Rennet is an animal enzyme used to curdle milk in making cheese. It is controversial to vegetarians since it is derived from the stomach lining of calves. Cottage cheese, ricotta nad some mozzerella are rennetless. There are also some rennet-like ingredients which do not require killing calves.

Rennet tablets can be used to make cheese, yogurt, custard, junkets, etc. Just search for rennet + recipe

http://www.wholefoodsmarket.com/products/rennet.html

A much easier way to make yogurt is to use a starter.

Scald milk - do NOT boil -
(To scald cook on medium heat until a film forms on the top.)
Then remove from the burner and let cool to approximately 110 degrees.
Pour into jars,
Add approximately 2 tablespoons per quart of either yogurt with live acidofilus cultures or yogurt starter.
You can either then let it rest in either a warm oven (approximately 110 - 120 degrees) or in a warm water bath (water at approximately 110 degrees) for 4 - 12 hours. Michelle Lennon set up a warm water bath in a igloo cooler and let it sit over night. It will start to thicken but will not completely firm up until it is cooled.
Place the jars in the fridge and it's ready to go as soon as it cools.

I've never heard of using rennet to make yoghurt.I have never seen it listed on the labels of commercial yoghurts either.

Can/are bacterial enzymes be used as a substitute for rennet?

You mean...you can specifically buy RENNET? Ewww...

I'm so glad that I'm a vegan

:)
Mark