The answer in a word is "poop" from healthy people: ;)

At this point, other than reestablishing "normal" balance of microbes in the gut, there doesn't seem to any other option. I recall that there was a company in Canada which was using "transplant" of fecal isolates from healthy people as the treatment. "RePOOPulate" was the name of the company in the early stage.  ;)

http://www.scientificamerican.com/podcast/episode/fake-fecal-transplants-for-gut-repo-13-01-10/

It may be time to develop a bacteriophage specific to clostridium difficile with an exaggerated lytic activity and do a bit of germ-war.  ;)

But then it does make spores so multiple cycles of growth and destruction may be needed. 

Thank you a lot for the very interesting answer. I have heard about this topic and it seems to have interesting development...

So from the other pharmaceutical products there are no measures to act selectively against clostridia,  with preservation of probiotic bacteria from microbiota?

I suspect the direct attack on exact genus or more than that species is impossible. Phage therapy sounds the most accurate one, but, indeed, I don`t know is there one existed or developed.

the other way - antibiotic therapy, however it is not so directed of course. Probably, there are Gram+ antibiotics, specific particularly to Firmicutes and Clostridiales. 

Moreover, I`m not sure it`s a good idea, :) but Clostridia are extremely sensitive to oxygen, probably the substances with reactive oxygen compounds will be useful. Just in very small concentrations to keep other, less sensitive microbes, alive. A bit fantastics, but who knows...

As an example of trying to be very specific, you can see what AvidBiotics is trying to do. (full disclaimer, I have no financial stake in this company). This might be a way forward. http://www.avidbiotics.com/products/antibacterial-product-development/

Very highl selective therapeutic substances are FIDAXOMICIN and SUROTOMICIN, first  substance also available in ceska republika since 2012 I guess, the second one in Phase 3 study , available hopefully in 2015,

Both do very less harm to the gut flora ! :-)

Fidaxomycin, a 20 tab pack costs upwards of £1350.

On highly selective and very little harm to normal gut flora...

Here is comparative information with older and current standard, vancomycin:

The study met its primary endpoint of clinical cure, showing that fidaxomicin was non-inferior to oral vancomycin (92.1% vs. 89.8%). In addition, the study met its secondary endpoint of recurrence: 13.3% of the subjects had a recurrence with fidaxomicin vs. 24.0% with oral vancomycin. The study also met its exploratory endpoint of global cure (77.7% for fidaxomicin vs. 67.1% for vancomycin). Clinical cure was defined as patients requiring no further CDI therapy two days after completion of study medication. Global cure was defined as patients who were cured at the end of therapy and did not have a recurrence in the next four weeks.

Yes, somewhat better than vancomycin - highly targeted, perhaps not! It is still a gram negative killer, just a matter of playing with doses to achieve somewhat desired endpoint (and there's nothing wrong with that in the absence of a truly specific treatment). However, for a marginal improvement of ~10%, almost doubling the price from already rather high cost of vancomycin seems high but that is free market in action! 

Clostridia? Which one? Single strain, species, genus, order or class level?

Looking on the heterogenicity within these groups it is highly unlikely that something like a selective therapeutic approach is possible.  In addition, many beneficial and immunomodulatory species necessary for a balanced microbiota are also part of the same groups and would be affected too.

Regarding recurrent C. difficile colitis; i agree that fecal transplantation looks most promising these days. In general, I would favor any approach which is not directly antibacterial but rather supports a balanced and 'healthy' microbiota and regulates the gut immune system, which will than indirectly clear infection. 

It is recently described that that few bacteriocins of lactic acid bacteria inhibit Clostridia but not any probiotics.You ca survey literature for that. Look also to my published papers herein.

All the answers are very useful to me - it helped me to defend my doctoral thesis last week! And it broughtened my knowledge in this topic.

In fact, we were searching for natural compound which would act selectivelly between bifidobacteria and clostridia. We found one in vitro - 8-hydroxyquinoline, and we would like to work on it further. Yes, I know that not all clostridia are "bad", and yes, I know that this substance is most probably not primarily of plant origin. Nevertheless we observed the selectivity here.

Thank you everybody for your valuable opinions and remarks!

Congratulations, Dr. Novakova!  :)

Thank you a lot !!