Settle Plate Testing for Settling Microorganisms in Laboratory Animal Rooms

Settle plate testing is the primary passive microbiological method used to quantify the deposition of viable airborne microorganisms in laboratory animal facilities. By exposing sterile agar plates for a defined period, operators and accredited third-party laboratories can verify that animal rooms, barrier systems and isolators meet the cleanliness requirements of standards such as GB 14925, GB/T 16294 and ISO 14698. This article explains the technical principles, sampling procedures, acceptance criteria and typical applications of settling microorganism testing in animal housing environments.

Technical Overview

Settling microorganism testing — commonly known as the settle plate, deposition plate or falling bacteria method — is a passive air sampling technique in which sterile nutrient agar plates are exposed at defined locations for a fixed time so that viable airborne particles deposit onto the agar surface through gravity and air currents. After incubation, visible colonies are counted and results are expressed as colony-forming units per plate per exposure period, typically CFU per plate per 30 minutes.

In laboratory animal facilities, deposited microorganisms can contaminate cages, feed, bedding, surgical instruments and working surfaces, threatening the health status of valuable colonies — especially specific pathogen free (SPF) animals — and compromising the validity of pharmacology, toxicology and infection studies. Settle plate testing therefore forms a core part of the environmental monitoring program specified in national and international standards:

• GB 14925 — Laboratory animal: requirements of environment and housing facilities, which defines environmental classes (conventional, barrier, isolation) and their microbiological limits.
• GB 50447 — Technical code for laboratory animal facilities, which governs facility design, construction and verification testing.
• GB/T 16294 — Testing method for settling microorganisms in cleanrooms (settle plate method), which specifies media, sampling points, exposure and incubation.
• ISO 14698-1 and ISO 14698-2 — Biocontamination control of cleanrooms and associated controlled environments, providing the framework for formal microbiological monitoring systems.
• GB/T 25915 / ISO 14644-1 — Cleanroom and controlled environment classification, referenced for the particulate cleanliness of barrier and isolator environments (typically ISO Class 7 for barrier rooms and ISO Class 5 for isolators).

As a passive method, settle plate testing measures the deposition rate of microorganisms onto surfaces, which correlates directly with the contamination risk to animals and materials. It is complementary to active air sampling (reported as CFU per cubic meter) and surface contact or swab testing, and it is inexpensive, simple and well suited to routine monitoring in animal rooms.

Testing Samples

Settle plate testing can be applied to every microbiologically controlled zone within an animal facility. The principal tested environments include the following.

Barrier (SPF) Animal Rooms

Breeding and holding rooms for SPF rodents and other species operating under positive pressure with HEPA-filtered supply air. These areas are subject to the strictest routine settle plate limits, commonly specified as no more than 3 CFU per plate per 30 minutes of exposure.

Conventional Animal Rooms

Open or semi-open rooms housing conventional animals, where environmental control is less rigorous. Limits are correspondingly relaxed and are often cited as no more than 10 CFU per plate per 30 minutes, reflecting the higher acceptable bioburden.

Isolation Environments (Isolators)

Gnotobiotic isolators, transfer chambers and their connecting ports, maintained at ISO Class 5 cleanliness. Routine settle plate testing is expected to show no detectable viable colonies, since any growth indicates a potential breach of containment integrity.

Support and Controlled Access Areas

Clean and dirty corridors, airlocks and pass-through chambers, cage wash and sterilization areas, bedding and feed preparation rooms, procedure and surgery rooms, and rooms housing individually ventilated cage (IVC) racks and cage-changing stations. These zones buffer the flow between clean and soiled areas and are monitored to help locate contamination sources.

Testing Items

The core testing items for an animal room settle plate campaign are:

• Total settling bacterial count: enumeration of mesophilic aerobic bacteria deposited on tryptic soy agar (TSA) plates, expressed as CFU per plate per 0.5 hour.
• Settling fungal count: deposition of yeasts and molds on Sabouraud dextrose agar (SDA) or equivalent fungal media, reported separately from bacteria.
• Colony identification (optional): representative or excessive colonies may be identified to genus or species level to trace contamination sources, for example skin-associated flora indicating personnel-related contamination or spore-forming environmental organisms indicating ingress issues.

Settle plate testing is normally performed together with the other environmental parameters required for animal facility qualification, so that a single testing campaign produces a complete compliance picture:

• Temperature and relative humidity
• Differential pressure between adjacent zones and air change rate
• Airborne particulate cleanliness class
• Illumination, noise and, where applicable, ammonia concentration

Testing Methods

Testing is performed according to the settle plate procedures of GB/T 16294, GB 14925 and GB 50447, supported by the biocontamination control principles of ISO 14698. A typical workflow consists of the following stages.

