Sterility and Bioburden

Overview

Sterility and bioburden are two distinct microbiological questions: how many viable organisms are present, and whether any viable organisms can be detected at all. Both are about microbes, not about the intended molecule.

Two different questions

Bioburden testing estimates the quantity of viable microorganisms in or on a material — a count. Sterility testing asks a yes/no-style question under defined conditions: can any viable organisms be detected? A material can have a measurable bioburden without being asked the sterility question, and the two tests are designed and interpreted differently.

What each result describes

A bioburden result characterises microbial load; a sterility result characterises whether viable organisms were detectable under the test’s conditions. Neither describes the identity, purity, or amount of the intended substance, and neither is the same as endotoxin testing — endotoxin can be present even where viable organisms are not, because it is a molecule rather than a living cell.

Limits of a microbiological result

Microbiological tests answer questions about organisms within the scope and conditions of the method used. They do not, on their own, describe the chemical make-up of the material, and they are not a statement about fitness for any use. They are one line of evidence among several that a full picture would consider.

What this test can and cannot establish

Key terminology

Bioburden

The quantity of viable microorganisms present in or on a material.

Sterility

The detectable absence of viable microorganisms under defined test conditions.

Viable organism

A microorganism capable of growth under the test conditions.