Research and new technologies are striving to minimize the impacts of the relentless lurker that is Listeria.
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Explore this issueAugust/September 2018
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Investigators at North Carolina State University (NCSU), Raleigh, have completed a proof-of-concept study in which they identified several compounds that may be effective in minimizing Listeria’s virulence.
Simply stated, a proof-of-concept study demonstrates that a particular theory has the potential for practical real-world application.
In this case the process started with the key understanding that inhibiting a particular enzyme of Listeria, glucose-1-phosphate uridylyltransferase (GalU), leads to rather dramatic modifications of the bacterial cell surface, according to Paul Orndorff, PhD, professor emeritus of microbiology.
“From this point we determined that these chemical modifications in turn rendered Listeria much less virulent than it normally is, and thus less able to cause illness,” Dr. Orndorff says. The work was published in Molecular Informatics in March 2018.
Dr. Orndorff and his collaborators in the NCSU Department of Chemistry, postdoctoral researcher Melaine Kuenemann, PhD, and Denis Fourches, PhD, an assistant professor of computational chemistry, embraced the task of identifying potential compounds that could inhibit the function of GalU. Using computers and cheminformatics methods, Dr. Fourches and Dr. Kuenemann characterized, analyzed, and virtually screened more than 88,000 drug-like compounds using a technique called 3D molecular docking.
All those computations, which were based on the three-dimensional structure of the GalU protein and virtual representations of each compound, took several weeks to accomplish, Dr. Fourches mentions.
“Through computer modeling, we prioritized 37 compounds predicted to bind the GalU active site and thus looked promising enough to be tested in vitro,” he relates. “Of the 37, three compounds showed good experimental activity and were deemed effective enough to warrant further study. This is a great result, considering we had no idea what type of chemical could actually bind the GalU pocket.”
Moreover, all those compounds, including the less active ones, yielded important information about how their chemical structures relate to their activity in inhibiting GalU’s function, Dr. Fourches notes. “We were able to derive several predictive structure-activity relationships based on those 37 compounds, and these relationships will help us design even more effective GalU inhibiting compounds,” he elaborates. “This study shows that one can develop small molecules to shut down the activity of one specific bacterial enzyme, leading to the suppression of virulence. This is a completely new avenue, especially for fighting antibiotic-resistant bacteria.”
This is true research at the interface of several complimentary disciplines, Dr. Fourches emphasizes. “The synergistic use of artificial intelligence, molecular modeling techniques, and in vitro confirmation is a game-changer in the way we rationally design chemicals,” he points out.
The next step, Dr. Fourches says, would be using computers to virtually generate thousands of new analogues, virtually screening them, and then selecting another batch of 50 molecules to be tested experimentally to determine their impact on GalU. “Unfortunately, this project is not funded at the moment, even though direct commercial applications from this study are reachable,” he mentions. “We need more support from both federal agencies and commercial partners to get this research moving forward.”
The NCSU research also determined that inhibiting GalU made Listeria more vulnerable to cefotaxime, an antibiotic to which it has a natural resistance.
“This antibiotic susceptibility suggests the possibility of viable therapies that could combine a GalU inhibitor and a known antibiotic such as cefotaxime,” Dr. Orndorff says. “However, we believe if the GalU inhibitor is effective enough, the host, whether human or animal, should be able to eliminate the Listeria population without antibiotics. This holds promise to be a great solution for farmers striving for antibiotic-free livestock and poultry operations.”
More Robust Risk Assessment
To benefit the dairy processing industry, the Midwest Dairy Foods Research Center and the South Dakota Agricultural Experiment Station are funding researchers in the South Dakota State University (SDSU), Brookings, Dairy and Food Science Department, who, in a cooperative effort with a commercial ice cream and frozen desserts manufacturer, are developing models to more accurately predict the risk from Listeria.