There are some simple ways that you can expose your baby to good bacteria.
Your Birth Plan:
Plan on having a vaginal birth. When your baby is born vaginally, he/she comes into contact with good bacteria in your vagina (lactobacilli). This is the first bacteria that colonizes your baby's gut microbiome. Also, as your baby passes through your vagina and past your rectum during a vaginal birth, your baby will come in contact with helpful bacteria near the rectum. If you have a c-section and the baby does not pass through the vaginal canal, the first bacteria your baby could be colonized by is negative bacteria (staphylococcus) in the operating room.
Skin to Skin:
If your baby is placed skin to skin on your chest immediately after birth, then your baby comes in contact with the good bacteria from your skin. If mom and baby have to be separated, then it is beneficial for dad to do skin to skin until mother and baby can be reunited. Interestingly, the vernix (white coating on baby after born) is very sticky and is food for good bacteria, so leaving this coating on is actually protecting your baby against bad bacteria (i.e. delaying the first bath may be a good idea).
Exclusive breastfeeeding is the most important thing that you can do to ensure that your baby's intestinal tract is colonized by helpful bacteria. Breastfeeding not only contains probiotics, or good bacteria, but also contains prebiotics, which is the food that probiotics eat. The good bacteria can essentially eat up all the food leaving no room for the bad bacteria.
So why is it so important to expose a baby to probiotics? Probiotics, or good bacteria, improve baby's immunity, reduce diarrhea, help digestion, and reduce the risk of other illnesses. Your baby will be get bacteria in their gut, skin and mucosal membranes. What type they get, is up to you.