Mannitol Motility Medium - Microbiology

What It Is

1. Motility (flagellar movement)
2. Mannitol fermentation (carbohydrate metabolism)

Principle

1. Motility Detection

• The medium contains a very low concentration of agar (0.3%), making it semisolid rather than fully solid.
• This semisolid consistency allows motile bacteria to freely migrate away from the stab line of inoculation using their flagella (or other locomotor structures like fimbriae/pili in gliding motility).
• Motile bacteria spread outward, making the medium appear turbid/cloudy.
• Non-motile bacteria cannot penetrate the loose agar network and remain confined to the stab line only.

2. Mannitol Fermentation Detection

• The medium contains mannitol as the sole fermentable carbohydrate.
• Organisms that can ferment mannitol produce acidic by-products (organic acids).
• Phenol red acts as a pH indicator:
  • Acidic conditions (fermentation) → medium turns yellow
  • Alkaline/neutral conditions (no fermentation) → medium remains red (or deepens in red)

Composition (per liter)

Procedure

1. Using a sterile straight inoculating needle, pick a colony from an 18-24 hour culture.
2. Stab the needle straight down through the center of the medium to about half its depth.
3. Withdraw the needle along the exact same line it entered (avoid fanning - this causes false positives).
4. Incubate at 35-37°C for 18-48 hours.
5. Read results for both motility and color change.

Interpretation

• Motility + / Fermentation + → diffuse yellow turbidity
• Motility + / Fermentation - → diffuse red turbidity
• Motility - / Fermentation + → yellow only at stab line
• Motility - / Fermentation - → red medium, growth only at stab line

Uses / Clinical Significance

• Differentiating Staphylococci: S. aureus is mannitol-positive (ferments mannitol); S. epidermidis is typically mannitol-negative.
• Differentiating enteric bacteria: Used alongside other tests to identify gram-negative rods.
• Separating motile from non-motile Enterobacteriaceae: e.g., Escherichia coli (motile) vs. Klebsiella (non-motile).

Key Points to Remember

• Low agar (0.3%) = semisolid = allows movement = detects motility
• Phenol red = pH indicator = detects acid from mannitol fermentation
• Always stab and remove needle in the same straight line to avoid false-positive motility
• Incubation at 35-37°C; read at 18-48 hours
• The medium combines two tests in one tube, saving time and resources in the diagnostic lab

Bacteria Differentiated Using Mannitol Motility Medium

Group 1: Motility (+) / Mannitol Fermentation (+)

Group 2: Motility (+) / Mannitol Fermentation (-)

Note: Proteus spp. are strongly motile but do not ferment mannitol - a key distinguishing feature from other Enterobacteriaceae.

Group 3: Motility (-) / Mannitol Fermentation (+)

Key point: This pattern helps distinguish S. aureus (mannitol +) from S. epidermidis (mannitol -). It also separates Shigella (non-motile) from Salmonella (motile).

Group 4: Motility (-) / Mannitol Fermentation (-)

Summary Table (All Organisms)

Key Differentiations This Medium Achieves

1. S. aureus vs. S. epidermidis - Both non-motile, but S. aureus ferments mannitol (yellow) while S. epidermidis does not (red). This is used alongside Mannitol Salt Agar for Staphylococcus speciation.
2. Salmonella vs. Shigella - Both are enteric pathogens causing diarrheal illness. Salmonella is motile and mannitol-positive; Shigella is non-motile but mannitol-positive. A single tube separates them.
3. Proteus vs. E. coli - Both are motile, but Proteus is mannitol-negative (red + turbid) while E. coli is mannitol-positive (yellow + turbid).
4. E. coli vs. Shigella - Both ferment mannitol, but E. coli is motile and Shigella is not.
5. Enteric pathogens vs. Staphylococci - The rich peptone base supports even fastidious gram-positive cocci like Staphylococcus, making this medium versatile across both gram-positive and gram-negative organisms.

AST Methods in Microbiology

1. Diffusion methods
2. Dilution methods
3. Gradient methods
4. Automated/molecular methods

1. DIFFUSION METHODS

Kirby-Bauer Disk Diffusion (Most Common)

• Prepare a 0.5 McFarland bacterial suspension (~1.5 × 10⁸ CFU/mL)
• Inoculate Mueller-Hinton Agar (MHA) by lawn method
• Place antibiotic disks (6 mm) at least 15 mm apart
• Incubate at 35-37°C for 16-18 hours
• Measure zone diameter in millimeters

2. DILUTION METHODS

A. Broth Macrodilution

• Serial two-fold dilutions of antibiotic in broth tubes (e.g., 0.5, 1, 2, 4, 8... µg/mL)
• Each tube inoculated with ~5 × 10⁵ CFU/mL
• Incubated 16-20 hours at 35°C
• Lowest clear (no turbidity) tube = MIC
• Largely replaced by microdilution; rarely used today

B. Broth Microdilution (Gold Standard - BMD)

• Same principle as macrodilution but performed in 96-well microplates
• Each well contains 100 µL of antibiotic at doubling concentrations
• Reference method endorsed by CLSI and EUCAST
• Results: MIC in µg/mL, then interpreted against breakpoints
• Can test multiple antibiotics simultaneously

C. Agar Dilution

• Antibiotic serially diluted and incorporated into molten agar before pouring
• Bacterial suspensions spotted/inoculated onto each agar plate
• Multiple organisms can be tested on same plate
• Lowest concentration agar with no visible growth = MIC
• Used in research settings, less common clinically

3. GRADIENT METHOD (Etest / MIC Strip)

• Etest (bioMérieux) - most widely used
• MIC Test Strip (Liofilchem)
• M.I.C. Evaluator (Oxoid)
• Ezy MIC Strip (HiMedia)

4. AUTOMATED SYSTEMS

5. MOLECULAR METHODS

Comparison Summary

Key Standards Bodies

• CLSI (Clinical and Laboratory Standards Institute) - USA
• EUCAST (European Committee on Antimicrobial Susceptibility Testing) - Europe