Difference Between Gram Positive and Gram Negative Bacteria

The key difference between gram positive and gram negative bacteria is that gram positive bacteria have a thick peptidoglycan layer, hence appear in purple colour while gram negative bacteria have a thin peptidoglycan layer, hence appear in pink colour at the end of the gram staining technique.

Bacteria are ubiquitous prokaryotes which are unicellular and microscopic. They belong to Kingdom Monera along with the Archaea. Moreover, they have a very simple cellular organization. Structurally, they lack internal compartments; nucleus and membrane-bound other organelles. Moreover, bacteria can be classified based on several characteristics such as shape, genetic makeup, cell wall composition, number of flagella, nutrition, biochemical reactions, etc. Gram staining is one of the techniques that commonly employ in identification and characterization of bacteria. According to the gram staining, there are two categories of bacteria such as gram positive and gram negative bacteria. These two groups of bacteria differ from each other by the cell wall composition. Hence, they stain in different colours during the grams staining technique.

CONTENTS

1. Overview and Key Difference
2. What is Gram Positive Bacteria
3. What is Gram Negative Bacteria
4. Similarities Between Gram Positive and Gram Negative Bacteria
5. Side by Side Comparison – Gram Positive vs Gram Negative Bacteria in Tabular Form
6. Summary

What is Gram Positive Bacteria?

Gram positive bacteria are a group of bacteria which stain in purple colour in the grams staining technique. The reason behind the staining in purple colour is that gram positive bacteria have a thick peptidoglycan layer in their cell walls. Normally, peptidoglycan layer of gram positive bacteria ranges between 20-80 nm thickness and teichoic acids are present on it.

Thick peptidoglycan layer retains the primary stain, which is crystal violet; hence, appear in purple or crystal violet colour under the microscope. Though decolourizing agent removes the primary stain, stain does not completely leave from the thick peptidoglycan layer. Staphylococcus, Streptococcus, Bacillus, Corynebacterium, Listeria and Clostridia species are some examples of gram positive bacteria.

What are Gram Negative Bacteria?

Gram negative bacteria are a group of bacteria which have a thin peptidoglycan layer in their cell wall. Hence, they are incapable of retaining the primary stain. Characteristically, gram negative bacteria have an extra membrane called as outer membrane, which is absent in gram positive bacteria. Also, this outer membrane contains lipopolysaccharides. Additionally, though an outer membrane is present, it degrades once the decolorizer is applied. Then, the peptidoglycan layer becomes porous, and the crystal violet stain completely leaves the cell wall.

Hence, gram negative bacteria appear in the secondary stain colour which is pink. When comparing with the gram positive bacteria, gram negative bacteria are more resistant to cell wall targeting antibiotics. This is due to the presence of an outer membrane. Furthermore, their cell walls have two layers, and there is a periplasmic space in the cell wall. And also, the cell wall is uneven and less rigid compared to gram positive bacteria.  Escherichia coli, Pseudomonas, Neisseria, Chlamydia, are some of the gram negative bacteria.

What are the Similarities Between Gram Positive and Gram Negative Bacteria?

• Gram Positive and Gram Negative Bacteria have a similar cellular organization.
• Also, both are prokaryotic unicellular microorganisms which possess capsules.
• And, they both possess a single chromosome and are ubiquitous.
• Furthermore, both may contain plasmids as their extrachromosomal DNA.
• Besides, both reproduce asexually by binary fission.
• Similarly, both undergo a transformation, transduction and conjugation.
• Moreover, Gram Positive and Gram Negative Bacteria are inhibited by antibiotics.
• Their cell walls contain peptidoglycan.
• And also both types of bacteria have a surface layer (S layer).
• They respond to grams staining procedure.
• Furthermore, they cause diseases to humans, plants and animals.

What is the Difference Between Gram Positive and Gram Negative Bacteria?

The major difference between gram positive and gram negative bacteria is that gram positive bacteria have a thick peptidoglycan layer in their cell wall while gram negative bacteria have a thin peptidoglycan layer in their cell wall. Apart from the peptidoglycan layer, gram negative bacteria possess an outer membrane and it is absent in gram positive bacteria. Hence, this is also a difference between gram positive and gram negative bacteria. Furthermore, gram negative bacteria have a periplasmic space and two layers in the cell wall while gram positive bacteria lack a periplasmic space and they have a single layered rigid and even cell wall.

The following infographic describes more facts regarding the difference between gram positive and gram negative bacteria.

Summary – Gram Positive vs Gram Negative Bacteria

Depending on the bacteria take up and retain the primary stain; crystal violet during the gram staining, there are two types of bacteria namely gram positive and gram negative. Gram positive bacteria have a thick peptidoglycan layer in the cell wall while gram negative bacteria have a thin peptidoglycan layer. This is the major difference between gram positive and gram negative bacteria. Due to this difference in the cell wall, gram positive bacteria stain in purple colour while gram negative bacteria stain in pink colour in the grams staining. Furthermore, gram negative bacteria have an outer membrane while it is absent in gram positive bacteria. Due to the presence of an outer membrane, gram negative bacteria are resistant to cell wall targeting antibiotics while gram positive bacteria are susceptible to them. Therefore, this summarizes the difference between gram positive and gram negative bacteria.

Reference:

1.Vidyasagar, Aparna. “What Are Bacteria?” LiveScience, Purch, 23 July 2015. Available here 
2.Bhimji, Steve S. “Bacteria, Gram Positive.” StatPearls [Internet]., U.S. National Library of Medicine, 6 Dec. 2017. Available here 
3.Gram-Negative Bacteria. Available here 

Image Courtesy:

1.”photomicrograph, spherical, cocci, gram, positive, staphylococcus aureus, bacteria, magnified, 320x” by Dr. Richard Facklam, USCDCP (Public Domain) via pixnio
2.”Pseudomonas aeruginosa Gram”By Y_tambe – Y_tambe’s file, (CC BY-SA 3.0) via Commons Wikimedia