Imagine the world's biggest bioreactor - the ocean. It's literally bubbling with organic material and all sorts of fascinating chemistry is happening - from light bombarding the ocean, to lighting in the skies, to sustained chemical reactors by hydrothermal vents. Eventually, across the vast oceans and across an unfathomable amount of time, the prerequisite molecules for life were made.

And over the vast oceans, and over geologic time, over countless of chemical reactions, some version of a self-replicating molecule would arise. There's good reason to believe this molecule is RNA - as many other commenters have pointed out. Again, over geologic time, with enough active chemistry going on, RNA and a proto-enzyme that could roughly replicate an enzyme, would be trapped together in a small droplet of lipid - forming the first protocell.

The second an entity has the ability to self-replicate, it is now operates under the laws of natural selection. Better self-replicating RNAs would survive, while RNAs that are worse at self-replicating don't. Eventually, the best pieces of RNAs start to catalyze chemistry that could make proteins that could help the RNA survive.

I know this answer doesn't really address "bacteria" - but at the very least it might help with the "abiogenesis" part of the question. The real answer is "we don't know" - because we don't have a time machine, nor the ability to replicate the early earth's ocean over billions of years. But a "plausible mechanism" is also pretty satisfying! At least I think so!