Purpose
To standardize counting and calculation of colonies for quantitative microbiological examinations.
Scope
This procedure applies to trained microbiology laboratory personnel working within an approved quality-management and biosafety system.
Principle
Under appropriate culture conditions, viable organisms form visible colonies that can be counted within the validated quantitative range of the method.
Specimen / Test Material
Appropriately inoculated culture plates from validated quantitative procedures.
Equipment and Materials
- Suitable culture plates
- Colony counter or manual counting aid
- Marker or approved recording system
- Calculator or validated laboratory information system
Procedure
- Confirm plate identification and culture integrity.
- Select plates meeting the validated counting criteria.
- Count colonies using the approved method.
- Record counts and dilution factors.
- Calculate the final result using the validated formula.
- Apply laboratory rules for plates outside the validated counting range.
Quality Control and Quality Assurance
- Verify counting criteria.
- Check calculation accuracy.
- Review unusual counts or contamination patterns.
Reporting
Report quantitative results in the approved units and include relevant limitations.
Safety
Use risk-based biosafety practices, appropriate personal protective equipment and approved decontamination and waste-disposal procedures. Treat clinical specimens and cultures according to the laboratory's biological risk assessment.
References
- World Health Organization (WHO): Toolkit for Standard Operating Procedures for Bacteriology, Mycology and Antimicrobial Susceptibility Testing.
- WHO: Laboratory Biosafety Manual, 4th edition.
- WHO guidance on collection, packaging and transport of infectious substances.
- EUCAST methodology and quality-control guidance for antimicrobial susceptibility testing, where applicable.
- Current locally approved laboratory procedures and validated manufacturer instructions.