Two microbes found to be very useful in genetic engineering are -

Escherichia coli is the most widely used host in genetic engineering. Its plasmids, such as pBR322, are used as cloning vectors, its restriction enzyme EcoRI is used to cut DNA, and it multiplies rapidly to produce the cloned gene or its product (for example human insulin).
Agrobacterium tumefaciens is a soil bacterium that causes crown gall disease in dicot plants. It naturally transfers a piece of DNA, the T-DNA of its Ti (tumour inducing) plasmid, into the plant genome.
The Ti plasmid has been modified into a cloning vector that is no longer pathogenic but can still deliver genes of interest into plants; Agrobacterium is therefore called the natural genetic engineer of plants.
Caenorhabditis elegans is a nematode, not a microbe used as a tool in this sense.
Vibrio cholerae and Diplococcus are not standard tools of genetic engineering.
So the two very useful microbes are Escherichia coli and Agrobacterium tumefaciens.