An investigation on fruit fly mortality and the carrier state in disease transmission with the bacterium Serratia marcescens.

This study focused on the bacterium Serratia marcescens; specifically to assay its effect on fruit fly mortality, its ability to use the fruit fly as a living reservoir, and its transmissibility between fruit flies. It was hypothesized that this insect pathogen would kill susceptible fruit flies leaving those more resistant in a carrier state capable of transmitting the bacteria to uninfected flies. One unique property of the bacterium is its ability to produce a distinct red pigment that can be used to trace its presence. Experimental methods involved infecting fruit flies with the bacterium by allowing them to ingest the bacterium mixed with their food supply. More susceptible flies developed an infection and died. Fruit flies that survived were tested for the presence of S. marcescens in and on their bodies. Survivors were then successfully used to transmit the bacterium to fresh uninfected flies. This study is important because: [1] it represents an animal model for studying innate immunity to infectious agents (S. marcescens is capable of causing human infections) and [2] it provides a relatively safe, inexpensive classroom exercise to demonstrate the significance of the carrier state in transmission of infectious disease.