Bacteria are unicellular prokaryotic organisms devoid of nuclei. The size varies from 0.2 µm to 50 µm having varied morphology in their appearance. Examples of gram-positive bacteria are Methicillin-resistant Staphylococcus aureus (MRSA), and examples of gram-negative bacteria are Salmonella, Pneumonia and Gonorrhea. These groups of bacteria are a serious threat as they cause major diseases in humans.
All prokaryotic cells are surrounded by a complex cell made of peptidoglycan, also known as murein. In 1884, Christian Gram invented the staining procedure to classify bacteria. The bacteria that retain the Gram stain are gram-positive while which does not retain the gram stain are gram-negative bacteria. This difference lies in morphological variation seen in the cell wall structure of both gram-positive and gram-negative bacteria. Gram-positive and gram-negative bacteria difference pdf can be downloaded for more information.
The two major divisions of gram-positive bacteria are:
Phylum Actinobacteria - Have high GC content. Example: Streptomyces.
Phylum Firmicutes - Have low GC content. Examples: Bacillus and Clostridium.
In gram staining procedures, they appear purple yielding positive results.
Gram Stain Results in Gram-positive Bacteria
This group is composed of a thick, homogenous, singular, peptidoglycan layer (20 to 80 nm thick).
Gram-positive bacteria lack cellular appendages and are non-motile.
They generally appear round, rod or filamentous.
This group has the ability to form endospores.
Cell Wall Structure of Gram-Positive Bacteria and Gram-Negative Bacteria
The gram-negative bacteria belong to the phylum Pseudomonadota. The two major groups are Enterobacteriaceae and the non-fermenters.
In gram staining, they appear red or pink as they do not retain the colour of crystal violet dye.
This group is composed of a 2 to 7 nm thick peptidoglycan layer surrounded by an outer membrane.
They are round, rod or spherical in shape.
Reproduction is either by binary fission or budding.
These bacteria do not form endospores.
The cell wall of gram-positive and negative bacteria is made of peptidoglycan, also called murein. Peptidoglycan is a polymer having two sugar derivatives.
NAG - N-acetylglucosamine
NAM - N-acetylmuramic acid
These derivatives are linked together by β-1,4 glycosidic bonds. A carboxyl group of NAM tetrapeptide chains made of alternate D and L amino acids is linked. The peptide interbridge if present connects tetrapeptide chains. This peptide cross-link reaction is called transpeptidation.
The cell wall of gram-positive bacteria contains acidic negatively charged substances called teichoic acid. These are the polymers of glycerol and ribitol linked by phosphate moiety. The presence of teichoic acid makes the cell wall rigid by attracting positive ions like magnesium and sodium. In gram-negative bacteria, teichoic acid is absent.
Linking of nucleotides with sugar precursors to form UDP-NAM and UDP-NAG; this step occurs in the cytoplasm.
Addition of amino acid group to UDP-NAM in a sequential manner to form NAM-pentapeptide.
In the third step, UDP-NAM pentapeptide forms a complex with bactoprenol to form lipid I. This step occurs at the plasma membrane.
A NAG molecule from a complex of UDP-NAG gets added to lipid I to form lipid II.
In the last stage, an outside plasma membrane polymerisation reaction occurs.
Gram-negative bacteria have an additional outer membrane composed of lipids and polysaccharides. This complex is called the lipopolysaccharide complex (LPS). LPS acts as an endotoxin layer as it contains lipid A which is toxic. The outer membrane has porins which make it more permeable than the plasma membrane.
Structure of Gram-positive Cell Wall
These are non-pathogenic and are found in the human body.
Some of the gram-positive bacteria help in the formation of cheese.
Corynebacteria are used in the large-scale production of enzymes, nucleotides and amino acids.
Bacillus amyloliquefaciens acts as a natural antibiotic protein called barnase.
According to WHO, a few pathogens, gram-positive bacteria are a serious concern and a health care problem, examples: multidrug-resistant (MDR) bacteria like methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium (VRE) and β-lactamase-resistant Streptococcus pneumonia.
A new antibiotic named teixobactin produced by Eleftheria terrae was discovered in 2015. It is highly effective against resistant bacteria.
What are the differences between the cell walls of gram-positive and gram-negative bacteria?
Ans: The gram-positive bacteria have single-layered cell wall while gram-negative bacteria cell wall is multilayered having outer LPS membrane. This makes gram-negative bacteria more resistant.
What are the major diseases caused by gram-negative bacteria?
Ans: Diseases caused by gram-negative bacteria are a major concern to the healthcare industry as they are highly resistant to the action of antibiotics. Major infections include pneumonia, bloodstream infections, wound or surgical site infections, and meningitis.
The cell wall of gram-positive and gram-negative bacteria is a chemical complex structure. It protects bacteria and plays a key role in adaptation.
The major component of the cell wall is peptidoglycan which is conserved in all prokaryotic organisms.
Gram-negative bacteria have outer membranes, thus, these groups of bacteria are human pathogens.
1. Which is more infectious, gram-positive or gram-negative?
The gram-negative bacteria have an LPS layer which makes them infectious. Lipid A present in the LPS layer is toxic. These bacteria are human pathogens that can cause severe diseases like cholera and pneumonia.
2. Why is it important to classify bacteria as gram-positive or gram-negative?
It is important to identify gram-positive bacteria from gram-negative bacteria to rule out the causative agent. This can help doctors to give proper medication.
3. What are endospores?
When conditions are unfavourable, the process of sporulation starts. The endospore is a complex structure having multiple layers. This makes bacterial cells heat-resistant and avoids dehydration in bacteria.