Here is a discussion of bacterial starter cultures from Peter Moller, a Danish cheese making friend of mine.  Note that he is using the recently changed scientific names for the genus Streptococcus.  Many former members of this genus have been renamed Lactococcus.  For instance,  Streptococcus thermophilus is now Lactococcus thermophilus.

Starter cultures:
When making dairy products there are generally speaking 3 different starter cultures to use depending on what product you are making. These cultures can be intermixed to some extent, but generally you will want to stick to the culture suggested here.

Yogurt Culture:
Lactococcus thermophilus and Lactobacillus bulgaricus
Optimum growth temperatures typically 40-45C.  [Editor's note:  I have had great success incubating yogurt at 50 C.]
Typical applications include Yogurt, but when mixed with an O-culture* it can be used for Mozzarella, Camembert and Feta.

This starter culture produces lactic acid and acetaldehyde. The acetaldehyde gives yogurt its characteristic flavour.

O-Culture:
Lactococcus lactis and Lactococcus cremoris
Optimum growth temperatures typically 20-30C
Typical applications include Cheddar and when mixed with a Yogurt culture: Feta, Mozzarella and Camembert.

The primary function of this starter is producing lactic acid.. It will not produce any Carbon Dioxide and because of this it is not suited for cheese making if you expect nice round holes.

DL-Culture ( Buttermilk starter ):

Lactococcus lactis, Lactococcus cremoris, Lactococcus diacetylactis* and Leuconostoc cremoris* ( Lactobacillus casei is added in some newer types of this culture to improve the ripening )

Optimum growth temperatures typically 20-30C
Typical applications include most hard and semi-hard yellow cheese types, butter making, sour cream, buttermilk.
The funny thing about this culture is that it actually has 2 functions:

1: Produce lactic acid

2a:  For cultured milk products: The bacteria marked with the * breaks the lactose down to diacetyl and acetaldehyde which produces the fresh taste known from buttermilk. The acetaldehyde taste would be best known from yogurt, while the diacetyl taste is dominant in cultured butter such as the Lurpak brand ( Danish export product ).

2b: For cheese making: The same bacteria mentioned in 2a are allowed to decompose the acetaldehyde and diacetyl even further, so you end up with the carbon dioxide which produces the holes ( Eyes, if you will ) in the cheese.
 

For Emmenthal type cheeses you need to cook the curd above 40C.
To be avoid problems with the starter culture you could use a thermophile culture such as a combination of Lactobacillus helveticus and Lactococcus thermophilus.

About names: Some years ago it was decided to rename the lactic part of the Streptococcus genus to Lactococcus to avoid confusion with the pathogenic part of the  Streptococcus genus, so if you cannot find the Lactococcus types mentioned here, try calling them Streptococcus instead..

Growing starter cultures:

Stages:
1:  Sterilize the medium
2:  Inoculate
3:  Incubate
4:  Check pH
5:  Cool the culture
6:  Leave for 4 hours before using

Practical, 1 liter of Buttermilk type starter:

Materials and equipment needed:

2 liter stainless steel cooler w. lid
Incubator ( Large tub of water w. temperature control )
1 liter of water and 200 grams of Skimmed milk powder, or 1 liter of fresh skimmed milk and 100 grams of skimmed milk powder

Process:

Mix the milk/water / milk powder in the cooler and place in incubator.

Set controller to 90C for 30 minutes

Cool to 20C and add 0.05 liters of fresh buttermilk

Leave for 16-22 hours until the pH is 4.6. VERY IMPORTANT: Keep the temperature constant during this period!

Leave for another 4 hours and cool to 5-7C

Before use stir the starter vigorously and taste it.  If the flavor is off something may have gone wrong, but you need to keep in mind that the milk flavor is slightly different compared to normal due to the elevated content of dry matter.

If you have trouble finding skimmed milk powder, or would like to grow your starter without it it should not be a problem doing so, but practical experience shows that the extra dry matter in the medium helps the starter grow.

Slight variations of the growing temperature can produce massive differences in the hole structure of the cheese due to the difference of optimum temperatures of the different bacteria.  Generally speaking: If you want more holes in the cheese you need to lower the growing temperature slightly. If you grow your own starter try growing 4 different batches at 4 different temperatures and keep track of which starter gave the best results.

In an industrial setup the typical practice is to make maybe 200 liters of starter ( mother culture ) from a frozen culture, use these to inoculate one or 2 larger batches of starter ( production starter). Additional batches are inoculated from the production starter. Regularly take samples from the production starter and freeze them as a backup, so you can start over if the production starter is contaminated.  Normally you would only increase the dry matter content of the skimmed milk in the mother culture.

P. N. Moller, 2005