Determination of Total Settling Microorganism Colony Count: Settle Plate Method for Cleanroom Air Mo

Determination of total settling microorganism colony count is a passive microbiological air monitoring method that quantifies viable airborne contaminants deposited onto agar plates exposed in cleanrooms, hospitals, and food production environments. Performed in accordance with GB/T 16294-2010, ISO 14698-1, and GMP requirements, the settle plate technique is a cornerstone of environmental monitoring and contamination control programs. This article explains the principle, sampling procedure, incubation conditions, acceptance criteria, and typical applications of settling microorganism testing.

Technical Overview

The settling microorganism colony count test, commonly known as the settle plate method, is a gravimetric (passive) microbiological monitoring technique used to evaluate the bioburden of air in controlled environments. Sterile Petri dishes filled with a nutrient culture medium are exposed at defined locations for a specified period, during which airborne viable particles carrying microorganisms settle by gravity onto the agar surface. After incubation, the resulting colonies are counted, and the result is expressed as colony-forming units (CFU) per plate per exposure period, commonly normalized to CFU per 4 hours.

Unlike active air sampling, which draws a defined volume of air through an impactor, the settle plate method measures the microbial deposition rate onto exposed surfaces. This makes it particularly relevant for assessing contamination risk to products, instruments, and critical work surfaces during aseptic operations. The method is specified in GB/T 16294-2010 (Testing method for settling microbes in cleanrooms of the pharmaceutical industry), ISO 14698-1 (Cleanrooms and associated controlled environments – Biocontamination control), and the GMP guidelines of major pharmaceutical jurisdictions.

Because the procedure requires no powered equipment inside the cleanroom, it is simple, low-cost, and non-disruptive, making it ideal for continuous monitoring during production. It is typically performed alongside active volumetric air sampling, surface contact plates, and swab testing to form a complete environmental monitoring program.

Testing Samples

Pharmaceutical and Biologics Cleanrooms

Air in Grade A, B, C, and D areas used for sterile and non-sterile drug manufacturing, including filling lines, laminar airflow hoods, restricted access barrier systems, and preparation rooms, is monitored for settling microorganisms during qualification and routine production.

Hospital and Healthcare Environments

Operating theatres, laminar-flow surgical suites, intensive care units, disinfection supply centers, and hemodialysis rooms are tested to verify hygienic air quality and support infection prevention and control programs, typically in accordance with hospital hygiene standards such as GB 15982.

Food, Beverage, and Cosmetics Production Areas

Packaging rooms, cold-chain processing zones, fermentation areas, and clean zones for ready-to-eat foods or low-preservative cosmetics are monitored as part of hygiene zoning, HACCP verification, and good manufacturing practice audits.

Medical Device and Electronics Controlled Environments

Cleanrooms for sterile device assembly, implantable products, and contamination-sensitive electronics manufacturing use settling microorganism counts to confirm that biocontamination remains within the specified class limits.

Testing Items

The core test item is the total viable settling microorganism count, reported as colony-forming units per plate for a defined exposure duration, for example CFU per 30 minutes or CFU per 4 hours. Depending on the monitoring program, accredited third-party laboratories may also provide the following related services:

• Total bacterial settling count on tryptone soy agar (TSA) or soybean-casein digest agar
• Total fungal (yeast and mold) settling count on Sabouraud dextrose agar (SDA) or rose bengal medium
• Identification of predominant or excursion-associated colonies using biochemical or molecular methods
• Trend analysis and conformance evaluation against cleanliness grade limits
• Combined environmental monitoring packages including active air sampling, surface contact plates, and personnel monitoring

Testing Methods

Reference Standards

The determination is generally carried out according to one or more of the following references, depending on the industry and jurisdiction:

• GB/T 16294-2010 – Testing method for settling microbes in cleanrooms of the pharmaceutical industry
• ISO 14698-1 – Cleanrooms and associated controlled environments: biocontamination control, general principles and methods
• China GMP (2010 Revision) and EU GMP Annex 1 – Manufacture of sterile medicinal products, environmental monitoring limits
• GB 15982 – Hygienic standard for hospital disinfection, air microbiological requirements
• GB 50457 – Code for design of pharmaceutical industry cleanrooms

Media, Reagents, and Equipment

The test relies on standard microbiological consumables and calibrated equipment:

• Sterile 90 mm (or 55 mm) Petri dishes containing approximately 15–20 mL of TSA for total aerobic counts; media may include neutralizers such as polysorbate 80 and lecithin where disinfectant residues are anticipated
• SDA plates where fungal monitoring is required
• Validated incubators capable of maintaining 30–35 °C and 20–25 °C
• Colony counter or illuminated counting aid, timer, and sterile handling tools
• Media qualified by sterility checks and growth promotion testing before use

Sampling Point Selection and Plate Placement

Sampling locations should represent the entire controlled area, with additional density at critical processing points. Under GB/T 16294-2010, the minimum number of sampling points is generally determined from the room area, consistent with the approach used for airborne particle testing, and at least two locations are recommended even for small rooms. Plates are placed at working height, typically 0.8–1.5 m above the floor or directly on critical work surfaces, away from direct discharge airflow, walls, and obvious obstacles that would shield or artificially accelerate deposition.

