Clostridium difficile represents a unique problem in health care system. C difficile is not affected by a wide range of antibiotics and they start showing up when gut microbiome is depleted due to antibiotics. C difficile is often treated with antibiotics such as vancomycin, metronidazole and fidaxomicin which cause further disruption of the resident microbiota leading thus increasing chances of relapse. A faecal microbiome transplant (FTM) is an option but it comes with some challenges. For example, It is nearly impossible to rule out every possible infection from a healthy donor and we are technologically incompetent yet to find the right best combination of microbiome for the gut. The best answer would be an antibiotic that kills only and only C diff. That means we have to be extremely specific.
Fig 1: Electron micrograph of negatively stained
purified diffocin particles isolated from CD4 strain.
C difficile is a highly competitive bacteria. C difficile strains compete with each other and they produce an R-type bacteriocin (High-molecular-weight or phage tail-like) which is called as diffocins. In a generalised sense, R-type bacteriocins attack target cells by specific receptor-binding on the surface. This is followed by sheath contraction and insertion of the core through the envelope of the target bacterium. The diffocins are ultra specific in their activity and their receptor binding activity can be modified to attack C difficile in general by modifying its binding to specific receptors.
|Table 1: Sensitivity of various bacteria to modified diffocins.|
Table 1 shows a summary of the sensitivity of various bacterial strains to heterologously expressed, recombinant diffocins. A company that is currently working on using these diffocins and their modified version- Avidbiotics Corp (California based Biotech company) have named them as Avidocin-CDs. One of such constructs Av-CD291.2 had been found to have wide spectrum activity against a range of hypervirulent C difficile strains tested, without affecting the microbiome.
There is sufficient literature evidence to indicate that avidocins are highly specific for C difficile and able to survive transit through mouse GI tract.
In the latest paper, the story becomes better now. The team of scientists from the University of Sheffield, AvidBiotics Corp, and the University of Glasgow have published a study on Avidocin-CD291.2 on how it could be extended for clinical use. Analysis of rare Av-CD291.2–resistant mutants enabled identification of S-layer protein A (SlpA) as the target. The paper showed that Av-CD291.2–resistant mutants lack an S-layer. The lack of S-layer also introduces a high sensitivity to innate immune molecules combined with sporulation defects. These S-layer mutant strains survived poorly in the standard charcoal medium which is used to transport C difficile strains. Interestingly, acquisition of Avidocin-CD resistance results in loss of toxin production and complete loss of virulence.
As Dr Robert P. Fagan, senior corresponding author comments, "We discovered that the weapons naturally produced by C difficile and those engineered by our colleagues at AvidBiotics were using certain proteins in the S-layer to identify which strains to target. The C difficile S-layer is unique to these bacteria, which explains why Avidocin-CD killing is so specific. Scientists at AvidBiotics Corp were then able to engineer different versions of Avidocin-CD to target 12 of the 14 known types of S-layer."
Joseph A. Kirk, Dana Gebhart, Anthony M. Buckley, Stephen Lok, Dean Scholl, Gillian R. Douce, Gregory R. Govoni, Robert P. Fagan. New class of precision antimicrobials redefines role of Clostridium difficile S-layer in virulence and viability. Science Translational Medicine, 2017; 9 (406): eaah6813 DOI: 10.1126/scitranslmed.aah6813