The objectives of this work are: 1) to determine whether plant essential oil components influence the ability of Escherichia coli and several Pseudomonas species to form biofilms, and inhibit bacterial quorum sensing; 2) to understand the role of autoinducer-2 (AI-2) in biofilm formation by E. coli W3110. The biofilm formation assays determined that cinnamon, cassia and citronella oils differentially affected growth-normalized biofilm formation by E. coli. Cinnamaldehyde (CA) also inhibited the swimming motility of E. coli. Subinhibitory concentrations of CA were effective at inhibiting two types of acyl homoserine lactone (HSL) mediated quorum sensing (QS), and also AI-2 mediated QS. Because CA is widely used in the food and flavor industries, its potential to affect bacterial QS regulated processes should be recognized. The role of AI-2 mediated QS expression in physiology of E. coli W3110 was pleiotropic, including carbon utilization, fimbriae production, and the biofilm development. Overall, the research presented in this dissertation supported the concept that QS, biofilm formation, and cell adhesion may be broadly correlated. The anti-biofilm and anti-QS capability of CA implies that plant essential oil components might be promising for preventing the formation of detrimental biofilms.