Glossary

Glossary of pharma microbiology terms

61 terms, explained the way you would in an interview.

A

ALCOA+: The data integrity principles that GMP records must meet: Attributable (who did it and when), Legible (readable for its whole life), Contemporaneous (recorded at the time), Original (the first record or a certified copy) and Accurate (correct and checked). The plus adds Complete, Consistent, Enduring and Available. In a micro lab, ALCOA+ applies to worksheets, plate counts, incubator records and instrument data, and auditors use it to judge whether results can be trusted.
ATCC: American Type Culture Collection, a supplier of reference microbial strains. Compendial tests specify strains by ATCC numbers, such as Staphylococcus aureus ATCC 6538 or Candida albicans ATCC 10231, or by equivalent numbers from other collections such as NCTC, NCIM, MTCC or NBRC. Labs buy these as reference cultures and maintain them through a seed-lot system, keeping working cultures within five passages of the original.
Action level: An established microbial or particle level that, when exceeded, requires immediate investigation and corrective action. In EU GMP Annex 1, action limits for each cleanroom grade are given in Table 5 for particles and Table 6 for microbes; for water, USP <1231> gives typical action levels. Exceeding an action level does not automatically mean a batch fails, but it does require a documented investigation, an impact assessment on product made during the period, and CAPA.
Alert level: A site-defined level, below the action level, that gives early warning of a drift from normal conditions. It is usually derived from the site's own historical data, for example the 95th percentile of a year of results, and should be reviewed at least yearly. Reaching an alert level triggers closer attention, trending and sometimes resampling, but not necessarily a full investigation. Repeated alert-level results at one location are often treated like an action-level excursion.
Aseptic process simulation (APS): Also called a media fill: sterile growth medium is processed through an aseptic process in place of product, with the same equipment, people, interventions and durations, to show the process does not introduce contamination. Filled units are incubated, usually for 14 days, and inspected for growth. Under EU GMP Annex 1, the target is zero contaminated units, and the APS is repeated twice a year for each line, process and shift.
Aseptic technique: Working practices that prevent micro-organisms from contaminating sterile materials. Examples include slow, deliberate movements in Grade A, never blocking first air over open containers, keeping hands below and away from critical sites, sanitising gloves frequently with sterile alcohol, using sterile tools rather than hands, and keeping the number of items in the workspace to a minimum. Aseptic technique is assessed during operator qualification and observed by inspectors on the shop floor.

B

Bioburden: The number and type of viable micro-organisms on or in a product, component or material before sterilisation. Bioburden is tested to set and justify sterilisation cycles, especially for heat-sensitive products using a bioburden-based approach, and to monitor the cleanliness of the manufacturing process. For aseptically filtered products, the bioburden of the bulk solution before filtration has a limit, often 10 CFU/100 mL, and is tested by membrane filtration.
Biofilm: A community of micro-organisms attached to a surface and embedded in a slimy matrix they produce themselves. Biofilms form readily in water systems, drains and wet equipment. They protect bacteria from disinfectants and heat, are very hard to remove once established, and shed bacteria and endotoxin into the water intermittently. Good design with continuous turbulent flow, no dead legs and regular sanitisation is the best way to prevent them.
Biological indicator (BI): A carrier, such as a paper strip, steel disc or self-contained vial, with a known population of highly resistant spores used to challenge and validate a sterilisation or decontamination process. Examples are Geobacillus stearothermophilus for steam and hydrogen peroxide, and Bacillus atrophaeus for dry heat and ethylene oxide. BIs are placed at the hardest-to-sterilise locations, and their failure to grow after the cycle shows the required lethality was achieved.
Bowie-Dick test: A daily test of air removal and steam penetration in pre-vacuum steam sterilisers. A standard test pack with an indicator sheet is processed in an empty chamber, usually first thing in the morning. A uniform colour change on the sheet shows that air was removed and steam reached the centre of the pack. A lighter patch in the centre indicates residual air, a leak or poor steam quality, and the steriliser must not be used until the problem is fixed.

