Which apparatus is specifically designed to grow bacterial cultures continuously at a defined growth rate and steady state by balancing nutrient feed and effluent removal?

Introduction / Context:
Continuous culture enables long-term study of microbes under constant conditions and is widely used to investigate physiology, evolution, and process optimization. The key device maintains a steady state by controlling dilution rate and limiting nutrients.

Given Data / Assumptions:

• Fresh medium is supplied at a constant rate.
• Culture volume stays constant via matched outflow.
• One essential nutrient is limiting and sets the specific growth rate.

Concept / Approach:
A chemostat achieves steady state when growth rate equals dilution rate. At equilibrium, biomass, residual substrate, and product concentrations remain constant, modeling balanced growth over extended periods. This contrasts with batch culture, which progresses through distinct phases and is not steady.

Step-by-Step Solution:
Identify the need: continuous, steady-state cultivation. Recall device function: feed plus effluent maintain constant volume and conditions. Match names to functions: chemostat fits; others do not. Select “Chemostat.”

Verification / Alternative check:
At steady state, measurements are reproducible across residence times, enabling precise parameter estimation (e.g., Ks, Yx/s).

Why Other Options Are Wrong:

• Coulter Counter: Counts particles electrically; not a culture device.
• Hemostat: Surgical clamp; unrelated to microbiology cultivation.
• Petroff-Hausser chamber: Microscopic counting slide; not continuous culture.

Common Pitfalls:
Confusing “turbidostat” (maintains constant turbidity via feedback) with “chemostat” (sets rate via limiting nutrient and dilution); both are continuous but operate differently.

Final Answer:
Chemostat.