Difference Between Gram Stain and Culture

Key Difference – Gram Stain vs Culture
 

Gram stain is a staining technique which is performed to differentiate bacteria into two groups according to the thickness of the peptidoglycan layer in their cell wall. Culture is a method of growing and maintaining microorganisms under laboratory conditions for different analysis. The key difference between grams stain and culture is their function; gram strain a staining technique of bacteria where culture is a method of growing and maintaining microorganisms.

CONTENTS
1. Overview and Key Difference
2. What is Gram Stain
3. What is Culture
4. Side by Side Comparison – Gram Stain vs Culture
5. Summary

What is Gram Stain?

Gram stain is an important differential staining technique used for bacterial identification in Microbiology. This technique was introduced by Danish Bacteriologist Hans Christian Gram in 1884. Gram staining categorizes bacteria into two major groups: gram positive and gram negative; these are very important in bacterial classification and identification. Gram staining is performed as an initial step for bacterial characterization.

Bacteria are grouped based on the differences in their cell wall. Gram positive bacteria contain a thick peptidoglycan layer in their cell wall while Gram negative bacteria contain a thin peptidoglycan layer in their cell wall as shown in figure 01. The outcome of the gram staining will be based on the thickness difference in the peptidoglycan layer of the cell wall.

Figure 1: Gram positive bacteria and Gram negative bacteria

Grams staining is performed using four different reagents namely; primary stain, mordant, decolorizing agent and counter stain. Crystal violet and safranin serve as the primary and counter stains, respectively while the grams iodine and 95% alcohol serve as the mordant and decolourizer, respectively.

Basic Steps of Grams Stain

1. A bacterial smear is prepared on a clean glass slide, heat fixed and cooled.
2. Smear is flooded with crystal violet for 1 – 2 minutes.
3. Smear is rinsed with slow running tap water to remove excess stains.
4. Grams iodine is applied to the smear for 1 minute.
5. Smear is rinsed with slow running tap water
6. Smear is washed with 95% alcohol for 2 – 5 seconds and rinsed with slow running tap water.
7. Smear is counter stained with safranin for 1 minute
8. Smear is rinsed with slow running tap water, dried and observed under the microscope.

At the end of the gram stain, gram negative bacteria will be observed in pink colour while the grams positive bacteria will be observed in purple colour as shown in Figure 02. The outcome of the grams stain is decided by the thickness of the peptidoglycan layer in their cell wall. During the decolorizing step, primary stain and the mordant are easily removed from the gram negative bacteria and become colorless since they have a thin peptidoglycan layer. The primary stain is retained in the grams positive bacteria since they have a thick peptidoglycan layer. Counter stain will not be effective for grams positive bacteria due to the retention of the primary stain. So grams positive bacteria will be visible in primary stain colour, i.e., purple colour. Counter stain will stain the gram negative bacteria and will be visible in pick colour which is the safranin colour. Hence, it is easy to categorize bacteria into two groups by the gram stain and it is valuable in bacterial differentiation and identification.

Figure 2: Gram Strain

What is Culture?

Microbial culture is a method of culturing and maintaining microorganisms under laboratory conditions for different purposes. Cultures are grown in solid, semi-solid and liquid media based on the type and the purpose of the microorganism culturing. Cultures are provided with necessary nutrients and growth conditions required by the microorganisms. There are different components of a culture medium such as energy source, carbon source, nitrogen source, minerals, micronutrients, water, solidifying agent, etc. Optimum temperature, oxygen and pH should be adjusted according to the type of the microorganism grown.

There are different types of microbial cultures; for example, batch culture, continuous culture, stab culture, agar plate culture, broth culture, etc. According to the composition of the growing medium, there are different types of culture media known as synthetic media, semi-synthetic media and natural media. Microbial cultures are prepared under sterile conditions inside a special chamber called laminar air flow. Growing medium and the glassware are sterilized prior to inoculation of the desired microorganism. Under proper sterile conditions, target microorganism is transferred into the sterilized nutrient medium and incubated at optimum temperature. Inside the medium, microorganism will grow and multiply using the provided nutrients.

Figure 3:  A bacterial culture on plate

What is the difference between Gram Stain and Culture?

Summary – Gram Stain vs Culture

Microbial cultures are prepared and maintained under laboratory conditions for different purposes such as storage, testing, chemical purification, etc. Grams stain is a staining procedure which differentiates bacteria into two main groups called grams negative bacteria and grams positive bacteria. Therefore, the difference between the grams stain and the culture is that grams stain is a staining technique of bacteria while the culture is a method of growing and maintaining microorganisms in the laboratories.

Reference:
1. Aryal, Sagar. “Gram Staining: Principle, Procedure, Interpretation, Examples and Animation.” Online Microbiology Notes. N.p., 20 Sept. 2015. Web. 01 Mar. 2017.
2. “Microbiological culture.” Wikipedia. Wikimedia Foundation, 01 Feb. 2017. Web. 01 Mar. 2017
3. Kumar, Sabarish. “What are the Basic Techniques of Microbial Culture?” What are the Basic Techniques of Microbial Culture? N.p., n.d. Web. 01 Mar. 2017

Image Courtesy:
1. “Gram-Cell-wall”Graevemoore inglise Vikipeediast (CC BY-SA 3.0) via Commons Wikimedia
2. “Gram Strain” By Y tambe – Y tambe’s file (CC BY-SA 3.0) via Commons Wikimedia
3. “Anthrax Culture” By U.S. Army Medical Research Institute of Infectious Diseases photo –  (Public Domain) via Commons Wikimedia

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