C

CAPA: Corrective and preventive action. Corrective action removes the cause of a problem that has happened so that it does not recur; preventive action removes the cause of a potential problem before it happens. Each CAPA should be linked to a root cause, have an owner and a due date, and include an effectiveness check with a measurable target. Regulators often criticise CAPAs that rely only on retraining without addressing system causes.
CFU: Colony-forming unit: a viable cell, or a cluster of cells, that grows into one visible colony on solid medium. Because clumps of cells can form a single colony, a CFU count estimates viable organisms rather than counting individual cells. Results are expressed per volume or area sampled, such as CFU/mL for water, CFU/m³ for air or CFU/plate for surfaces. Countable ranges, such as 25–250 colonies for TAMC plates, keep counts reliable.
Change control: A formal system to propose, evaluate, approve, implement and verify changes that could affect product quality or the validated state. Typical changes include new equipment, a different disinfectant, a new media supplier, a revised test method or changes to EM locations. The impact on validation, other products and regulatory filings is assessed before approval, and the change is checked after implementation. Uncontrolled changes are a frequent audit finding.
Cleanroom: A room in which the concentration of airborne particles and micro-organisms is controlled to specified limits, and in which temperature, humidity and pressure are also controlled. Cleanrooms are supplied with HEPA-filtered air, kept at positive pressure relative to less clean areas, and classified under ISO 14644-1 and EU GMP grades A to D. People entering must follow gowning procedures, because they are the largest source of contamination.
Contact plate: A plate with a raised, convex agar surface, typically 55 mm in diameter (about 25 cm²), pressed onto a flat surface to sample micro-organisms. The agar usually contains neutralisers such as lecithin and polysorbate 80 to inactivate disinfectant residues. Contact plates are used for surfaces, gowns and gloves, applied with even pressure for a few seconds at the end of operations. The surface is wiped afterwards to remove agar residue.
Contamination Control Strategy (CCS): A planned, site-wide set of controls for micro-organisms, endotoxins and particles, required by EU GMP Annex 1 (2022). It covers facility and equipment design, utilities, raw materials, personnel, cleaning and disinfection, process controls, monitoring and validation, and explains how they work together. The CCS is a living document, reviewed regularly and after significant changes or adverse trends. Inspectors often ask for it first when reviewing a sterile site.

D

D-value: The time or dose needed under stated conditions to reduce a microbial population by 90%, which is one log. For example, a D-value of 2 minutes at 121 °C means that every 2 minutes at that temperature reduces the surviving population tenfold. D-values depend on the organism, the temperature and the material the organism is in. They are used to choose biological indicators and to calculate the lethality needed for a target sterility assurance level.
Dead leg: A section of piping where water does not circulate, such as a branch leading to a rarely used valve. Stagnant water in a dead leg allows bacteria to attach and form biofilm, which can then contaminate the main loop. Good design keeps dead legs as short as possible, commonly less than three pipe diameters from the main flow, and uses zero-dead-leg valves at points of use. Dead legs are checked during installation qualification and system walk-downs.
Depyrogenation: A process that removes or destroys pyrogens such as bacterial endotoxins. The most common method is dry heat at high temperature, for example 250 °C for at least 30 minutes, in an oven or tunnel for glassware and metal parts. Rinsing with WFI can remove endotoxin from some components. Depyrogenation is validated with endotoxin indicators placed in the coldest locations, and must show at least a 3-log reduction of endotoxin.
Deviation: A departure from an approved instruction, procedure, specification or standard, whether planned or unplanned. Deviations are recorded promptly, assessed for impact on product quality, classified by risk, investigated in proportion to that risk and closed with corrective and preventive actions where needed. Examples in micro labs include incubator temperature excursions, missed samples and media used past expiry. Deviation trends help reveal recurring system problems.
Direct inoculation: A sterility test method in which the product is added directly to the culture media, FTM and SCDM, and incubated for at least 14 days. It is used when the product cannot be filtered, such as some suspensions, ointments and medical devices. The product volume must not exceed 10% of the medium volume. If the product is antimicrobial, it must be neutralised or diluted enough not to inhibit growth, as shown by method suitability.
Disinfectant efficacy test: A study proving that a disinfectant, at its in-use concentration and contact time, reduces relevant organisms on representative surfaces. It usually combines suspension tests, surface (coupon) tests on materials such as stainless steel, glass and vinyl, and in-situ EM comparisons. The challenge panel includes standard strains and in-house isolates, with at least one spore-former and one mould. Typical targets are 3-log reduction for vegetative bacteria and 2-log for spores.

