Student Presenter(s): Manuel Ortiz
Faculty Mentor: Bryan Gibb
Department: Biological and Chemical Sciences
School/College: College of Arts and Sciences, Long Island

Bacteria are microscopic organisms. They can grow quickly in cultures under laboratory conditions where they double every 20–30 minutes. Many experiments with bacteria require that one knows how many bacteria are in a culture prior to starting an experiment. However, enumerating bacteria is a time-consuming and tedious process involving many petri dishes and overnight incubation. We needed a fast, efficient way to enumerate a bacterial culture as it growing rather than after the fact. As bacteria grow, the culture turns cloudy, and bacteria scatter light (much like a fog). If a culture of bacteria is put into a spectrophotometer, the turbidity can be measured, which is quick and easy but is an indirect measure of population size. Although turbidity increases as with bacterial growth, dead cells and cellular debris also contribute to turbidity. We hypothesized that in a young, healthy culture most of the cells will be alive so the turbidity will more accurately reflect the bacterial population. We developed a method for rapidly determining bacterial cell population based on optical density using a calibration curve based on bacterial cell counts. This technique enables us to conduct experiments that demand that we know how many bacteria are in a growing culture at the time we are conducting an experiment. This validated technique is already being put to good use as a key component of experiments characterizing bacteriophages in Dr. Gibb's lab.