Bdellovibrio is predatory deltaprotobacteria , This bacterium predates gram-negative bacteria and lysis them (growing inside of them) forming clearing are on their lawns

Bdellovibrio can only attack gram negative bacteria , however it attaches to both gram-nigative and positive

The reason of that isn't known clearly yet !

Unfortunately not! Bdellovibrio are only able to prey upon Gram-negative bacteria, ie cells with both a cell wall and an outer membrane layer, as they grow in the area between the two. Gram-positive bacteria such as Staphlococcus aureusonly have a cell wall, (which is different to those in Gram-negative bacteria), and no outer membrane, so Bdellovibrio cannot eat them.

Bdellovibrio can prey on many Gram-negative pathogenic bacteria, including E. coli, Salmonella and Pseudomonas.

That would be Bdellovibrio bacteriovorus. It is a predatory bacterium, meaning that it kills other bacteria to live. It travels at speeds of 160 micrometers per second, Which would be an equivalent of someone swimming three Olympic-size swimming pools in a single second.

Source: The Invisible Kingdom by Idan Ben-Barak.