Pharmaceutical Microbiology Quality Control

Bioburden testing counts the viable microorganisms in or on a material before sterilization or further processing. It is required at raw material receipt, at in-process hold points, on water systems, and immediately before sterilization or sterile filtration.

Since 1 December 2025, USP <1119> - Bioburden Monitoring* and <1119.1> - Bioburden Test have governed this testing, replacing chapter <1229.3>. They exist to correct a common error: USP <61> is not interchangeable with bioburden testing. USP <61> covers release testing of non-sterile products, while <1119> covers process monitoring and extends to sterile products, in-process material, containers, and pharmaceutical water. Unlike USP <61>, it is risk-based sampling sites, frequencies, and limits must be scientifically justified. Ph. Eur. 2.6.12 and JP 4.05 apply outside the US.

Sterility tests must detect aerobic bacteria, anaerobic bacteria, yeasts, and molds. Fluid Thioglycollate Medium is incubated at 30-35°C for anaerobes; Soybean Casein Digest Medium at 20-25°C for aerobes and fungi. Incubation runs 14 days.

Growth promotion testing confirms the media support these organisms. FTM is challenged separately with Clostridium sporogenes, Pseudomonas aeruginosa, and Staphylococcus aureus; SCDM with Aspergillus brasiliensis, Bacillus subtilis, and Candida albicans, each at no more than 100 CFU. The test is read visually and is pass/fail: any macroscopic growth within the 14 days fails and triggers a sterility failure investigation before batch disposition. USP <71> and Ph. Eur. 2.6.1 have been harmonized since 2002.

A sterility test confirms the absence of any viable microorganism in finished sterile product: a pass/fail release assay under USP <71>. A bioburden test counts the organisms present before sterilization: a quantitative process control under USP <1119> series.

Products with antimicrobial activity are filtered through a 0.45 µm membrane and rinsed with sterile diluent to remove inhibitory residue before incubation. Oily preparations are first dissolved in a sterile, non-inhibitory solvent such as isopropyl myristate.

The rinse is the step most often under-validated. USP <71> permits up to five 100 mL portions per membrane, with three a common starting point. Fluid A is the general-purpose rinse; Fluid D adds polysorbate 80 for oils and poorly wettable matrices; beta-lactams require penicillinase alongside rinsing rather than instead of it. Rinse volume, rinse fluid, and any inactivator must be fixed by the method suitability test, not adjusted per batch. Closed systems such as Sterisart perform these steps through a sealed fluid path, reducing exposure during the longer handling these matrices require.

A MST proves your product does not inhibit microbial growth under your test conditions.  This was formally called the bacteriostasis and fungistasis (B&F) test.  Without conducting this test, one cannot tell whether nothing grew because nothing was there, or because the product suppressed it.  It also must be conducted on every new or reformulated product.

Requirements changed in 2026. Ph. Eur. 2.6.7 version 12.2 came into force on 1 April 2026, and USP <77> becomes official on 1 October 2026. Both recognize NAT methods as equivalent to culture and indicator cell culture, rather than as alternatives requiring comparability.  In Ph. Eur. 2.6.7, there is more emphasis on the workflow.

The sensitivity criterion is now a limit of detection of 10 CFU/mL or <100 genomic copies/mL, introducing genomic copies as a standardized unit for the first time. Validation material must have CFU determined at harvest without prior freezing, with a GC/CFU ratio not exceeding 10. Testing should cover both cells and supernatant, defined controls including an external positive control are mandatory, and emphasis has shifted from method comparison toward whole-workflow validation.

VBNC mycoplasmas are metabolically active but will not form colonies under standard culture conditions, so a 28-day culture test can return a clean result on a contaminated batch. Ph. Eur. 2.6.7 now explicitly requires detection of both culturable and non-culturable mycoplasmas.

That effectively ends culture-only strategies: a method blind to the non-culturable fraction no longer satisfies the chapter on its own. qPCR, RT-qPCR, and digital PCR are the recognized route and may replace culture, indicator cell culture, or both when properly validated. Exposure is highest in cell and gene therapies, where shelf life is shorter than a 28-day test, and in any process using serum-derived raw materials.

USP <62> is a qualitative absence test rather than a count. It requires specified organisms to be absent from a defined sample quantity, with the organism panel set by the product's route of administration.

Note that "objectionable" may extends beyond the listed organisms as the FDA has taken enforcement action over Burkholderia cepacia complex (BCC) in aqueous non-sterile products so compliance to USP <62> result is a floor rather than a ceiling. Results are read alongside USP <61>: USP <61> tells you how much, USP <62> tells you what.

Closed systems keep the fluid path sealed from sample introduction through to incubation, removing the manual membrane transfer that is the largest single source of laboratory-induced contamination.

The cost of a false test result is easy to underestimate. One false positive triggers a full sterility failure investigation for that batch; while one false negative triggers a full sterility failure investigation since the last validation of the test method.  Under EU GMP Annex 1, that investigation must produce a defensible root cause; if it cannot conclusively demonstrate laboratory error, the batch cannot be released. Open canister methods require an operator to open the system and transfer the membrane by hand in the critical zone. Closed systems such as Sterisart eliminate that step.

Inspectors assess microbiology laboratories against 21 CFR 211.194, EU GMP Annex 1, and ICH Q10. The recurring finding is rarely a missing document, it is broken traceability, where records exist but cannot be linked back to a result.

Expect requests for validated SOPs and method validation packages; current per-product method suitability tests; IQ/OQ/PQ, calibration, and maintenance records for incubators, autoclaves, isolators, and cabinets; media preparation records with lot traceability and growth promotion testing of every lot; reference cultures within five passages of the reference lot; analyst training and aseptic qualification; environmental and personnel monitoring with trending; and ALCOA+ raw data with audit trails. A sterility result should trace to its medium lot, analyst, qualification status, session monitoring, and equipment without anyone having to reconstruct it.

USP <63> defines what a mycoplasma test must achieve. USP <77> - Mycoplasma Nucleic Acid Amplification Tests, official 1 October 2026, defines how to prove a NAT method achieves it. USP <77> does not replace <63>; it closes a gap <63> left open.

USP <63> prescribes two compendial procedures: Method A, the agar and broth media procedure, and Method B, the indicator cell culture procedure. It permits a validated NAT or enzymatic method to be used instead, but it never sets out how that validation should be conducted only that the method must be shown comparable to Method A or B. Molecular method development ran well ahead of the chapter, and laboratories were left interpreting comparability for themselves.

USP <77> supplies the missing criteria: validation design, definition of intended use, control strategy, and specificity demonstrated across more than 100 mollicute species. It formalizes the GC-to-CFU ratio as an accuracy metric, aligning with the 10 CFU/mL or <100 genomic copies/mL sensitivity requirement in Ph. Eur. 2.6.7 v12.2 and JP 18 G3. In practice, if you run NAT for release testing, USP <63> remains the reference your method is measured against and USP <77> becomes the yardstick for your validation package.