inokulum

Inokulum is a term used in microbiology and related fields to denote the material used to initiate a culture or infection in a controlled setting. The inoculum is the small quantity of viable microorganisms or biological material that is introduced into a growth medium, host organism, or experimental system to begin growth or a biological process. It can be derived from a pure culture, a colony fragment, a clinical specimen, an environmental sample, or a standardized preparation of cells or spores.

Preparation and standardization of the inoculum involve selecting the source material and suspending it in a sterile diluent or growth medium. To achieve reproducible results, laboratories often adjust the suspension to a defined density, using turbidity standards such as the 0.5 McFarland standard, which for many bacteria corresponds to about 1.5×10^8 CFU per milliliter. Depending on the application, the inoculum may be a small, precisely measured volume or a portion of a culture.

Inoculation methods vary and include introducing the inoculum into agar plates, broth cultures, or susceptible host organisms. Inoculum size influences growth dynamics, detection time, and experimental outcomes; too little may fail to establish growth, while too much can cause confluent growth or distort results. In antimicrobial susceptibility testing and other standardized assays, specific inoculum densities are mandated by guidelines from organizations such as CLSI and EUCAST.

Applications span diagnostic microbiology, basic and applied research, vaccine and fermentation industries, and plant pathology. Safety, sterile technique, and accurate labeling are essential to manage risks and ensure traceability.

Etymology: inoculum comes from Latin, meaning a thing to be planted or implanted.