I'm not a biologist nor in any sense of the word am I qualified to answer your question. However, I feel that I might be able to lend some perspective on that matter that might otherwise be useful in gaining a firmer level of comprehension on the issue at hand.
Onto the question regarding the transfer of some of the bacteria from mother to child I'm almost certain that someone may be able to shed some light on this puzzle.
As noted earlier, I'm almost nearly certain that I am in no way shape or form the person who could assist in this conundrum.
Don't hesitate to ask should you require further assistance.
I wonder if the mom's in Japan test out the baby's bottles with their mouths to see if they are too hot. That might explain bacteria transfer.