  An analysis of adiabatic perturbations of a perfect fluid is performed to
first-order about a general FLRW background using the 1+3 covariant and
gauge-invariant formalism. The analog of the Mukhanov-Sasaki variable and the
canonical variables needed to quantise respectively the scalar and tensor
perturbations in a general FLRW background space-time are identified. The
dynamics of the vector perturbations is also discussed.
