Stabs, Slants, and Streaks: A Microbe’s (Mini) Guide to Changing Colors

Microbiology feels like chemistry with a "biologic pulse". When we look at a rack of tubes or a stack of plates, we’re observing the visible evidence of complex, evolutionary adaptations in real-time. While bacteria may seem incredibly simple, they are far more complex than we realize.

In the search for novel soil antibiotics, these biochemical tests act as our first look into a microbe’s identity and competitive arsenal. These organisms have spent thousands, if not millions of years developing specialized enzymatic toolkits to thrive in high-stakes environments, and each shift in color or release of gas is a signature of those metabolic pathways in action. By mapping these hardwired behaviors through standardized biochemical protocols, we can better identify the unique candidates that might just hold the key to the next generation of antimicrobial discovery.

Selective and Differential Agars

Mannitol Salt Agar (MSA)

Goal:To identify halotolerant (salt-loving) bacteria and determine if they can ferment mannitol.

Protocol:

1. Use a sterile loop to streak your sample onto the MSA plate using the quadrant streak method

2. Incubate at 37°C for 24–48 hours.

Results/Interpretations:

• Growth: The organism is halotolerant (likely Staphylococcus species).

• Yellow Halo: Positive for mannitol fermentation (acid produced drops the pH).

• Pink/Red Media: Negative for mannitol fermentation.

5% Sheep's Blood Agar

Goal: To test for hemolytic activity (the ability to break down red blood cells).

Protocol:

1. Streak the plate for isolation.

2. Incubate at 37°C for 24 hours (often in a CO2 incubator).

Results/Interpretations:

• Beta: Complete lysis. Clear, transparent zone around colonies.

• Alpha: Partial lysis. Greenish or "bruised" discoloration around growth.

• Gamma: No lysis. The agar remains red and unchanged.

DNAse Agar

Goal: To see if an organism produces the enzyme deoxyribonuclease to break down DNA.

Protocol:

1. Spot inoculate a heavy "dime-sized" circle of bacteria onto the agar.

2. Incubate at 37°C for 24 hours.

3. If using agar without an indicator dye, flood the plate with 1N HCl.

Results/Interpretations:

• Positive: A clear halo appears around the growth (where DNA was eaten away).

• Negative: The agar remains cloudy (DNA is still intact).

Eosin Methylene Blue (EMB) Agar

Goal: To select for gram-negative bacteria and differentiate lactose fermenters.

Protocol:

1. Streak for isolation.

2. Incubate at 37°C for 24–48 hours.

Results/Interpretations:

• Metallic Green Sheen: Vigorous lactose fermentation (classic for E. coli).

• Pink/Purple/Dark Centers: Slow to moderate lactose fermentation (coliforms).

• Colorless: Non-lactose fermenters.

Vigorous lactose fermentation resulting in the shiny green appearance.

Tube-Based Tests

S.I.M. Medium (Sulfur, Indole, Motility)

Goal: A three-in-one test for hydrogen sulfide production, indole formation, and movement.

Protocol:

1. Stab the medium 2/3 of the way down with an inoculating needle.

2. Incubate for 24–48 hours.

3. After incubation, add five drops of Kovac’s Reagent to the top.

Results/Interpretations:

• Sulfur (S): Black precipitate = hydrogen sulfide positive.

• Indole (I): Red/Pink ring at the top = Indole positive (tryptophan was broken down).

• Motility (M): Cloudiness radiating away from the stab line = Motile.

Kligler’s Iron Agar (KIA)

Goal: To differentiate Enterobacteriaceae based on glucose/lactose fermentation and sulfur reduction.

Protocol:

1. Stab the "butt" (bottom) of the tube and streak the "slant" (surface).

2. Incubate with a loose cap for 18–24 hours (timing is critical!).

Results/Interpretations:

• Red Slant/Yellow Butt: Indicates glucose fermenter and lactose non-fermenter.

• Yellow Slant/Yellow Butt: Indicates lactose fermentation (either lactose alone or both lactose and glucose fermentation).

• Red Slant/Red Butt: Indicates both lactose and glucose non-fermenter.

• Blackening of Agar: Hydrogen sulfide produced.

• Cracks/Bubbles in Agar: Gas produced.

Gelatinase

Goal: To detect the enzyme gelatinase, which liquefies gelatin.

Protocol:

1. Heavily stab a nutrient gelatin tube.

2. Incubate at 37°C for 48 hours to a week.

3. Crucial: Place the tube in the refrigerator for 20 minutes before reading.

Results/Interpretations:

• Liquid: Positive for gelatinase (the protein "glue" is broken).

• Solid: Negative

Methyl Red/Voges-Proskauer (MR/VP)

Goal: To determine which metabolic pathway is used to ferment glucose.

Protocol:

1. Inoculate MRVP broth and incubate for 48 hours.

2. Split the broth into two tubes.

   1. MR: Add five drops of Methyl Red.

   2. VP: Add Barritt’s Reagents A and B; shake well.

Results/Interpretations:

• MR Positive: Red color (stable acid produced)

• VP Positive: Red/pink color after 15-30 minutes; acetoin produced!

Rapid ("Spot") Tests

Catalase

Goal: To detect the enzyme catalase, which breaks down hydrogen peroxide (H2O2).

Protocol:

1. Place a drop of H2O2 on a glass slide.

2. Use a plastic loop or sterile toothpick to smear a colony into the drop.

Results/Interpretations:

• Bubbles: Positive (Oxygen is being released).

• No Bubbles: Negative.

Oxidase

Goal: To identify bacteria that use Cytochrome c oxidase in their electron transport chain.

Protocol:

1. Apply a drop of Oxidase reagent to a piece of filter paper.

   1. You can use Oxidase disks if you have them

2. Rub a colony onto the damp paper using a plastic loop (metal loops can cause false positives!).

Results/Interpretations:

• Positive: Turns dark blue/purple within 20 seconds.

• Negative: No color change or change after 30 seconds.

---

All information and photos taken from their respective MicrobeNotes.com articles; a wonderful resource for students!