New 'metal-made' antimicrobials show high kill rate with low resistance

Researchers have discovered a new group of antibiotics that not only showed significant antimicrobial activity against both Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) strains, but also may overcome the antibiotic resistance of S. aureus, according to a study published online in Medicinal Chemistry Communications. 

Unlike conventional antibiotic drugs, these potential new antibiotics contain the transition metal iridium. Transition metal complexes don’t break down easily, and they have already proven useful in diagnostic and chemotherapeutic drugs. But the inclusion of such a material in antibiotics is still a novel application.

Because the new compounds contain iridium, the researchers tested them for safety as well as efficacy, and found that the antibiotics effectively kill the bacteria without inhibiting mammalian cells.

One compound, for instance, demonstrated strong antimicrobial activity, with a minimum inhibitory concentration (MIC) of 4 μg mL⁻¹ against S. aureus and MRSA strains. It killed 99% of S. aureus cells in 6 hours.

“So far, our findings show that these compounds are safer than other compounds made from transition metals,” said the study’s corresponding author Joseph Merola, PhD, professor of inorganic chemistry in the College of Science at Virginia Tech, in Blacksburg, VA. “One of the reasons for this is that the compounds in this paper that target MRSA are very specific, meaning that a specific structure-function relationship must be met in order to kill the bacteria.”

The researchers noted that the compounds they investigated have very specific activity against S. aureus strains, but lack activity against other microorganisms such as Escherichia coli, Mycobacterium smegmatis, Candida albicans, and Aspergillus niger.

“We are still at the beginning of developing and testing these antibiotics but, so far, our preliminary results show a new group of antibiotics that are effective and safe,” said co-author Joseph Falkinham, PhD, professor of microbiology in the College of Science at Virginia Tech and an affiliate of the Virginia Tech Center for Drug Discovery. “Within the next few years, we hope to identify various characteristics of these antibiotics, such as their stability, their distribution and concentration in animal tissue, their penetration into white blood cells, and their metabolism in animals.”