I've seen some mention here, maybe by Dave (Likespace), about spending a small fortune on Propionibacteria. Francois and I have both posted some methods for propagating cultures, such as making bulk mother culture and duplicating cheese styles by using commercial rind flora. I wanted to follow that up with a discussion on how to propagate Propionibacteria. This process applies to the common strains such as shermanii, as well as unclassified strains. In most cases, you will use the process to take a small amount of commercial DVI starter and multiply it to make your dollars stretch.
A few more notes and then the process. One, this does take a moderate initial investment. Two, if you failed biology/chemistry, don't worry, you can still do this, but pay very close attention to having a sterile environment. Three, propagation works best if you can use the culture quickly, like you would for a bulk culture. You can freeze this, but it will decrease viability. If you want to increase viability, I can post a followup of how to centrifuge the broth and preserve in glycerin in the freezer.
OK, enough theory (almost). The basic idea of propagating any culture is to give the bacteria enough food and the right temperature so that it can thrive. With Propionibacteria, the right food is a sodium lactate broth, and the right temp is 30 degrees C (86 F). Meaning to propagate, you need to inoculate the broth and keep it at 30 degrees. The length of time to propagate depends on a few factors. One, it depends on how much you start with. With a 1% inoculum, you can achieve culture saturation in as little as 3-5 days. With less, it may take 7-9 days. If you are money conscious, it is better to err on the side of too long than too short, within reason. Don't go too far past a week.
To prepare sodium lactate broth (SLB):
10 g tryptone, or similar pancreatic digest of casein (this is 1% wt/vol)
10 g yeast extract (this is 1% wt/vol)
10 g sodium lactate (again, this is 1% wt/vol)
(optionally) .5 g of tween 80, this is an emulsifier, aka polysorbate 80
(optionally) .25 g K2HPO4, this is a buffer
1 L distilled H2O
1) Mix everything together in water and stir to dissolve a bit.
2) If you have an autoclave, heat to 121 C for 15, or follow your lab's procedure for broth prep. If not, boil for 20 mins and transfer to aseptic container, or as aseptic as you can find.
To culture Propionibacteria:
1) Take the broth, heat to 30C, put in anaerobic container that lets off gas, such as a bottle with a stopper and wine bubbler lock filled with alcohol, and add 1% inoculum. Measure the powder out beforehand. If you're using an anerobic kit, you're fine, but if you're using a stopper with bubbler, purge with CO2 or argon to get out the oxygen.
2) Keep at 30 degrees C for at least 5 days.
3) Put in fridge and use up as soon as you can. At 1C, you should still have some viability even after 6-9 months. At least enough to reculture the next batch.
I'm anticipating some questions, so I will try to answer them beforehand.
Q: Do I have to use sodium lactate broth? That's a lot of stuff to buy.
A: No, you can use MRS broth. It's almost as good. Propionic is picky, so those are your two best choices. For anyone who is a Simpsons fan: "Troy: If you can't find metal stucco lath, Homer: Uh-huh? Troy: Use carbon-fiber stucco lath! Homer: Uh-hohh! Troy: Now parge the lath!"
Q: Where do I get this stuff?
A: Look online. The major biology supply houses like cole palmer and VWR should have culture media. If all else fails, just buy MRS broth. Make sure you get the broth and not agar.
Q: How much should I use?
A: This one is tougher to answer because your colony size will depend on how much inoculum you use, how many days you let it grow, the style of cheese, etc. As a starting point, try 10-20 grams per 2-4 gallons for an emmenthaler style. This is about a teaspoon per gallon.
Q: What is the least amount of equipment that I will need?
A: Gram weight scale for solids, graduated measuring device for liquids, bottle with stopper and bubble lock. Also storage containers for prepared broth and cultured broth.
Q: Can I reculture from an existing cheese to isolate a swiss-type wild or native strain?
A: Yes, but this is time consuming, requires more equipment, and for this one you really should take a few classes in biology that have lab sessions.
Q: How much money can I save?
A: You really do need to be a serious swiss cheesemaker for this to be worthwhile considering that you can make about 80-100 lbs of cheese for $10 in Propionic culture. But, to make the same amount in broth form would take something like 10-30 grams of broth base, which costs about 10 cents/gram. Meaning for the average person, not very much. And it's a lot of hassle and takes time. For the maker of huge wheels or for someone who already has the equipment and also cultures using bulk starter, it's more worthwhile.
Q: Can I use this process to boost viability of my propionic and to add broth instead of powder to the milk?
A: Of course. This may be a sweet spot for the home cheesemaker. Use 1/5-1/10 the amount of DVI and make up a small batch of broth to use for a batch of cheese. No need to maintain cultures that way and you can stretch the commercial stuff. For this method ferment the propionic for 2-4 days beforehand.
Q: Can I use this method to reculture other single-strain commercial starters or propogate lactic bacteria.
A: Yes, in theory. At home, it's easier to use skim milk or reconstituted skim milk enriched with some yeast extract, though. Cheaper, too.
Hope this helps. I know some here have asked about more details on reculturing and propagation, and that's the gist of it.