Changing pathogens

What is meant by strains of bacteria and mutation?
Changing pathogens as i dont understand this.

Strains = subtypes of a bacterial species, or virus.

In a immunology setting, mutations give rise to different strains. These strains vary from each other - some may be more pathogenic/likely to cause illness, whilst others may have less effects. For example, Staphylococcus aureus is a gram positive bacteria associated with epidermal surfaces, and highly treatable in cases of infection. On the other hand, MRSA (a methicillin-resistant strain of Staphylococcus aureus) is a major issue in a clinical setting due to antibiotic resistance.