Hello, it was a big challenge to read the original paper, and I want to point out some details that were a bit vague:

I'm still contemplating its capability of being the primary source of electricity, somewhere in the future

No. The idea is a century old and it was more about the catchy title than about practical battery. It could be used as a biosensor or similar.

were found to have at least 8 genes associated with its capability to produce electricity by using a method called forward genetic screening. This would allow them to produce a specific type of coenzyme

To avoid misunderstanding, forward genetic screening is not a method to produce coenzyme, it was the experimental workflow o understand what is going on:

Forward genetics (or a forward genetic screen) is an approach used to identify genes (or set of genes) responsible for a particular phenotype of an organism. Reverse genetics (or a reverse genetic screen), on the other hand, analyzes the phenotype of an organism following the disruption of a known gene.

Than you said

I've come to a conclusion that, the capacity of a bacterium to produce electricity might have depended on its capability to cause harm to a human host. Listeria monocytogenes might be benign for people with an intact immune system, but they were quite deadly with people who have an immature immune system (like babies who are less than 6 months old)

It's not from the original research, but from the LifeScience and it was not in the context of electricity.

To make it simple, let's imagine, gram-positive bacteria have one layer of cell wall while gram-negative bacteria have two. This would make it difficult to transfer electron out in the gram-negative bacteria so technically speaking, they must have a more sophisticated electron transport system or they just have a lousy capability to generate an electrical potential.

Well, our cells have none... It's not the right dirrectionwhen thinking about this topic

vitamin B2 to survive, they also speculated that this particular vitamin is important for a bacterium to generate a specified amount of electricity.

The more they eat the more electrons need to pass. Electrons are analogues to our exhaled CO2, or poop. It's expected...

From the original study:

To determine whether flavins could be used as electron shuttles, we tested the effect of exogenous riboflavin, FMN and FAD on EET activity. Injection of FMN into an L. monocytogenes- inoculated electrochemical chamber resulted in a pronounced increase in electric current

could generate a different kind of potential and probably by a different set of genes

Not really, because it's the final result is nothing but a chemistry

Through this means, we can observe whether we can enable the same gene across all bacteria to produce the same desired effect; they must exert some kind of physiological beneficial effect, don't they?

Very, very brave...