The behaviour of a bacterial population as a whole has traditionally been viewed as the sum of identically behaving cells. This assumption stemmed from the lack of tools and techniques to study bacteria at the single-cell level. The development of microscopy and fluorescent tools has recently challenged this common belief. During this seminar, I will present the bacterial phenotypic heterogeneity associated with two important bacterial processes: (i) horizontal gene transfer of DNA and (ii) virulence of a marine bacterium infecting the European abalone Haliotis tuberculata. The single cell behaviour of bacteria will be described, and the evolutionary origins of these individual cell fates will be discussed.