From what I understand about the BBB, it's practically impermeable to anything the size of a bacteria (mainly intended for gating small molecules like fats and drugs), until an individual has some sort of disease or other health problem that effects the BBB's integrity. But I can easily imagine that some bacteria (harmful, or helpful) could have exactly the right surface proteins to indicate it's permitted.

I guess this review/writeup https://asm.org/articles/2020/april/how-pathogens-penetrate-... covers most of the known mechanisms for traveral of the BBB by pathogens.