This article presents a decentralized controller/observer for nonlinear large- scale interconnected systems with actuator fault. The proposal integrates a robust proportional-integral-derivative (PID) controller with the unknown input observer (UIO) to achieve closed-loop robustness against the interactions and the actuator faults. In this scheme, the PID controller is tuned using the Bacterial foraging optimization algorithm (BFO) algorithm. On the other hand, the unknown input observer can diagnose the actuator faults from the controller input. A numerical example consisting of two subsystems is adopted to clarify the effectiveness of the suggested method with a guarantee that the state estimation error is asymptotically converged to zero. The actuator faults have been added to the second subsystem, keeping the first subsystem free of fault. The simulation results demonstrated the influence of the interactions between subsystems, verifying that the unknown input observer can detect the actuator faults despite the presence of these interactions between the subsystems.