Media Preparation and Quality Control

Sterile 90 mm settle plates are prepared with TSA for total bacterial count and SDA for fungal count. Each media lot is checked for sterility using unexposed control plates and verified for growth promotion performance before use. Plates are transported in sealed containers, protected from external contamination, and equilibrated to room temperature before exposure to avoid condensation on the agar surface.

Sampling Point Layout

The minimum number of sampling points is derived from the floor area of the room — commonly the square root of the area in square meters, with a minimum of three points per room — and points are placed at working height, typically 0.8 to 1.5 meters, aligned with the cage racks or operating plane. Points should be distributed evenly across the room, positioned away from walls (generally at least 0.5 meters), and should not be located directly under supply air diffusers or adjacent to return air inlets, as these positions yield unrepresentative results. High-risk locations such as cage-changing stations and material transfer hatches may be sampled in addition.

Exposure Procedure

The HVAC system is operated in its normal working condition, and testing is carried out either in the static (at-rest, after cleaning and disinfection) or dynamic (operational, with normal husbandry activity) state, as defined by the testing plan. Temperature, humidity and differential pressure are recorded at the time of sampling. Personnel wear appropriate cleanroom garments and minimize movement during exposure. At each point the lid is carefully removed, the agar surface is exposed upward for 30 minutes, and the lid is then replaced with the plate labeled with location, date and exposure time. Negative control plates remain unexposed throughout and are incubated together with the samples.

Incubation and Enumeration

Bacterial plates are incubated at 30 to 35 degrees Celsius and examined after approximately 48 hours, extended to 72 hours where slow growth is suspected. Fungal plates are incubated at 20 to 25 degrees Celsius for 5 to 7 days. Colonies are counted using a colony counter, and results are reported as CFU per plate per exposure period together with the arithmetic mean for each room.

Evaluation and Reporting

The average settle plate count for each area is compared with the applicable limit for its environmental class. Where limits are exceeded, the report should trigger an investigation, typically including review of cleaning and disinfection procedures and agent rotation, verification of differential pressures and HEPA filter integrity, examination of gowning and personnel practices, and repeat testing after corrective action. Trending of results across successive campaigns is more informative than isolated values and is expected under a formal monitoring program aligned with ISO 14698-2.

Applications

• Commissioning and qualification of new or renovated animal facilities in the as-built, at-rest and operational states.
• Routine periodic environmental monitoring of SPF breeding farms, research animal centers and pharmaceutical animal facilities, typically at monthly to quarterly intervals defined by internal procedures.
• Support for GLP-compliant preclinical studies, where documented environmental control is part of study integrity and data acceptance.
• Preparation for institutional animal care and use accreditation assessments and regulatory inspections.
• Re-qualification after HEPA filter replacement, HVAC modification, deep disinfection or a change of disinfection regime.
• Root cause investigation after suspected contamination events, abnormal colony morphology or pathogen-positive sentinel monitoring results.
• Independent verification by accredited third-party laboratories whenever an objective, externally documented result is required.

FAQ

• Q: How often should settle plate testing be performed in animal rooms? A: Frequency depends on the environmental class and institutional procedures. Barrier (SPF) facilities are commonly tested at least quarterly, often monthly, while conventional rooms may be tested semi-annually. Additional testing is recommended after maintenance, renovation or contamination events, and during facility qualification.
• Q: Why is the standard exposure time 30 minutes? A: The limits in GB 14925 are expressed as CFU per plate per 0.5 hour, so a 30-minute exposure aligns the measurement with the acceptance criterion. Longer exposures of up to several hours are used in some pharmaceutical cleanroom applications under GB/T 16294, but results must then be interpreted against the corresponding limits.
• Q: What is the difference between settling microbe testing and active air sampling? A: Settle plates passively measure the deposition of microorganisms onto surfaces, which best reflects the contamination risk to cages, feed and work surfaces. Active air samplers draw a known volume of air onto a collection medium and report CFU per cubic meter. ISO 14698 recommends combining both approaches for a complete picture of biocontamination.
• Q: Can testing be performed with animals present? A: Yes. Dynamic (operational) testing with normal husbandry activity represents the most realistic worst-case condition. The state of the facility, static or dynamic, must be recorded in the report and considered when results are evaluated.
• Q: What should be done if settle plate counts exceed the limit? A: The area should be re-cleaned and disinfected, ventilation parameters such as pressure differentials, airflow and filter integrity should be verified, personnel practices reviewed, and testing repeated. Colony identification can help trace the source, and consecutive results should be trended to confirm the corrective action was effective.
• Q: Who can perform the testing? A: Trained facility personnel can perform routine internal monitoring, while commissioning, qualification and compliance testing is frequently outsourced to accredited third-party laboratories with microbiological competence and quality-controlled media.