Exposure Procedure

The exposure is performed in a manner that minimizes operator-induced contamination:

• Equilibrate the plates to room temperature to prevent condensation on the agar surface
• Transport plates into the area using sterile handling technique; open one unexposed control plate as a negative control
• At each sampling point, remove the lid carefully without touching the agar and place the lid face down beside the dish
• Expose the agar surface for the defined duration, commonly 30 minutes for static qualification testing and up to 4 hours for operational or continuous monitoring of higher-grade areas
• Replace the lid, retrieve the plates, and transfer them promptly to the incubator; record locations, exposure time, room state (at rest or in operation), temperature, and humidity

Incubation, Counting, and Result Expression

Bacterial plates are incubated inverted at 30–35 °C for at least 48–72 hours, while fungal plates are incubated at 20–25 °C for 5–7 days. After incubation, colonies are counted with the aid of a colony counter, and results are reported as CFU per plate for the actual exposure period. Where the monitoring plan requires, results may be extrapolated to CFU per 4 hours using a proportional conversion, provided the approach is scientifically justified and consistently applied. Colony counts at each location, the mean value for the area, and the growth observed on the negative control are all documented in the test report.

Quality Control and Precautions

• Exposure should not exceed approximately 4 hours, because progressive desiccation of the agar reduces microbial recovery and biases results downward
• Operators should remain still and downwind of the plates during exposure and avoid talking, coughing, or passing directly over open dishes
• Avoid placing plates in direct laminar air streams that either scour the surface or create unnatural deposition
• Discard the run and repeat sampling if the negative control shows growth or if plates are damaged, contaminated during handling, or over-dried
• All media must pass growth promotion and sterility checks, and incubators must be within calibration

Applications

Settling microorganism determination is widely used across regulated and hygiene-sensitive industries:

• Cleanroom qualification and periodic requalification of pharmaceutical facilities against GMP Grade A–D limits
• Routine environmental monitoring during aseptic filling, sterilization preparation, and sterile compounding
• Hospital operating theatre and ICU hygiene surveillance for infection control accreditation
• Food and beverage factory hygiene zoning, HACCP verification, and customer or regulatory audits
• Cosmetics and medical device cleanroom biocontamination control
• Evaluation of disinfection procedures, HVAC and airflow performance, and root-cause investigation after contamination excursions
• Third-party test reports for facility acceptance, regulatory inspection support, and supplier qualification by accredited third-party laboratories

FAQ

What is the difference between the settle plate method and active air sampling?

The settle plate method passively measures the number of viable particles that deposit by gravity onto an exposed surface over time, reflecting contamination risk to exposed products. Active air sampling draws a known volume of air through an impactor onto agar and reports microbial concentration as CFU per cubic meter. The two methods are complementary and are normally both included in a comprehensive environmental monitoring program.

How long should settle plates be exposed?

Static qualification testing commonly uses 30-minute exposures to achieve adequate sensitivity. For operational monitoring of Grade A and Grade B areas, GMP practice frequently uses continuous or semi-continuous exposure for up to 4 hours per plate. Exposure beyond about 4 hours is not recommended because agar drying impairs recovery.

What are the typical GMP acceptance limits for settle plates?

Under China GMP and EU GMP Annex 1, the commonly applied settle plate limits during operation are:

• Grade A: less than 1 CFU per 4 hours
• Grade B: 5 CFU per 4 hours
• Grade C: 50 CFU per 4 hours
• Grade D: 100 CFU per 4 hours

Specific products and processes may require stricter internal alert and action limits established through a documented contamination control strategy.

How many sampling points are required?

The minimum number of locations is generally derived from the room floor area according to GB/T 16294-2010, consistent with particle-count sampling practice, and is increased at critical processing positions. Points should be distributed uniformly, with working-height placement between 0.8 m and 1.5 m above the floor.

Why is exposure limited to about four hours?

During prolonged exposure, the agar surface progressively loses moisture, which reduces the viability recovery of deposited microorganisms, particularly Gram-negative bacteria. Validation studies have shown that recovery declines significantly after 4 hours, so this duration is accepted as the practical upper limit in most standards and GMP guidance.

How should results be reported?

Results are reported as CFU per plate for the actual exposure time, for example CFU per 30 minutes or CFU per 4 hours. The report should also state the room state (at rest or in operation), sampling locations, exposure duration, media and incubation conditions, negative control results, and a comparison against the applicable acceptance criteria.

How often should settling microorganism monitoring be performed?

Frequency depends on the cleanliness grade and risk assessment. Grade A and Grade B areas are typically monitored every production shift or batch, while lower grades are monitored weekly or monthly. Full requalification testing is performed after facility modifications, HVAC changes, or as required by the site's contamination control strategy.