A microbial detection assembly is disclosed which includes a tubular body, a pierceable septum, and a plunger. The tubular body defines a longitudinal axis and has a first end and a second end. The pierceable septum is configured to seal the first end of the tubular body. The plunger head is configured to substantially close the second end of the tubular body. Together, the plunger head, the pierceable septum, and the tubular body define a reservoir dimensioned to receive a medium and a sample substance. Additionally, the plunger head is positioned and is movable within the tubular body in response to changes in pressure within the reservoir.