Observation of Pneumococcal Phase Variation in Colony Morphology
Related Experiment Video
Updated: Oct 19, 2025
Experimental Human Pneumococcal Carriage
Published on: February 15, 2013
Jing Li1, Juanjuan Wang1, Fangfang Jiao1
1Center for Infectious Disease Research, School of Medicine, Tsinghua University, Beijing, China.
Abstract:
Streptococcus pneumoniae (pneumococcus) is an important human pathogen that causes pneumonia, meningitis, sepsis, and otitis media. This bacterium normally resides in the nasopharynx as a commensal, but sometimes disseminates to sterile sites of humans and causes local or systemic inflammation. This biphasic behavior of S. pneumoniae is correlated with a reversible switch between the opaque and transparent colony forms on agar plates, a phenomenon referred to as phase variation. The opaque variants appear to be more virulent in animal models of bacteremia but are deficient in nasopharyngeal colonization animal models. In contrast, the transparent variants display higher levels of nasopharyngeal colonization but relatively lower virulence in animal models. We have recently demonstrated that pneumococcal phase variation between these two colony types is caused by a reversible switch of genome DNA methylation (or epigenetic) patterns, which is driven by DNA inversions in the DNA methyltransferase genes. Observation of colony morphology is a simple and useful method to differentiate colonies with different characteristics, such as size, color, and opacity. This protocol describes how to study pneumococcal phase variation in colony morphology with a dissection microscope.
Insights
Streptococcus pneumoniae exhibits phase variation between opaque and transparent colony forms, impacting virulence and colonization. This switch is driven by reversible changes in DNA methylation patterns, offering insights into pneumococcal pathogenesis.
Area of Science:
- Microbiology
- Pathogenesis
- Epigenetics
Background:
- Streptococcus pneumoniae (pneumococcus) is a significant human pathogen causing pneumonia, meningitis, sepsis, and otitis media.
- Pneumococcus exists as a commensal in the nasopharynx but can cause invasive diseases.
- The bacterium exhibits biphasic behavior linked to reversible colony morphology changes (phase variation).
Purpose of the Study:
- To investigate the underlying mechanisms of pneumococcal phase variation.
- To correlate colony morphology with virulence and colonization capabilities.
- To describe a method for studying pneumococcal phase variation.
Main Methods:
- Observation of colony morphology (opaque vs. transparent) on agar plates using a dissection microscope.
- Analysis of genome DNA methylation patterns.
- Investigation of DNA inversions in DNA methyltransferase genes.
Main Results:
- Pneumococcal phase variation between opaque and transparent colony forms is linked to virulence and colonization.
- Opaque variants show higher virulence but lower colonization in animal models.
- Transparent variants exhibit enhanced colonization but reduced virulence.
- Phase variation is driven by reversible switches in DNA methylation patterns due to DNA inversions.
Conclusions:
- Pneumococcal phase variation is an epigenetic phenomenon regulated by DNA methylation.
- Understanding phase variation is crucial for developing strategies against pneumococcal infections.
- Colony morphology observation is a practical method to study this variation.
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