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Mannitol Motility Medium - Microbiology

What It Is

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

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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)

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Composition (per liter)

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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.

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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

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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).

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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

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Bacteria Differentiated Using Mannitol Motility Medium

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

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

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

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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).

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

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Summary Table (All Organisms)

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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.

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AST Methods in Microbiology

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

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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

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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

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3. GRADIENT METHOD (Etest / MIC Strip)

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

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4. AUTOMATED SYSTEMS

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5. MOLECULAR METHODS

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Comparison Summary

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Key Standards Bodies

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

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Triple Sugar Iron (TSI) Agar

What It Is

1. Carbohydrate fermentation (glucose, lactose, sucrose)
2. Gas production (CO₂ and H₂)
3. Hydrogen sulfide (H₂S) production

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Composition (per liter)

Key ratio: 10:10:1 - Lactose:Sucrose:Glucose = 10:10:1. This ratio is the entire basis of interpretation (explained below).

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Principle

The Slant vs. Butt Logic

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A. Carbohydrate Fermentation

• ALL Enterobacteriaceae ferment glucose
• Small amount → acid produced quickly in both slant and butt → both turn yellow (A/A) initially
• But: in the aerobic slant, acid is rapidly neutralized by oxidative deamination of peptones (which releases alkaline NH₃)
• Since glucose is only 0.1%, the acid in the slant is exhausted first → slant reverts to alkaline (red/K)
• The butt (anaerobic) retains the acid → stays yellow (A)
• Result after 18-24 h: K (red slant) / A (yellow butt) = glucose-only fermenter

• Organisms that also ferment lactose/sucrose produce far more acid (10× more sugar available)
• The massive acid production overwhelms the alkaline neutralization in the slant
• Both slant and butt remain yellow
• Result: A (yellow slant) / A (yellow butt) = glucose + lactose/sucrose fermenter

• No acid produced at all
• Peptone deamination makes everything alkaline/red
• Result: K (red) / K (red) = non-fermenter

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The Reversion Phenomenon (Critical Concept)

If only glucose is fermented (K/A), the slant turns yellow initially, then reverts back to red as the small amount of glucose acid is used up and peptone alkalinity dominates. This is why TSI must be read at exactly 18-24 hours - reading at 48+ hours can give a false K/K result for a glucose-only fermenter.

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B. Gas Production

• Fermentation of sugars produces CO₂ and H₂ gas
• Gas detected by:
  • Cracks or fissures in the agar
  • Displacement of the agar plug upward
  • Bubbles within the medium
• Note: Heavy H₂S (black precipitate) can mask cracks - look carefully

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C. Hydrogen Sulfide (H₂S) Production

• Substrate: Sodium thiosulfate (Na₂S₂O₃)
• Mechanism: Organisms with thiosulfate reductase convert thiosulfate → H₂S gas
• Detection: H₂S reacts with ferric ammonium citrate → ferrous sulfide (FeS) = black precipitate in butt
• H₂S only forms in acidic conditions (butt must be yellow/acid for black to appear)
• If butt is alkaline (K/K), H₂S may be suppressed even if organism is H₂S-producing

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Procedure

1. Pick a well-isolated colony with a sterile inoculating needle
2. Stab the butt to ¾ depth, then streak the slant surface
3. Leave cap slightly loose (to allow aerobic conditions on slant)
4. Incubate at 35-37°C for 18-24 hours
5. Read: slant color / butt color / H₂S / gas

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Result Notation

• A = Acid (yellow) - fermentation occurred
• K = Alkaline (red/pink) - no fermentation or reversion
• NC = No change

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Complete Interpretation Table

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Identification of Clinically Important Organisms

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Key Points to Remember

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TSI vs KIA (Kligler Iron Agar)

• M - Motility
• M - Mannitol fermentation (or MR - Methyl Red)
• T - Triple Sugar Iron (TSI)
• P - Phenylalanine deaminase (or Peptone water indole)
• C - Citrate utilization
• U - Urease

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The MMTPCU Battery - Overview

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Indole Test (the "P" in MMTPCU)

What It Detects

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Principle

Tryptophan  →  Indole + Pyruvic acid + Ammonia
              (tryptophanase enzyme)

• Tryptophan is present in the peptone water (tryptone broth) medium - it is rich in this amino acid
• Organisms with tryptophanase cleave tryptophan at the indole nucleus → indole accumulates in the medium
• Indole is detected by adding Kovacs' reagent (p-dimethylaminobenzaldehyde in amyl alcohol + HCl)
• Indole reacts with p-dimethylaminobenzaldehyde → forms rosindole dye = cherry-red ring at top of medium

Indole + p-dimethylaminobenzaldehyde (Kovacs') → Red rosindole complex

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Medium Used

Alternatively: SIM medium (Sulfide-Indole-Motility) is used when you want to detect indole + motility + H₂S simultaneously in one tube - very common in practice.

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Reagents

• p-dimethylaminobenzaldehyde - 5 g
• Amyl alcohol (isoamyl alcohol) - 75 mL
• Concentrated HCl - 25 mL
• Forms a yellow layer on top of medium; turns red if indole is present

• Same principle, but uses ethanol instead of amyl alcohol
• More sensitive for organisms producing small amounts of indole

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Procedure

1. Inoculate peptone water / tryptone broth (or stab into SIM medium) with test organism
2. Incubate at 35-37°C for 24-48 hours
3. Add 3-5 drops of Kovacs' reagent directly to the surface of the broth (it forms a separate layer on top)
4. Observe immediately (within seconds)

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Result Interpretation

Read immediately after adding Kovacs' - the red color develops within seconds. A delayed orange color can be a false positive.

