A student investigated the effectiveness of a household disinfectant spray designed to clean kitchen worktops. The student swabbed a section of the worktop before and after using the disinfectant spray, then transferred the samples onto nutrient agar plates and incubated them. After 48 hours, some colonies were still present on the plate inoculated with the 'after' swab. Suggest two reasons why this investigation does not prove that the disinfectant kills all bacteria on the kitchen worktop.
