Part of the CURIS Validation Series

Some microorganisms can survive boiling water, radiation, and even the vacuum of space.

These microbial survival experts are bacterial spores, and they are among the most resilient biological structures known to exist. Their remarkable durability is exactly why they play such a critical role in sterilization validation.

When laboratories and pharmaceutical facilities validate a sterilization or biodecontamination process, they are essentially asking one question:

Can this system reliably destroy the toughest microorganisms under the most challenging conditions?

To answer that question, validation protocols rely on carefully designed microbial challenges using:

• Biological indicators (BIs)
• Highly resistant test organisms
• Defined microbial populations
• Measured log reductions

In this article—part of the CURIS Validation Series—we’ll explore how these elements work together to demonstrate sterilization efficacy and why they are essential elements of robust contamination control strategies (CCS) in animal laboratories and pharmaceutical manufacturing environments.

Why Sterilization Validation Matters

Contamination in controlled environments can have serious consequences.

In animal research laboratories, microbial contamination may compromise experimental data, affect animal health, or invalidate study results.

In pharmaceutical manufacturing and aseptic processing, contamination can lead to:

• batch failures
• regulatory observations
• costly investigations
• product recalls

Because of these risks, contamination control strategies depend on validated sterilization and biodecontamination processes.

Validation answers a critical question:

Can this system consistently destroy microorganisms under worst-case conditions?

To demonstrate this, validation studies intentionally challenge systems using highly resistant organisms and very large microbial populations.

The Role of Biological Indicators

A biological indicator is a standardized test system containing a known population of microorganisms with well-characterized resistance properties.

Rather than assuming a process works, biological indicators allow facilities to measure microbial destruction directly.

These indicators typically contain bacterial spores, which are among the most resistant forms of microbial life.

If a sterilization process can reliably destroy these spores, it provides strong evidence that less resistant microorganisms will also be eliminated.

Why Spores Are Used for Validation

In our earlier article in this series, “3 Common Misconceptions About Validation Using Spores,” we discussed why spores are widely used in sterilization validation.

Bacterial spores possess several features that make them ideal challenge organisms:

• thick protective outer layers
• extremely low metabolic activity
• resistance to chemical and environmental stress
• the ability to survive harsh conditions for long periods

Because of these traits, spores are significantly harder to kill than most bacteria, fungi, or viruses.

Using spores in validation, therefore, creates a conservative benchmark for sterilization performance.

If a sterilization or biodecontamination process can destroy highly resistant spores, it is extremely likely to eliminate the microorganisms typically encountered in laboratory or pharmaceutical environments.

Challenge Organisms Vary by Sterilization Chemistry

An important nuance in sterilization validation is that the most resistant organism depends on the sterilization technology being used.

Different chemistries destroy microorganisms through different mechanisms. Because of this, validation protocols select organisms that represent worst-case resistance for the specific sterilization method being evaluated.

For example:

This is why the idea of a single “hardest organism to kill” can be misleading. Instead, validation focuses on organisms that are particularly resistant to the chemistry being tested.

Challenge Organisms and Hydrogen Peroxide Vapor

For hydrogen peroxide vapor or vaporized hydrogen peroxide systems, several spore-forming organisms are commonly discussed when evaluating microbial resistance.

Although resistance levels can vary with cycle conditions, studies generally indicate the relative resistance trend observed under hydrogen peroxide vapor exposure, as shown in the graphic to the right.

Research comparing hydrogen peroxide vapor exposure shows that Geobacillus stearothermophilus spores often demonstrate greater resistance than Bacillus atrophaeus spores under comparable hydrogen peroxide vapor conditions, supporting their widespread use as biological indicators for hydrogen peroxide sterilization validation.

Because of this resistance, G. stearothermophilus spores are commonly used as worst-case challenge organisms when validating hydrogen peroxide-based sterilization processes.

However, resistance rankings are not absolute. Microbial survival depends on several environmental factors, including:

• hydrogen peroxide concentration
• humidity conditions
• exposure time
• surface materials
• airflow and sterilant distribution

This is why sterilization validation must consider both organism selection and environmental conditions.

Population Levels: Why the Numbers Matter

The organism used for validation is only part of the challenge.

The number of microorganisms present in the test also plays a critical role.

Biological indicators typically contain very high microbial populations—often 10⁶ spores (one million microorganisms).

Why such a large number?

Because sterilization effectiveness is measured using log reduction values.

What Is Log Reduction?

Log reduction describes how effectively a sterilization process decreases microbial populations.

Each “log” represents a tenfold reduction.

Log Reduction	Microbial Reduction
1-log	90% reduction
3-log	99.9% reduction
6-log	99.9999% reduction

A 6-log reduction means reducing one million microorganisms to one or fewer survivors.

This level of reduction is widely considered the benchmark for sporicidal sterilization efficacy.

Why Worst-Case Testing Matters

Validation studies intentionally create worst-case conditions to challenge biodecontamination systems.

Biological indicators may be placed in locations that represent difficult-to-treat areas, such as:

• inside complex equipment
• behind structural obstructions
• within isolators or biosafety cabinets
• beneath equipment decks
• inside airflow-restricted areas

These placements simulate the kinds of contamination challenges that occur in real facilities.

In both animal laboratories and pharmaceutical environments, microorganisms rarely exist in readily accessible open spaces—they may reside within equipment housings, airflow pathways, or other protected locations.

Validation must prove that sterilization processes work even in these difficult conditions.

How Automated Biodecontamination Systems Support Validation

Manual cleaning and spray-and-wipe disinfection can be difficult to validate because application methods vary between operators.

Automated biodecontamination systems help solve this problem by delivering controlled, repeatable cycles.

For example, systems using CURIS’ hybrid hydrogen peroxide™ vapor technology can distribute disinfectant throughout enclosed environments while maintaining precise cycle parameters.

These systems can undergo extensive testing during:

• Factory Acceptance Testing (FAT)
• Site Acceptance Testing (SAT)
• Installation Qualification (IQ)
• Operational Qualification (OQ)
• Performance Qualification (PQ)

Once validated, automated cycles help ensure consistent performance while generating detailed treatment records to support quality systems and regulatory compliance.

Automation also reduces the potential for human error by standardizing cycle parameters and treatment protocols.

Validation Provides Scientific Confidence

Microorganisms have evolved extraordinary survival mechanisms.

Sterilization validation ensures that modern contamination control technologies can overcome those defenses.

By combining...

• resistant test organisms,
• high microbial populations, and
• worst-case environmental placement,

validation studies demonstrate that processes are not only effective, but scientifically proven to work.

For animal research laboratories and pharmaceutical facilities alike, this validation process provides something invaluable:

confidence that contamination risks are under control.

Continue the CURIS Validation Series

Explore other articles in the CURIS Validation Series:

• 3 Common Misconceptions About Validation Using Spores
• Bacterial vs. Fungal Spores: Understanding the Titans of Contamination Control
• Choosing the Best BI for Your Facility

Together, these resources provide a deeper understanding of the science behind biological indicators, microbial resistance, and sterilization validation.