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Indole Results of Common Organisms in MMTPCU Context

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Key Differentiations the Indole Test Achieves

1. E. coli vs. Klebsiella pneumoniae - Both are common UTI causes; E. coli is indole (+), K. pneumoniae is indole (-)
2. Proteus vulgaris vs. Proteus mirabilis - P. vulgaris is indole (+), P. mirabilis is indole (-); both are H₂S positive/motile on TSI
3. Shigella dysenteriae vs. other Shigella spp. - S. dysenteriae is indole (+); others are indole (-)
4. E. coli vs. Enterobacter/Klebsiella - E. coli is indole (+); the others are indole (-)

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How Indole Fits into the Full MMTPCU Identification Scheme

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Important Points to Remember

• Medium must be rich in tryptophan (peptone water/tryptone broth) - ordinary nutrient broth does not work
• Read immediately after adding Kovacs' - a false orange tinge can appear with delay
• Kovacs' reagent is toxic (HCl + amyl alcohol) - handle with care
• In SIM medium, indole is tested last (after reading H₂S and motility) by adding Kovacs' to the top of the semisolid tube
• Indole alone is not diagnostic - always interpret within the full MMTPCU battery

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Citrate Utilization Test (the "C" in MMTPCU / IMViC)

What It Tests

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Medium Used: Simmons Citrate Agar

Composition (per liter)

Key design: No peptone, no glucose, no organic nutrients - only citrate and ammonium. Only organisms with the full metabolic machinery to use these can grow.

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Principle

Biochemical Pathway (Step by Step)

• Citrate enters the cell via a specific citrate permease (membrane transporter)
• Only organisms possessing this permease can import citrate

Citrate  →  Oxaloacetate + Acetate
            (citrate lyase enzyme)
Oxaloacetate  →  Pyruvate + CO₂

• Products (CO₂, acetate, pyruvate) are used for biosynthesis and energy
• CO₂ combines with water and sodium → forms sodium carbonate (Na₂CO₃) - an alkaline compound

• Organism uses ammonium salt (NH₄H₂PO₄) as nitrogen source
• NH₃ is released → ammonium hydroxide (NH₄OH) → further raises pH

• Both Na₂CO₃ and NH₄OH are alkaline → pH rises above 7.6
• Bromothymol blue changes from green (neutral) → blue (alkaline)
• Visible color change = positive test

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Procedure

1. Inoculate using a light inoculum - use a sterile wire loop or needle with a cell suspension in sterile water

   Important: Do NOT carry over rich medium - even a tiny amount of peptone/glucose from nutrient agar causes false positives

2. Streak the surface of the slant in a zig-zag motion (do not stab the butt)
3. Leave cap slightly loose to allow gas exchange (CO₂ needs to form)
4. Incubate at 35-37°C for 24-96 hours (up to 4 days - some slow citrate users need extra time)
5. Observe color and growth

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Result Interpretation

Key rule: Growth alone (even without color change) = POSITIVE. Because if there's no other carbon source in the medium, any visible growth means citrate was utilized.

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Organisms and Their Citrate Results

Citrate POSITIVE (+) → Blue / Growth

Citrate NEGATIVE (-) → Green / No growth

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How Citrate Fits in MMTPCU Identification

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Important Points to Remember

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Alternative Citrate Media

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Urease Test

What It Detects

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Principle

Biochemical Reaction

Urea + H₂O  →(urease)→  2 NH₃ + CO₂

2 NH₃ + CO₂ + H₂O  →  (NH₄)₂CO₃  (ammonium carbonate)
                          ↓
                    Alkaline medium (pH rises)

• Ammonia + CO₂ form ammonium carbonate - a strongly alkaline compound
• The rising pH is detected by phenol red indicator:
  • Phenol red is yellow/orange at pH 6.8 (neutral-acidic)
  • Phenol red turns bright pink/magenta at pH 8.1-8.2 (alkaline)
• The color change from yellow-orange → bright pink/magenta = urease positive

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Medium Used: Christensen's Urea Agar

Composition (per liter)

Critical: Urea is heat-labile - it must be filter-sterilized and added aseptically to cooled (50°C) agar. Autoclaving destroys urea.

Why Low Peptone + Reduced Buffer?

• Less peptone = less buffering capacity = even small amounts of ammonia can shift pH
• This allows detection of weakly urease-positive organisms (Klebsiella, Enterobacter)
• Also allows detection of slow/delayed urease producers

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Procedure

1. Streak the entire surface of the urea agar slant heavily with a well-isolated colony
2. Leave the cap loosely closed (CO₂ exchange needed)
3. Incubate at 35-37°C
4. Read at 15 min, 1h, 4h, 6h, 24h, 48h, up to 7 days
   • Early readings identify rapid producers
   • Delayed readings identify slow producers

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Result Interpretation

Compare slant color to the unstabbed butt (internal control). If butt stays orange/yellow but slant is pink = true positive. This rules out non-specific color changes.

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Speed of Reaction - Three Categories

Proteus mirabilis is the classic rapid urease positive - entire medium bright pink within 15-30 minutes. This is so characteristic it is almost diagnostic for Proteus.

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Urease Results of Clinically Important Organisms

POSITIVE (+)

NEGATIVE (-)

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Types of Urease Test Media

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Urease Test in MMTPCU Identification

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Key Points to Remember