Probiotics are great and help keep the bacteria in your intestines healthy (i.e more good bacteria than bad ones!), and new studies have shown that that bacteria actually plays a HUGE roll in behaviour and mood. If you ever take anti biotics or have a period of lots of stress then i suggest everyone to buy probiotic pills and take them for a while, though the unfortunate thing is that just a 2 week course of anti biotics can take up to 1-2 years to regain all those good bacteria, which is one of the reasons i am very anti antibiotics and prefer to take other options. The capsule form/probiotic pills are supposedly very good, i take them everyday anyway but i also eat yoghurt with probiotics.. but i think it can be very hard to get enough probiotics through just food as there is a very limited amount which actually has them. But some of the foods which have probiotics are:
sourkraut, fermented foods and sourdough bread apparently, as well as yoghurt.