E

EU (endotoxin unit): The unit of endotoxin activity, defined against the reference standard endotoxin (RSE). Because endotoxins from different bacteria vary in potency, measuring them by weight would be misleading, so activity units are used instead. Roughly, 1 EU corresponds to about 0.1–0.2 ng of reference endotoxin, but the exact relationship for a control standard endotoxin depends on the lysate lot and is stated on its certificate. Limits are written as EU/mL, EU/mg or EU per unit.
Endotoxin: Lipopolysaccharide from the outer membrane of Gram-negative bacteria, released when cells die or divide. Endotoxin is heat-stable and pyrogenic: when injected, it causes fever and, at high doses, shock. It survives normal steam sterilisation, so it is controlled by preventing Gram-negative growth in water and materials and by depyrogenation. Parenteral products and WFI are tested for endotoxin using the bacterial endotoxin test.
Endotoxin limit: The maximum permitted endotoxin in a product, calculated as K/M. K is the threshold pyrogenic dose, 5 EU/kg for most parenterals and 0.2 EU/kg for intrathecal products, and M is the maximum dose per kg of body weight given in one hour. The limit is expressed per mg, per mL or per unit of product. Together with the lysate sensitivity, it determines the maximum valid dilution for the test.
Environmental monitoring (EM): Routine sampling of air, surfaces and personnel to verify that cleanrooms remain in a state of microbial and particulate control. EM uses active air sampling, settle plates, contact plates, swabs, glove prints and particle counters, at locations and frequencies set by risk assessment. Results are compared with alert and action levels and trended over time. EM does not make product sterile, but it shows whether the controls that protect product are working.

F

First air: Filtered air that has not touched any surface or object before reaching the critical zone, such as open vials or the stopper bowl. In unidirectional airflow, first air sweeps particles away from exposed product. Placing a hand, tool or container between the HEPA filter and the critical site blocks first air and can carry contamination downstream onto the product. Protecting first air is a core principle of aseptic technique and is checked with smoke studies.
Fluid Thioglycollate Medium (FTM): A sterility test medium that supports anaerobic and aerobic bacteria, incubated at 30–35 °C for at least 14 days. Thioglycollate and a small amount of agar lower the oxygen level, so anaerobes can grow in the deeper part while aerobes grow near the surface. Resazurin acts as an oxygen indicator, turning pink when oxidised. If more than one third of the medium is pink before use, it can be restored once by heating.
F₀ value: The equivalent sterilisation time in minutes at 121.1 °C delivered by a heat process, calculated using a z-value of 10 °C. F₀ adds up the lethality of the whole cycle, including heating and cooling, converted to time at 121.1 °C. It allows cycles at different temperatures to be compared. Overkill cycles often target an F₀ of at least 15 minutes, while heat-sensitive products may use a lower F₀ justified by low bioburden.

