Abstract: An adaptor cap for a container. The adaptor cap includes a skirt constructed to engage a fluid container and to retain the adaptor cap on the fluid container. The adaptor cap further includes a dome projecting outwardly from said skirt. The dome defines an interior chamber. The dome further defines a spike port providing fluid communication between the chamber and an exterior environment of the dome, the spike port being constructed to receive and retain a hollow spike therein. The dome also defines at least one vent aperture therethrough, the vent aperture providing fluid communication between the chamber and the exterior environment of the dome. The adaptor cap further includes a microbial filter mounted on the dome across the vent aperture.