Bacteria grow/reproduce if given the correct conditions ie food and temperature. You only need a sample of the bacteria such as a small sample size of a few capsules and then they can be cultured fairly easily.
A well made yoghurt can have considerably higher numbers of bacteria than most probiotic capsules. There are other forms of fermentation if milk is a problem. You would need a method to maintain a suitable temperature around body temperature is ideal but a bit higher 42C is okay.
If you make yoghurt, getting/making a temperature controlled chamber where the temperature can be set to a higher temperature of high 40's C for an hour and then lowered after that to around 40C can produce quicker results. The company linked below have a device which can do it but you could build a device yourself by using a temperature controller STC-1000 and a chamber and heating source to maintain the temperature. There is a updated firmware for the STC-1000 which allows it to be programmed with temperatures and times so that it can adjust the temperature automatically which could give a perfect way of culturing bacteria quickly. I am currently looking for a chamber to use with the STC-1000. It needs to be big enough to culture large amounts of yoghurt in some large jars but finding something suitable is proving difficult. I did think about a mini fridge but finding a none working mini fridge is also difficult and I don't want to pay for a new mini fridge due to the costs involved.
Note that only certain versions of the STC-1000 can be reprogrammed with new firmware. The firmware is available from the following link and so are details on where to acquire the correct version of the STC-1000.
https://github.com/matsstaff/stc1000phttp://brodandtaylor.com/I did see a few problems with the above pre made device where people were complaining of parts melting and for the money they charge I did not fancy wasting money on it.