G

Gel clot method: A bacterial endotoxin test method in which endotoxin causes Limulus amebocyte lysate to form a firm gel. Sample and lysate are mixed in tubes and incubated undisturbed at 37 °C for 60 minutes. The tube is then inverted 180°: a firm gel that stays intact is positive, while no gel or a broken gel is negative. It is simple and robust, and is the referee method when results from different BET methods disagree.
Glove print: A personnel monitoring sample taken by pressing all five fingertips of a gloved hand onto an agar contact plate or 90 mm plate. Glove prints are taken after critical interventions and when operators leave the aseptic area, and they show how well operators keep their gloves free of contamination. Under EU GMP Annex 1, the limit is no growth in Grade A and 5 CFU per glove in Grade B. Gloves are changed after sampling.
Grade A: The cleanroom grade for high-risk operations such as aseptic filling, stoppering, open ampoules and aseptic connections, usually under unidirectional airflow within a RABS or isolator. It must meet ISO 5 particle limits at rest and in operation, and under EU GMP Annex 1 no microbial growth is expected in any sample. Any recovery is investigated and identified to species. Grade A must be monitored continuously during critical processing.
Growth promotion test (GPT): A test showing that a batch of culture medium supports growth of specified organisms at a low inoculum, not more than 100 CFU. On solid media, recovery must be within a factor of 2 of an inoculum control or approved batch; in liquid media, growth must be clearly visible. Selective and differential media also need inhibitory and indicative checks. GPT is done on every batch, whether prepared in-house or bought ready to use.

H

HEPA filter: High-efficiency particulate air filter; removes at least 99.95–99.97% of particles at the most penetrating particle size, depending on the standard used, such as EN 1822 or IEST. HEPA filters supply clean air to cleanrooms, LAF units, biosafety cabinets and isolators. Their integrity is tested after installation and periodically, typically every 6 or 12 months, by challenging them with an aerosol and scanning the downstream face for leaks.

I

Inhibition/enhancement test: A bacterial endotoxin test suitability study proving that the product does not interfere with detection of endotoxin. The product, at the intended dilution, is spiked with known amounts of endotoxin. For the gel clot method, the endpoint must be between 0.5λ and 2λ; for photometric methods, recovery must be 50–200%. If the product inhibits or enhances, it is diluted within the maximum valid dilution or treated, and the test is repeated.
Isolator: A sealed enclosure that separates the operator from the critical zone, decontaminated with a validated sporicidal process such as vaporised hydrogen peroxide, and operated through gloves or half-suits. Isolators are used for aseptic processing and sterility testing because they greatly reduce contamination from people. They are usually placed in a Grade C or D background. Glove integrity, decontamination cycles and pressure are monitored and documented.

L

LAL (Limulus amebocyte lysate): A reagent made from the blood cells of the horseshoe crab, Limulus polyphemus, that reacts with bacterial endotoxin through an enzyme cascade. Depending on the formulation, the reaction produces a gel, turbidity or colour. LAL is the basis of most bacterial endotoxin test methods described in USP <85> and Ph. Eur. 2.6.14. Each lysate lot has a stated sensitivity, which the lab confirms before use.
Laminar (unidirectional) airflow: Air moving in a single direction, usually vertically downward, at a uniform velocity across the working area, sweeping particles away from the critical zone. Unidirectional airflow is used in Grade A zones, LAF benches and biosafety cabinets. Annex 1 gives a guidance velocity of 0.36–0.54 m/s at the working position. Smoke studies show whether the flow stays unidirectional around equipment and during interventions.

