With a disease like MS very little is known definitively about the direct cause. The broad theory is that something triggers the immune system to attack the nerve cells causing severe scaring (sclerosis) which degrades or eventually completely blocks neuron function. The biggest puzzle has been what causes the blood brain barrier breach in this autoimmune reaction. However, given how much we are now uncovering regarding the gut microbiome link with brain function and mental health, it’s plausible that a Clostridium infection in the gut could open such a pathway.

After posting my comment above I went back to the literature and confirmed that doxycycline does indeed cover clostridium perfringens. I would be very interested to see a patient trial, but I don’t expect many pharmaceutical companies are lining up to discover a new disease treatment for a drug that has been in commercial use for more than 50 years.