what is the role of HCL in the DNase experiment

Serratia genus as a whole is dnase positive.

There are no reagents added when a Dnase test is performed. The test is done in a methyl green medium with a pure inoculum culture. If halos form around the culture than Dnase is present.

nope.

it is chealeting agent and has great affinity with metal ions and mg- ions present in dnase as a cofactor and responsible for dnase action that degreded DNA hear edta bide with mg- ions and stop the action of dnase.