M

MVD (maximum valid dilution): The maximum dilution of a product at which the endotoxin limit can still be detected, calculated as (endotoxin limit × concentration of the test solution) ÷ λ. Diluting a product reduces interference with the BET, but diluting beyond the MVD would mean endotoxin at the limit could no longer be detected. A more sensitive lysate, with a lower λ, gives a larger MVD and more room to overcome interference.
Media fill: See aseptic process simulation: an aseptic process run with sterile growth medium instead of product, using the same equipment, people, interventions and durations as routine production. Filled units are incubated and inspected for growth, and the target is zero contaminated units. Media fills validate aseptic processes and qualify the operators who work in them, and are repeated at least twice a year for each line and shift.
Membrane filtration: A method in which a liquid is filtered through a membrane, typically 0.45 µm, that retains micro-organisms, which are then incubated on agar or in liquid media. It is used for sterility testing, water testing, bioburden and MLT of antimicrobial products. Rinsing the membrane removes inhibitory substances, and large volumes can be tested to improve sensitivity. The membrane type and rinse procedure are confirmed in method suitability.
Method suitability: An experiment showing that a microbiological method recovers organisms in the presence of the product. Low numbers of test organisms, not more than 100 CFU, are added to the product prepared as in routine testing, and recovery is compared with a control without product. For counts, recovery must be within 50–200%; for sterility tests, growth must be visibly comparable. If not, dilution, neutralisers or filtration are used until the method passes.
Microbial limit test (MLT): Tests for the number of micro-organisms, TAMC and TYMC, and for the absence of specified organisms such as E. coli, Salmonella, P. aeruginosa and S. aureus in non-sterile products. MLT follows USP <61> and <62>, with acceptance criteria by dosage form from USP <1111>. It is performed on raw materials, intermediates and finished products such as tablets, syrups and creams, after method suitability has been shown for each product.

N

Neutraliser: A substance that stops the antimicrobial activity of a product, preservative or disinfectant residue so that surviving organisms can grow. Common examples are lecithin and polysorbate 80 for quaternary ammonium compounds and parabens, sodium thiosulfate for chlorine and iodine, and histidine for aldehydes. Neutralisers are added to diluents, rinse fluids and contact plates. Their efficacy and lack of toxicity to organisms must be shown in method suitability or disinfectant studies.

O

OOS (out of specification): A test result that falls outside an established specification for a product or material. In microbiology, examples include a TAMC above the limit, the presence of a specified organism, a positive sterility test or an endotoxin result above the limit. An OOS triggers a phased investigation: first a laboratory investigation, then a full investigation of manufacturing if no clear lab error is found. A result can only be invalidated with documented evidence.
Objectionable organism: An organism that could harm patients or the product, considering its type, number, the product, the route of administration and the patient population. The idea goes beyond the pharmacopoeia's list of specified organisms. For example, Burkholderia cepacia complex is objectionable in oral liquids and nasal sprays even though it is not on the USP <62> list. Decisions are documented in a risk assessment and often reflected in product specifications.
Overkill cycle: A sterilisation cycle designed to deliver at least a 12-log reduction of a highly resistant biological indicator, such as Geobacillus stearothermophilus spores, regardless of the actual bioburden. Because real bioburden is usually much lower and less resistant, an overkill cycle gives very high sterility assurance. It is used for heat-stable items like equipment parts, glassware, garments and culture media. Heat-sensitive products use a bioburden-based approach instead.

P

PUPSIT: Pre-use post-sterilisation integrity testing of a sterilising-grade filter, expected by EU GMP Annex 1. The filter is integrity tested after it has been sterilised and installed, but before product is filtered, to detect damage from sterilisation or handling. Critics argue the extra connections add contamination risk downstream. Where PUPSIT is not feasible, the site must justify it with a risk assessment and other controls, and always test after use.
Passage: One subculture of a micro-organism from one medium to fresh medium. Every passage carries a small risk of genetic change, so reference cultures used for GPT, method suitability and identification are limited to five passages from the original reference culture. Labs track passages with a seed-lot system: master stocks, working stocks and a log recording the passage number of every culture used.
Purified Water (PW): Pharmaceutical-grade water for non-parenteral uses, such as oral and topical products, cleaning and feed for WFI generation. It is usually produced by reverse osmosis and electrodeionisation from potable water, and must meet conductivity and TOC requirements. The typical microbial action level is 100 CFU/mL. PW systems are monitored at points of use and trended, and controlled by circulation, UV and periodic sanitisation.

