Bioburden testing

Bioburden is the count of viable microorganisms in or on a material, reported as colony forming units per gram or per milliliter. It answers a narrow question: how many organisms are alive in this lot. It is distinct from sterility, which is a pass or fail claim about the absence of any viable organism, and from endotoxin, which measures a bacterial cell wall fragment that remains active long after the bacteria are dead.

Those three tests are routinely confused, and the confusion causes real problems, because a material can pass one and fail another. Autoclaved buffer has zero bioburden and can carry enough endotoxin to activate every cell in a plate.

How the measurement works

The sample is dissolved or suspended, filtered or plated, and incubated on growth media. Total aerobic microbial count uses a general medium at around thirty degrees for three to five days. Total yeast and mold count uses a selective medium at a lower temperature for five to seven days. Both numbers are usually reported.

The method has to be validated for the specific material, because the material itself can inhibit growth. A peptide with antimicrobial activity, and many cationic amphipathic peptides have some, will suppress the colonies it is supposed to be reporting and produce a reassuring zero. Suitability testing spikes a known low count of reference organisms into the sample and confirms they are recovered. Without that control, a bioburden result on an antimicrobial peptide means nothing.

Where the organisms come from

For a synthetic peptide the synthesis itself is not the source. Organic solvent, trifluoroacetic acid and acetonitrile are hostile environments. Contamination enters downstream, in the aqueous steps: purification buffers, the lyophilizer, the vialing environment and the operator.

For a recombinant protein the picture differs. Expression is in a live culture, so the question becomes how completely the host and the fermentation environment were cleared, and bioburden after each purification step is a process control rather than only a release test.

Storage matters in both cases. A lyophilized powder held dry is a poor growth environment. The same material reconstituted in an unpreserved aqueous buffer and left at four degrees for a month is a different proposition, and the bioburden on the certificate describes the powder rather than your working stock.

When to specify it

For a peptide or protein used in biochemical assays, in a plate reader or on a chip, bioburden is generally not worth paying for. The material is dilute, the assay is short, and contamination shows up as an obvious failure.

For anything entering cell culture it matters, and endotoxin matters more. Antibiotics in the medium mask low-level bacterial contamination while the endotoxin those bacteria produced continues to drive cytokine responses, differentiation and apoptosis, which then get attributed to the molecule. For primary cells and immune assays this is the single most common cause of results that will not reproduce.

For in vivo work both bioburden and endotoxin are required, and the endotoxin limit is the binding one. The general guidance is five endotoxin units per kilogram of body weight per hour, which for a mouse study translates to a limit that is easy to exceed with research-grade material that was never tested.

For any material intended to support a regulatory filing, bioburden becomes a specified in-process and release test with justified limits, and the validation of the method becomes as important as the result.

What to ask a supplier

Ask whether bioburden was tested or assumed, because research-grade certificates frequently carry neither. Ask whether method suitability was performed on this material rather than on a generic reference. Ask for endotoxin separately and in endotoxin units per milligram, not as a pass or fail. Ask what the material was lyophilized and vialed into, since that is where the organisms enter.

If the answer to the first question is that it was not tested, that is workable information. Testing an aliquot yourself costs little and takes a week, and it is a great deal cheaper than a failed animal study.