WEST LAFAYETTE, Ind.—Antibiotic-resistant bacteria present one of the most serious issues both in the United States and globally, particularly as the majority of cases occur in hospital or urgent care settings. As antibiotics are overprescribed and bacterial strains continue developing resistance, more of the most commonly used antibiotics become less effective every year, making the search for new options a leading priority in the field.
Along those lines, a Purdue University team recently shared news of a compound that offers similar effectiveness to approved antibiotics—with the benefit of bacteria seemingly being less likely to develop resistance to it. Their work appeared in the European Journal of Medicinal Chemistry in a study titled “N-(1,3,4-oxadiazol-2-yl)benzamide analogs, bacteriostatic agents against methicillin- and vancomycin-resistant bacteria.”
The compound in question is called F6, and was tested against clinical isolates of both methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Staphylococcus aureus (VRSA). Researchers at Purdue discovered the compound via screening a chemical library for compounds that displayed antibacterial activity. All told, F6 proved to be effective against MRSA, VRSA, Enterococcus faecalis (bacteria that live in the gut), vancomycin-resistant Enterococcus (VRE) and Listeria monocytogenes, which are frequently associated with unpasteurized dairy products.
Equally encouraging, F6 “was found to be non-toxic against mammalian cells,” as noted in the paper’s abstract, and “was equipotent to the antibiotic fusidic acid in reducing MRSA burden” in a mouse with a skin wound infection. (Fusidic acid is commonly used to treat skin infections caused by staphylococcal bacteria.)
Once the Purdue team was assured of its antibacterial activity, F6 was tested to determine how easily common strains of bacteria might develop resistance to it. To do this, they repeatedly exposed F6 to the same strains of MRSA USA400 in vitro.
“The idea is that if you keep adding increasing concentrations to bacteria and then you keep regrowing the bacteria, after so many cycles you are going to develop resistance,” explained Herman Sintim, drug discovery professor in Purdue’s Department of Chemistry and corresponding author for the study. “Scientists do this to figure out whether whatever they have created develops resistance quickly.”
The minimum inhibitory concentration (MIC), or the lowest possible concentration of a compound needed to block visible bacteria growth, of F6 remained the same through nine passages of the MRSA isolate, and doubled on the tenth passage. The MIC stayed at that level up to the fourteenth passage over a two-week period. When the research team performed the same tests with the antibiotic ciprofloxacin—which is used to treat respiratory, skin, urinary and kidney infections, as well as sinusitis—the MIC tripled after just eight passages, and increased to more than 2,000-fold by the fourteenth passage.
“We are not saying there will never be resistance to the F6 molecule or analogs thereof,” Sintim cautioned. “What we are saying is that here is a new molecule that works and when we try to force resistance, we couldn’t generate resistance.”
As for why it’s more difficult for bacteria to develop resistance to F6, Sintim tells DDNews that “Our current hypothesis is that F6 probably inhibits two or more essential targets in bacteria. It is more difficult for resistance to develop against a compound that hits more than one essential target. Alternatively, the compound could be binding to a site that has conserved residues. Although resistance to drugs could evolve via protein mutations, some mutations are actually detrimental to protein function.”
Though F6 was effective against gram-positive bacteria, the team found that it was not effective against gram-negative strains. In the paper, the authors note that “The lack of activity against gram-negative bacteria appears to be due to F6 being a substrate for efflux. This can be seen by the shift in the MIC observed for compound F6 against wild-type E. coli BW25113 (MIC > 128 μg/mL) in comparison to a mutant strain (E. coli JW5503-1) where the AcrAB-TolC multidrug-resistant efflux pump is knocked out (MIC for F6 improves to 2 μg/mL). A similar result was observed with linezolid and erythromycin, two antibiotics known to be substrates for the AcrAB-TolC efflux pump in gram-negative bacteria.”
As noted in the 2003 paper “The important of efflux pumps in bacterial antibiotic resistance” in the Journal of Antimicrobial Chemotherapy, “Efflux pumps are transport proteins involved in the extrusion of toxic substrates (including virtually all classes of clinically relevant antibiotics) from within cells into the external environment. These proteins are found in both gram-positive and -negative bacteria as well as in eukaryotic organisms.1 Pumps may be specific for one substrate or may transport a range of structurally dissimilar compounds (including antibiotics of multiple classes); such pumps can be associated with multiple drug resistance (MDR).
“There is some debate as to the ‘normal’ physiological role of efflux transporters, as antibiotic susceptible as well as resistant bacteria carry and express these genes … Although genes encoding efflux pumps can be found on plasmids, the carriage of efflux pump genes on the chromosome gives the bacterium an intrinsic mechanism that allows survival in a hostile environment (e.g. the presence of antibiotics), and so mutant bacteria that over-express efflux pump genes can be selected without the acquisition of new genetic material. It is probable that these pumps arose so that noxious substances could be transported out of the bacterium, allowing survival. Indeed it is now widely accepted that the ‘intrinsic resistance’ of gram-negative bacteria to certain antibiotics relative to gram-positive bacteria is a result of the activity of efflux systems. Efflux systems that contribute to antibiotic resistance have been described from a number of clinically important bacteria, including Campylobacter jejuni (CmeABC7,8), E. coli (AcrAB-TolC, AcrEF-TolC, EmrB, EmrD9), Pseudomonas aeruginosa (MexAB-OprM, MexCD-OprJ, MexEF-OprN and MexXY-OprM9), Streptococcus pneumoniae (PmrA10), Salmonella typhimurium (AcrB11) and Staphylococcus aureus (NorA12).”
Sintim says that the team has a number of plans for additional work with F6. They hope to generate compounds that are more potent, he says, adding that “We already have newer generation of F6 that are more potent” than the original iteration. Sintim notes that they hope to produce analogs that are active against gram-negative bacteria, and “Our ultimate goal is to translate the compounds into antibiotics for human or animal use.”