Gram Positive Bacteria – Classification And Characteristics

Bacteriology describes gram positive bacteria as bacteria that has a positive result when exposed to a gram stain test.

These tests are traditionally done so that the bacteria can be quickly classified into two umbrella groups – gram positive and gram negative. 

Depending on which group the bacteria falls into, each bacteria will have a different type of cell wall. Gram positive bacteria will show up in the crystal violet stain during the test.

Then, it will show to be purple in color once it is examined under a microscope. 

The reason for this is because the peptidoglycan layer is thick within the cell wall and even when washed away, the stain is retained.

Gram Positive Bacteria – Classification And Characteristics

Alternatively, gram negative bacteria doesn’t retain this violet stain after decolorization. 

The peptidoglycan layer is much thinner in the gram negative bacteria than in the gram positive bacteria.

These bacteria appear pink or red when examined under a microscope because of this. 

Due to the fact that gram positive bacteria has a thicker peptidoglycan layer, it is able to be more receptive to antibiotics that target cell walls because of the lack of outer membrane.

We are going to have a look in more detail at the classification and characteristics of gram positive bacteria. 


Gram staining is a quick method that is used to help differentiate different species of bacteria. Cell shape is also important in determining the different species as well. 

A full classification of bacteria is able to be determined once a stain test is done along with the relevant antibiotic susceptibility tests and growth requirements.

On top of this, other physiologic and macroscopic tests can be done. 

Outer Cell Membrane

Traditionally, bacteria are separated into two groups – gram positive and gram negative – classified by their gram stain retention properties.

This can be quite an ambiguous classification method due to the fact that it refers to three aspects. 

These aspects are the staining result, taxonomic group and envelope organization. Unfortunately, some of these aspects don’t actually help to indicate the species of bacteria. 

On top of this, the gram positive and gram negative staining responses are also not necessarily a great indicator for bacteria species.

To combat this, we also rely on the structure of the outer cell membrane of bacteria to be able to classify the species. 


Generally, you can find the following characteristics when examining gram positive bacteria. These characteristics are: 

  1. A Cytoplasmic lipid membrane
  2. Lipoids and teichoic acids are present which form lipoteichoic acids. These then serve as chelating agents. 
  3. A thick peptidoglycan layer 
  4. Smaller volumes of periplasm compared to gram negative bacteria
  5. Rigid cell walls are formed due to peptidoglycan cross linked chains

Some species of gram positive bacteria have capsules that contain polysaccharide.

You may also find some species which are flagellates. If they do have flagella then they will only have two basal body rings that exist to support them. 

In comparison to this, gram negative bacteria have four basal body rings. Both gram negative and gram positive bacteria are commonly known to have an S-layer. 

Gram positive bacteria has an S-layer that is attached to its peptidoglycan layer. In gram negative bacteria, the S-layer is actually attached to the bacteria’s outer membrane. 

You will only find teichoic acids present within the cell walls of gram positive bacteria.

On top of this, some of the acids are actually lipoteichoic acids due to their lipid component that is within the cell membrane. These aid in anchoring the peptidoglycan. 

Gram Positive Vs. Gram Negative Bacteria

Gram positive bacteria will always have a peptidoglycan layer that is thick as well as a distinct lack of an outer lipid membrane.

Opposite to this, gram negative bacteria have a thinner peptidoglycan layer as well as an outer lipid membrane. 

Due to the fact that gram positive bacteria have no outer lipid membrane, their structure is referred to as monoderms. This is only applicable to their structure and not their staining properties. 

Bacteria that are gram negative will have their physical structure referred to as diderms because of the presence of the outer lipid membrane. 

The staining technique used to determine whether bacteria is gram positive or gram negative was first developed in 1884 by Hans Christian Gram, a Danish bacteriologist.

A gram stain is never used to determine which specific species of bacteria you are dealing with. 

However, it is used to narrow down the list of potential species that the bacteria could be.

Having a narrow list of candidates means that other follow up tests can be done to determine which specific species the bacteria is, if necessary. 

Gram Stain

When it comes to the differences in staining, this is a little easier to determine. Gram positive bacteria will appear purple when viewed under a light microscope once the gram staining test is complete. 

The reason for this color is because of the cell wall’s thick peptidoglycan layer.

Gram negative bacteria will appear to be more reddish in color when it is viewed under a light microscope once the gram staining test is complete. 

The color of gram negative bacteria is this way due to the fact that the thinner peptidoglycan layer is unable to retain the violet stain and is therefore only left with the reddish stain. 


To find out whether bacteria is gram positive or gram negative, you will need to carry out a gram staining test. This will help to narrow down the list of potential bacteria species that you could be dealing with. 

If the bacteria is gram positive then the stain will appear violet purple in color.

This test is not used to find an exact species, other tests must be completed to determine the details. However, it does help to narrow it down. 

We hope this has helped you understand what gram positive bacteria is and how to determine whether bacteria are gram positive or gram negative.

Jennifer Dawkins

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