Thermostable DNA polymerase used in PCR was isolated from 

The thermostable DNA polymerase commonly used in PCR (polymerase chain reaction) is isolated from Thermus aquaticus, a thermophilic bacterium. The specific DNA polymerase isolated from this organism is named Taq DNA polymerase. It is well-suited for PCR as it can withstand high temperatures required for DNA
denaturation (usually around 95°C) without being denatured itself.

(2) Escherichia coli is the best-characterised bacterial species that has been extensively used in both basic
molecular biology and biotechnology laboratories. It is often used as a host organism for cloning and expression of genes of interest. 

(3) Agrobacterium tumefaciens is a soil bacterium that causes crown gall disease in dicot plants (eukaryotic
cells). Due to its ability to transfer genes into plant cells, it is used as a tool for genetic engineering experiments with plants. Agrobacterium tumefaciens is genetically engineered to carry a modified Ti plasmid (disarmed) that contains the desired gene of interest but lacks the ability to induce a tumour in host cells. This disarmed vector can successfully transfer foreign DNA into plant cells in a process called transformation.

(4) Bacillus thuringiensis is a soil bacterium that produces cry proteins which are toxic to certain groups of insects. The cry genes cloned from this bacterium into crop plants can provide resistance to insects without the need for insecticides. Thus, Bt crops become naturally resistant to certain groups of insects.