they have a variety of defense mechanisms that can prevent antimicrobial peptides from reaching the bacterial cell membrane. “The bacteria will send out enzymes to protect themselves,” says Gorr.  If the peptide makes it past the enzymes, it encounters a thick cell wall that protects the bacteria.  “Our peptide, in its natural left-handed form, was both destroyed by the enzymes and, in separate experiments, unable to breach the cell wall.  The right-handed peptide defeated both of these bacterial defenses. The enzymes could not destroy the right-handed peptide and the peptide was also able to slip past the cell wall defenses and kill the bacteria.

 “There is a difference in the way the cell wall sees the L- and D- versions,” says Gorr.  “Although we have identified some of the cell wall molecules involved, we don’t yet know how the right-handed peptide slips by.  What we do know, though, is that we can overcome the bacterial resistance. Most important, the bacteria do not seem to gain resistance to the new peptide”  Gorr is quick to add that investigations to-date have been done in a test tube where  experimental variables could be controlled.  

In addition to vancomycin-resistant Enterococci, the new peptide also kills multi-drug-resistant Pseudomonas, which often cause infections in those with existing diseases or conditions such as cystic fibrosis and traumatic burns, and MRSA, a superbug that can cause painful skin infections and serious complications such as pneumonia and sepsis. According to the CDC, these three drug-resistant bacteria alone are responsible for 35 deaths daily in the U.S.

After 17 years and investigations that led from the BPIFA2 salivary protein to the development of the two new peptides, Gorr says the investigation is just beginning to get really interesting.  His eyes light up when he discusses his team’s most recent findings and the hope that the new peptide may one day help some of the patients with multi-drug-resistant infections.

In collaboration with the University’s Clinical and Translational Sciences Institute and Center for Translational Medicine, Gorr’s team is now exploring future clinical uses of the new peptide. Promising early data show that the peptide has low toxicity and can be effective in an infection model, when applied as a topical antibiotic ointment.