Survive Time and Kill Time Calculation: Complete Guide for Pharmaceutical and Microbiological Applications
In pharmaceutical microbiology and sterilization studies, determining Survive Time and Kill Time is essential for validating disinfectants, sterilization processes, and microbial resistance. These parameters help establish the effectiveness of sterilization and disinfection procedures by identifying the duration required to kill microorganisms or their survival time under specific conditions.
Understanding and accurately calculating these times ensures that sterilization cycles, disinfectant exposures, and microbial control methods meet Good Manufacturing Practice (GMP) and regulatory standards set by authorities such as WHO, USP, and EU guidelines.
1. What is Survive Time?
Survive Time refers to the period during which microorganisms can survive exposure to a sterilizing or disinfecting agent before they are completely killed. It indicates the resistance level of the microbial population to a particular process, such as steam sterilization, dry heat, or chemical disinfection.
Definition:
The Survive Time is defined as the maximum time duration at a specific temperature or disinfectant concentration where viable microorganisms can still be recovered from the sample.
Example:
If bacterial spores survive up to 8 minutes at 121°C during an autoclave validation test, then the Survive Time = 8 minutes.
2. What is Kill Time?
Kill Time is the time required to completely eliminate or kill all viable microorganisms under defined conditions of temperature, pressure, or disinfectant concentration. It represents the effectiveness endpoint of a sterilization or disinfection process.
Definition:
Kill Time is the minimum exposure duration at which no microbial growth is observed after incubation under appropriate conditions.
Example:
If all spores are killed within 10 minutes at 121°C in an autoclave validation study, then the Kill Time = 10 minutes.
3. Difference Between Survive Time and Kill Time
| Parameter | Survive Time | Kill Time |
|---|---|---|
| Definition | Time during which microorganisms can still survive the sterilization or disinfection process. | Time required to completely destroy all microorganisms. |
| Purpose | To determine microbial resistance. | To confirm sterilization effectiveness. |
| Microbial Growth | Detected in samples. | No growth detected in samples. |
| Application | Used to identify D-value and resistance profile. | Used to define process completion time. |
| Relation | Occurs before complete microbial death. | Occurs after complete microbial death. |
4. Calculation of Survive Time and Kill Time
Step 1: Preparation of Spore Suspension or Microbial Culture
Prepare a known concentration of microorganisms or bacterial spores (usually Bacillus stearothermophilus or Bacillus subtilis for steam and dry heat studies).
Step 2: Exposure to Sterilization or Disinfection Conditions
Expose biological indicators (BIs) or inoculated carriers to the sterilization cycle or disinfectant for various time intervals — for example, 5, 6, 7, 8, 9, and 10 minutes.
Step 3: Recovery and Incubation
After exposure, transfer each BI or sample to a suitable growth medium and incubate at recommended conditions (e.g., 55°C for B. stearothermophilus for 48 hours).
Step 4: Observation of Growth
Check for visible turbidity or colony formation after incubation:
- Growth observed → microorganism survived
- No growth → microorganism killed
Step 5: Interpretation
The last time interval showing growth = Survive Time
The first time interval showing no growth = Kill Time
Example Calculation:
If the results are as follows:
| Exposure Time (min) | Growth Observation |
|---|---|
| 6 | Growth Present |
| 7 | Growth Present |
| 8 | No Growth |
| 9 | No Growth |
➡️ Survive Time = 7 minutes
➡️ Kill Time = 8 minutes
5. Importance of Survive and Kill Time Determination
- Validates sterilization and disinfection cycle parameters.
- Ensures product and environmental microbial control in pharma manufacturing.
- Determines microbial resistance and D-value (decimal reduction time).
- Supports GMP, GLP, and validation documentation requirements.
- Ensures safety and sterility assurance level (SAL) compliance.
6. Applications in Pharmaceutical and Microbiological Studies
- Autoclave and dry heat sterilizer validation
- Disinfectant efficacy and kill-time tests
- Depyrogenation tunnel validation
- Biological indicator qualification
- Sterilization cycle optimization studies
7. Conclusion
Survive Time and Kill Time are crucial parameters in the validation of sterilization and disinfection processes. They help in establishing the microbial resistance pattern and ensure that sterilization conditions are sufficient to achieve a Sterility Assurance Level (SAL) of 10⁻⁶. Accurate determination of these values contributes to product safety, process validation, and compliance with international regulatory standards.
In summary, Survive Time indicates microbial resistance, while Kill Time defines sterilization efficiency — together forming the scientific foundation of sterilization validation in the pharmaceutical and microbiological industry.
💬 About the Author
Siva Sankar is a Pharmaceutical Microbiology Consultant and Auditor with extensive experience in sterility testing, validation, and GMP compliance. He provides consultancy, training, and documentation services for pharmaceutical microbiology and cleanroom practices.
📧 Contact: siva17092@gmail.com
Mobile: 09505626106
