What Is a Bioreactor?

A bioreactor is a vessel that provides a controlled environment in which living cells or microorganisms grow and produce a biological product such as a therapeutic protein, monoclonal antibody, vaccine, or enzyme. It regulates temperature, pH, dissolved oxygen, and agitation so the culture stays productive and viable. In biomanufacturing, the bioreactor is the core of the upstream process, and its design and installation are governed by cGMP and ASME BPE hygienic-equipment practice.

How a bioreactor works

A bioreactor holds a liquid culture medium that supplies nutrients to the cells or microorganisms growing inside it. To keep that population productive, the vessel actively controls four core parameters: temperature, pH, dissolved oxygen, and mixing. Temperature is typically maintained through a jacket or coil that circulates heating or cooling fluid around the vessel wall. pH is held within a narrow band by dosing acid or base, and dissolved oxygen is supplied by sparging gas into the medium through a distributor at the base of the tank.

Agitation is central to how the vessel performs. In a stirred-tank design, one or more impellers on a central shaft mix the medium so that nutrients, gases, and cells stay evenly distributed and no part of the culture becomes starved or overheated. The agitation rate is balanced carefully: enough to keep oxygen and nutrients dispersed, but gentle enough to avoid shear damage to fragile cells, which matters especially for mammalian cell lines. Sensors and probes feed pH, dissolved oxygen, temperature, and sometimes cell density back to a control system that adjusts sparging, dosing, jacket temperature, and stirring in real time.

Because the product is a biological molecule made by living cells, contamination is the primary threat. The vessel and all connected piping must be sterilizable and cleanable to hygienic standards, which is why sanitary vessel design, clean-in-place (CIP) and steam-in-place (SIP) capability, and crevice-free surfaces are as important to a working bioreactor as the process controls themselves.

Types of bioreactors and their upstream role

The most common configuration is the stirred-tank bioreactor, used across both microbial and mammalian processes. When the culture is bacteria or yeast run at high growth rates, the vessel is often called a fermenter; when it grows mammalian cells such as CHO lines to produce antibodies, it is generally called a cell-culture bioreactor. Both share the same basic principle but differ in oxygen demand, agitation intensity, and how gently the cells must be handled.

A major distinction is between reusable stainless-steel bioreactors and single-use bioreactors (SUBs), which use a pre-sterilized disposable plastic bag inside a support structure. Single-use systems remove the need for cleaning and cleaning validation between batches and reduce cross-contamination risk, making them well suited to multi-product facilities and smaller volumes. Stainless-steel vessels remain the workhorse at large production scale, where their durability and capacity are more economical over many batches. The bioreactor sits at the heart of the upstream portion of biomanufacturing: cells are expanded through a seed train of progressively larger vessels, cultured to the target density in the production bioreactor, and then handed off to downstream steps such as clarification, filtration, and chromatography for purification.

Single-use vs stainless-steel bioreactors

Standards & references