Determining Bacterial Motility: Methods and Interpretation

Determining Bacterial Motility

Introduction to Bacterial Motility

Bacterial motility is an important characteristic used to differentiate and identify microorganisms. Motility is primarily achieved through specialized structures called flagella, which vary in arrangement and number among different bacterial species.

• Monotrichous: A single flagellum at one end.

• Lophotrichous: A tuft of flagella at one end.

• Amphitrichous: Tufts of flagella at both ends.

• Peritrichous: Flagella distributed around the entire cell.

Methods for Determining Motility

There are two primary laboratory methods for assessing bacterial motility: the wet mount technique and the motility medium (MTM) with TTC method.

Wet Mount Technique

The wet mount method involves placing a suspension of bacteria in broth onto a glass slide, covering it with a cover slip, and observing under a microscope. Because the bacteria are unstained and transparent, they are difficult to visualize, requiring careful adjustment of the microscope's diaphragm.

• Advantages: Direct observation of living, moving bacteria.

• Limitations: Unstained bacteria are nearly invisible due to their refractive index being similar to water.

Motility Medium (MTM) with TTC

This method uses a semi-solid agar deep containing TTC (2,3,5-triphenyltetrazolium chloride). As motile bacteria grow, they reduce TTC to formazan, an insoluble red pigment, which marks the areas of bacterial growth.

• Procedure: Inoculate MTM tubes with bacteria using a needle. Incubate at 37°C.

• Interpretation: Red color diffuses from the stab line if bacteria are motile; nonmotile bacteria grow only along the stab line.

Types of Motility Observed

• True Motility: Active movement of bacteria, such as Pseudomonas aeruginosa.

• No Motility: Bacteria remain stationary, such as Staphylococcus aureus.

• Browning Motion: Random movement due to water molecules, not true motility.

Laboratory Procedure Summary

1. Label and inoculate MTM tubes with Pseudomonas aeruginosa and Staphylococcus aureus.

2. Incubate tubes at 37°C.

3. Prepare wet mounts for microscopic observation.

4. Discard slides in designated containers after use.

Interpretation of Motility Medium Results

The motility medium provides a visual indicator of bacterial motility based on the distribution of red formazan pigment.

Tube	Appearance	Interpretation
Uninoculated MTM	Clear, no color change	No bacterial growth
Nonmotile (S. aureus)	Red color only along stab line	Bacteria grow only where inoculated
Motile (P. aeruginosa)	Red color diffuses throughout medium	Bacteria move away from stab line

Key Terms and Definitions

• Flagella: Whip-like appendages used for bacterial movement.

• Motility Medium (MTM): Semi-solid agar used to test bacterial motility.

• TTC (2,3,5-triphenyltetrazolium chloride): A dye reduced by bacteria to form red formazan pigment.

• Formazan: Insoluble red pigment indicating bacterial growth.

Example Application

Motility testing is essential in clinical microbiology for distinguishing pathogenic bacteria. For instance, Pseudomonas aeruginosa is motile and can be identified by its ability to spread through motility medium, while Staphylococcus aureus is nonmotile and remains confined to the stab line.