The present disclosure provides a culture device for enumerating colonies of microorganisms. The device can comprise a base, a coversheet, and a nonporous spacer member disposed therebetween. The spacer member comprises an aperture that defines a growth compartment. Disposed in the growth compartment are a cold water-soluble gelling agent, a dry oxygen-scavenging reagent, a dry buffer system, and an effective amount of a dry carbon dioxide-generating reagent. The buffer system is selected such that, when the growth compartment is hydrated with a predetermined volume of deionized water, an aqueous mixture with a pH less than or equal to 6.35 is formed.