What is meant by mutant strains?

Question

What is meant by mutant strains?

Answer 1

A mutant strain of bacteria has a new genetic character arising or resulting from  undergoing mutation.

Hence, those bacteria or viruses have a mutant gene which expresses itself in the phenotype.

Also, a mutant strain is genetically different in one or more characteristics from an arbitrarily defined, previously existing ‘wild-type’ strain.

Answer 2

A mutant strain is a variation of a line of biological entity (say a virus or a bacteria) that has changed some of its characteristics.  It is like having a bacteria that was vulnerable to penicillin and then somehow acquired the ability to stand it.  The new bacteria is similar to the old one except for the new defense against penicillin.  Then we say the new bacteria is a mutated strain of the old one.