R

R2A agar: A low-nutrient medium used to recover stressed, slow-growing bacteria from water systems. Organisms adapted to pure water often grow poorly on rich media, so R2A with longer incubation, typically 5–7 days, gives more accurate counts. It is used for Purified Water and WFI testing, often by membrane filtration. As with any medium, the choice and incubation conditions should be supported by a comparison study and growth promotion testing.
RABS: Restricted access barrier system: a rigid wall with glove ports around a Grade A zone, operated within a Grade B room. Operators work through gloves instead of reaching directly into the critical zone, which reduces contamination from people. An active RABS has its own air supply; a closed RABS is not opened during processing. RABS sit between conventional cleanrooms and isolators in terms of protection and are widely used for aseptic filling.
Recombinant Factor C (rFC): An animal-free reagent for endotoxin testing that uses Factor C, the first enzyme of the LAL cascade, produced by recombinant technology. When endotoxin activates Factor C, it cleaves a fluorescent substrate, and the signal is measured. rFC avoids harvesting horseshoe crab blood and gives consistent lot-to-lot performance. It is described in Ph. Eur. 2.6.32 and USP. Switching from LAL needs a comparability study for each product.

S

SCDM (Soybean-Casein Digest Medium): A general-purpose medium, also called tryptic soy broth (TSB) or tryptic soy agar (TSA), that supports a wide range of bacteria and fungi. The broth is used in sterility testing at 20–25 °C and in media fills, and the agar is used for TAMC in MLT and for environmental monitoring, often with added neutralisers. It is also the enrichment broth in USP <62> tests for specified organisms.
Settle plate: A 90 mm agar plate left open to collect micro-organisms falling from the air by gravity, for up to 4 hours. Settle plates are used in Grade A and B throughout critical operations because they give continuous coverage. Results are in CFU per 4 hours: no growth in Grade A and 5 CFU in Grade B under Annex 1. The site must show that the agar does not dry out and lose recovery over the exposure time used.
Sporicidal agent: A disinfectant that kills bacterial spores, such as hydrogen peroxide combined with peracetic acid, or sodium hypochlorite. Routine disinfectants like alcohols and quaternary ammonium compounds do not kill spores, so EU GMP Annex 1 requires periodic use of a sporicide in cleanrooms. Its frequency should be based on EM data and the contamination control strategy. Sporicides can be corrosive and leave residues, so rinsing may be needed.
Sterility assurance level (SAL): The probability that a single unit is non-sterile after sterilisation, usually 10⁻⁶, one in a million, for terminally sterilised products. SAL cannot be measured by testing finished product; it is calculated from the bioburden, its resistance and the validated lethality of the process. Aseptically processed products cannot claim a numerical SAL in the same way, and rely instead on process validation, media fills and environmental control.

T

TAMC: Total aerobic microbial count, determined on Soybean-Casein Digest Agar at 30–35 °C for 3–5 days, as described in USP <61>. Any fungi that grow on this medium are included in the count. TAMC is reported as CFU per gram or mL and compared with the acceptance criterion for the dosage form, for example 10³ CFU/g for non-aqueous oral products, which allows a maximum of 2,000 CFU/g.
TYMC: Total combined yeasts and moulds count, determined on Sabouraud Dextrose Agar at 20–25 °C for 5–7 days, as described in USP <61>. Any bacteria that grow on SDA are included in the count, and SDA with antibiotics may be used if justified. TYMC is reported as CFU per gram or mL and compared with the acceptance criterion, for example 10² CFU/g for non-aqueous oral products, allowing a maximum of 200.

W

Water for Injection (WFI): Water for parenteral preparations and for final rinsing of product-contact equipment in sterile manufacturing. It has a typical microbial action level of 10 CFU/100 mL, an endotoxin limit below 0.25 EU/mL, and must meet conductivity and TOC requirements. WFI is traditionally produced by distillation, though membrane-based methods are now accepted in many markets, and is usually stored and circulated hot at 65–80 °C.

Z

z-value: The temperature change needed to change the D-value by a factor of 10. If an organism's z-value is 10 °C, raising the temperature by 10 °C makes it ten times faster to kill. The z-value lets you convert time at one temperature into equivalent lethality at another, which is the basis of F₀ calculations. By convention, 10 °C is used for steam F₀ calculations and 20 °C is common for dry heat.