INTRODUCTION

A brief history

In the 1920s Frederick Griffith, a medical officer at the Ministry of Health in Britain, made a significant discovery regarding Streptococcus pneumoniae, a bacterium that caused a pneumonia epidemic in London. While examining the strain variability within different groups of pneumococci, Griffith noted that an avirulent strain of the bacterium could revert to the virulent type or remain unchanged following subculture (37). Because this phenomenon enabled the bacterium to acquire a novel heritable phenotype, Griffith coined the term “transformation principle” to describe the phenotypic changes he observed.

In his classic experiment, Griffith studied a highly infective, encapsulated S strain that formed smooth colonies, and an avirulent R strain, which had no capsule and formed rough colonies when grown on blood agar (37). When healthy mice were injected with the S strain, they died of septice- mia, whereas separate admission of the R strain or the heat-killed S strain appeared to be harmless. However, when the live R strain and the heat-killed S strain were injected simultaneously, the mice died. Surprisingly, when blood samples drawn from these dead animals were analyzed, both R and S live strains were detected. Based on these results, Griffith concluded that a “transforming factor”, present in the heat-killed S strain, was able to “transform” an avirulent R strain into a capsulated, virulent S strain.

Over the next few decades, Griffith's inspiring work on transformation was followed up by a number of scientists.