Bacteria are important examples of active colloids or self-propelled colloids. Because of their directed motion, they accumulate at solid or fluid boundaries. We study the well characterized bacterium Pseudomonas aeruginosa PAO1 on or near hexadecane-water interfaces in a dilute system that allows us to study individual bacteria. We characterize their ensemble behavior. In addition, we study individual bacteria, and observe four distinct trajectory types, including one type that is not adsorbed and three types of motion for adsorbed bacteria. The mobility of the fluid interface allows adsorbed bacteria to swim in the interfacial plane. However, the interface imposes important constraints that alter the bacteria motion. We characterize these swimming modalities and relate the observed motion to the adsorbed bacteria state. This research has implications in the fields of synthetic active colloids, microrobotics and interface engineering.