Bacteria can be classified broadly into two different groups: gram-positive and gram-negative bacteria. These classifications are based on their appearance after gram staining, a four-step procedure invented by scientist Hans Christian Gram in the 1880s. Gram-positive bacteria have certain characteristics that we must know and that scientists use to help develop medications that have gram-positive anti-bacterial activity.
What are Gram-positive bacteria?
Gram-positive bacteria are a type of bacteria with a thick cell wall made of peptidoglycans which turns purple after Gram staining.
It's as simple as that: the Gram-positive/Gram-negative classification is just a way to differentiate bacteria in terms of their reaction to a really simple and common staining method, the Gram stain. That reaction, though, has a biological reason behind it.Gram-positive and Gram-negative bacteria are morphologically different: Gram-positive bacteria have a thick cell wall, while Gram-negative bacteria have a thin cell wall and a lipid outer membrane.
Gram-positive bacteria colour
Gram-positive bacteria appear purple after they go through the gram stain procedure. Gram staining involves four steps:
Primary staining with crystal violet
Fixing the sample
Decolorizing with alcohol wash
Counterstaining with safranin
The four substrates that are used during each stage are, respectively:
Crystal violet as the primary stain - this turns bacteria purple.
Iodine as the fixing agent - this doesn't change the bacteria's colour.
Ethyl alcohol as the decolourizer - this washes out the purple colour of gram-negative bacteria, but doesn't change the colour of gram-positive bacteria
Safranin as the counterstain - this turns the now colourless gram-negative bacteria pink, but cannot overwhelm the purple colour of gram-positive bacteria, so they are unaffected by safranin and stay purple.
Gram-positive bacteria remain purple following all four steps of gram staining because of certain unique properties of their cell walls. We will explain how and discuss these properties below.
Gram-positive bacteria cell wall features
Gram-positive bacteria have a cell membrane comprised of a phospholipid bilayer. External to this membrane there are a few additional features. In order of closeness to the cell membrane, they are:
- the periplasmic space,
- the thick peptidoglycan cell wall, and
- the teichoic acids and lipoteichoic acids (Fig. 1).
Thick peptidoglycan cell wall
Gram-positive bacteria have a cell wall comprised of peptidoglycan. As the name suggests, peptidoglycan is comprised of both amino acids (peptides) and sugars (glycans).
Specifically, NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid) are the two amino acid sugars that make up the backbone of peptidoglycan.
NAM can carry a chain of four amino acids, called a tetra-peptide side chain. NAG and NAM are cross-linked together, and tetra-peptide side chains are linked to each other as well, giving peptidoglycan its strong but porous structure (Fig. 2).
This basic peptidoglycan structure is present in both gram-positive and gram-negative bacteria, but what differentiates the two? Well, in gram-positive bacteria this peptidoglycan cell wall is very thick.
The thickness of the cell wall allows iodine, the fixing agent (also called a mordant) to properly affix crystal violet to the bacteria during gram staining. Also, the pores within the cell wall, with the addition of ethyl alcohol, cause dehydration of the cell, as well as shrinking and closing of the pores. This helps trap the purple colour from the crystal violet dye and prevents gram-positive cells from being decolourized by the alcohol wash.
Lipoteichoic acids (and teichoic acids)
Lipoteichoic acids, unlike peptidoglycans, are only seen in Gram-positive bacteria. You will not find lipoteichoic acids in gram-negative bacteria.
Lipoteichoic acids and teichoic acids are virulence factors. Their exact methods of action are not yet completely known to scientists, however, we do know they are analogous to lipopolysaccharides (LPS) in gram-negative bacteria.
They arise from the plasma membrane (lipoteichoic acids) or the peptidoglycan wall (teichoic acids) and stick out above the cell wall. Here, they can activate enzymes or illness-inducing cytokines, that increase the virulence and pathogenicity of the bacteria that contain them.
Periplasmic space
Gram-positive bacteria have a small periplasmic space between their plasma membrane and their cell wall. The most important thing to note here is that the periplasmic space of gram-positive bacteria is much smaller than that of gram-negative bacteria. In fact, you may not see periplasmic spaces mentioned for gram-positive bacteria in all sources, but you should be aware that it is there.
Difference between Gram-positive and Gram-negative bacteria
Gram-positive and Gram-negative bacteria have quite a few differences in structure. These differences help decide what organisms and what organs within those organisms they can infect. We will outline the differences between gram-positive and gram-negative bacteria below.
- Difference #1: Gram-positive bacteria have thick peptidoglycan cell walls. Gram-negative bacteria on the other hand have thin peptidoglycan cell walls.
- Difference #2: Gram-positive bacteria have lipoteichoic acids and teichoic acids. Gram-negative bacteria do not have either.
- Difference #3: Gram-positive bacteria do not have lipopolysaccharides (LPS) on the outside of their cell walls, while gram-negative bacteria do.
- Difference #4: Gram-positive bacteria do not have an outer membrane. They only have a single cell membrane, beneath their cell wall. Gram-negative bacteria do have an outer membrane.
- Difference #5: Gram-positive bacteria are defined by their purple colour after the process of gram staining. Gram-negative bacteria are defined by their pink colour after gram staining.
| Characteristic | Gram-positive (+) | Gram-negative (-) |
| Peptidoglycan cell wall | Thick | Thin |
| Lipoteichoic and teichoic acids | Yes | No |
| LPS | No | Yes |
| Outer membrane | No | Yes |
| Gram stain colour | Purple | Pink |
Table 1. Differences between Gram-positive and Gram-negative bacteria.Examples of Gram-positive bacteria
There are a number of Gram-positive bacteria that are relevant medically. These are the ones that can cause illness in humans and non-human animals.
Typically, bacterial agents that can cause disease are classified by two things. The first is their Gram stain, and the second is their shape. We will show here genuses of Gram-positive bacteria with two different shapes: rod-shaped (bacilli) or spherical (cocci). Listeria is the exception to this, because it has an intermediate shape, in between rod and sphere.
You will notice that sometimes the shape of the bacteria is depicted in the name (some genus names include the words bacillus or coccus), but not always.
Gram-positive bacilli
Bacillus
Corynebacterium
Clostridium
Gram-positive cocci
Staphylococcus
Streptococci
Enterococci
Gram-positive coccobacilli
List of diseases caused by Gram-positive bacteria
The bacilli and cocci that we listed above can all be quite virulent pathogens and can cause many diseases in human hosts. We will make a sample list of some of the diseases that can be caused by these bacteria.
Bacillus - this genus has three important species: B. anthracis, B. cereus, and B. subtilis.
B. anthracis = causes anthrax.
This can present in different ways - with black ulcers on the skin, diarrhoea, vomiting, or a severe lung infection.
B. cereus = causes acute food poisoning.
B. subtilis = this is a model organism and is the main organism used to study gram staining.
Clostridium - this genus has important species including C. botulinum, C. tetani, and C. perfringens.
C. botulinum = causes paralysis.
C. tetani = causes tetanus.
C. perfringens = causes massive wound necrosis.
Streptococcus = this genus has important species, including S. pyogenes and S. pneumonia.
Listeria = this genus has one very important species, capable of causing severe disease in human beings of all ages - L. monocytogenes.
Gram-Positive Bacteria - Key takeaways
- Gram-positive bacteria are purple after gram staining.
- Gram-positive bacteria have thick peptidoglycan cell walls.
- Gram-positive bacteria have lipoteichoic and teichoic acids, but do not have lipopolysaccharides.
- Gram-positive bacteria do not have outer membranes.
- There is a wide range of Gram-positive bacteria.
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