The lack of new antibiotics for Gram-negative bacteria is one of the cornerstones of the global crisis of antibiotic resistance. The quest is finding a molecule with antibacterial activity that can pass the double-layered cell wall and that manages to remain in the cell long enough to kill. New lab-based studies suggest that such antibiotics may already exist, and that the solution to activate them is widely available, and for free. As these findings were published in not-so-well-known-and-hardly-read journals for clinicians, such as EMBO journal and Scientific Reports, here follows the summary for dummies (written by a dummy).

The biology is simple: there are bacteria with one cell membrane and bacteria with two membranes. That was already known to doctor Hans Christian Gram, a Danish bacteriologist, who used this difference to differently stain bacteria in 1884. The bacteria with one membrane turned blue and became known as Gram-positive, the bacteria with two membranes turned red and became famous as Gram-negative bacteria. Similarly, men discovered antibiotics that could and could not pass the double-layered cell wall of the Gram-negative bacteria. Those that could not included vancomycin and rifampin, only useful to kill (single-layered) Gram-positive bacteria.

Medical students (and laymen) think that only antibiotics are needed to kill bacteria to treat infection. We all know that antibiotics help, but also that some activity of the immune system is required to finish the job. Yet, how this interaction occurs is not fully understood. PhD student Dani Heesterbeek in our department, supervised vy professor Suzan Rooijakkers, elucidated the essential role of the complement system in bacterial killing. Upon activation (of C5 to C9 to be precise) the Membrane Attack Complex (MAC) is formed which makes a pore in the outer bacterial cell membrane. With this the killing starts. Naturally, this process results from a long evolutionary tract, but luckily it also enhances the activity of antibiotics. They can now easily pass the first hurdle.

So what? This insight may have 3 important consequences for how to treat future infections.

First, in our way of choosing the best antibiotic. For that we rely on the MIC, the minimal inhibitory concentration. This is determined in the lab, by growing bacteria (mostly in broth) and exposing them to different concentrations of antibiotics. The lowest concentration without growth is the MIC. The higher the MIC, the more resistant the bug. Yet, this MIC has been called a brainless measure, as it hardly reflects the in vivo situation. As different levels of complement activation lead to different levels of MAC formation, it probably also leads to different MICs for bacteria in the human body (in vivo). The consequence could be that we currently neglect potentially useful antibiotics, as we – unjustly –  consider them inactive based on the in vitro MIC.

Second, in treating infections. Passing the first cell membrane, leaves only one cell membrane to be passed. And guess what, with activated MACs adding an antibiotic usually active only for Gram-positive bacteria synergistically enhances the killing activity of antibiotics effective against Gram-negatives (experiments were with MDR Klebsiella). This would open up a whole new avenue of combination therapy: a carbapenem plus vanco (or rifampin) for Carbapenemase producing pathogens.

Third, in modulating the immune system. What if we could stimulate the complement system in order to assist our failing antibiotics, for instance with monoclonal antibodies.

Now relax, and breath quietly. All this needs to be translated into clinical research first, to undergo the torture of evidence-based medicine. But to me, it feels like a whole new box of possibilities has been opened, and it may not be Pandora’s box.

Suzanne Rooijakkers

Prof. Suzan Rooijakkers and Dani Heesterbeek, who (succesfully) defended her PhD thesis August 29, 2019