Of E. coli and classrooms: stories of persistence

Abstract

Plasmids exist in bacteria and are small, extrachromosomal pieces of DNA that often encode accessory genes such as antibiotic resistance genes. They are largely responsible for spreading antibiotic resistance genes through bacterial populations via their ability to conjugate into different bacterial hosts or species. In environments without selection for the plasmid, the proportion of plasmid-containing cells is expected to decrease in the population due to fitness costs associated with plasmid carriage. Yet because of these costs, we also expect that beneficial genes located on plasmids should eventually transition to the chromosome. Thus, the existence of plasmids is puzzling. I explored reasons for their existence and found that 1) co-evolution of hosts and their plasmid can increase plasmid persistence, which has further consequences for the increased emergence of multi-drug resistance when these co-evolved pairs are in bacterial communities, and 2) environments with alternating selection for the plasmid can allow even costly, conjugative plasmids to be maintained in bacterial populations. I also explored the effects of values-affirmation on the reduction of stereotype threat in introductory biology classrooms, and how completing a classroom exercise in which students affirm values they find important to them can reduce the achievement gap in exam scores between underrepresented minority and white students.