Gram Positive Bacteria

Introduction

Bacteria are microscopic organisms and are not visible to the naked eye. Studying the internal cell structure is much needed to understand the bacterial disease cycles. Most bacteria lack colour and are stained with staining agents for efficient observation under a light microscope. Gram staining is one such staining technique which imparts colour to the bacterial cell wall based on the cell wall biochemistry and its structural properties. Gram-positive bacteria retain the primary stain even after the discolouration step in Gram’s staining technique. Cell wall biochemical constituents allow the primary stain retention in the step-wise staining according to the Gram’s staining technique.

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Classification

Bacteria acquire different colours when stained with the Gram’s staining technique. Gram-positive bacteria appear violet-coloured in a microscopic observation when stained with the Gram’s staining technique. The thick cell wall retains crystal violet and hence they appear violet-coloured. Gram-negative bacteria appear in red colour after losing the crystal violet because of the relatively thin peptidoglycan layer.

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Gram-positive bacteria are differentiated based on cell shape.

• Cocci − spherical or round-shaped bacteria. Staphylococcus species occur as clusters of small spheres, while Streptococcal species occur as long chains. Examples, Staphylococcus aureus, and Streptococcus pyogenes.

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• Bacilli − rod-shaped (singular bacillus) bacteria. Bacilli are again spore-forming and non-spore-forming based on their ability to generate endospores. Examples, Bacillus anthracis, and Clostridium tetani.

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Features of Gram-positive bacteria

• The Gram-positive bacteria appear violet when stained with Gram’s staining technique.

• Similar to all other living cells, Gram-positive bacteria are enclosed by a plasma membrane made up of lipids.

• A thick cell wall composed of a peptidoglycan layer lies outside of the lipid cell membrane. This is the most distinguishing feature and differentiates it from Gram-negative bacteria.

• The presence of cell wall teichoic acids is another marked feature.

• Locomotion is performed by the flagella or cilia.

• Another distinctive characteristic is their ability to produce endospores to survive in harsh conditions.

Cell wall structure

Gram-positive bacteria possess a cell wall composed of peptidoglycan.

• Although peptidoglycan is the major component of most bacterial cell walls, gram-positive bacteria contain a thick and broad peptidoglycan cell wall.

• Peptidoglycan (murein) is a polysaccharide of N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM). Both NAG and NAM are glucose derivatives that alternate in long chains.

• Tetrapeptide chains composed of L-aminoacids cross-link the polysaccharide chains. The dense cross-linking in the peptidoglycan layer imparts rigidity to the bacterial cells.

• The presence of teichoic acids is an additional feature in Gram-positive bacterial cell walls. They are either linked covalently to peptidoglycan layers or attached to the lipid membrane. In the second case, they are referred to as lipoteichoic acids.

• Teichoic acids add more rigidity to the cell walls and help in maintaining bacterial cell shape even in adverse conditions of high temperatures and high saline environments. They are anionic glycopolymers that impart pathogenic factors and antibiotic resistance to the bacteria.

Examples

Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pyogenes, Streptococcus pneumoniae, Streptococcus viridans, Bacillus anthracis, Clostridium difficile, Clostridium botulinum, Clostridium perfringens, Clostridium tetani, Listeria monocytogenes, Corynebacterium diphtheria.

Benefits, risks and diseases

Gram-positive bacteria are widely distributed and have a diverse role in the living world.

Benefits

• The harmless species inhabit human skin, mouth, intestines, and upper respiratory tracts.

• Several non-pathogenic Bacillus and Corynebacterium species have wide applications in the biotechnology industry. They are employed to produce enzymes, antibiotics, nucleotides, amino acids and metabolites.

• Some gram-positive strains help plants to easily absorb unavailable soil nutrients and have wide importance in the agricultural industry.

• Several species are known for the degradation of chemicals, industrial production of cheese etc.

Risks

Bacteria are ubiquitous in nature and gram-positive bacteria live in the human body. They are resident flora of the skin, nose, mouth, throat and vagina. During surgeries or some low immune conditions, there are potential risks of infections leading to severe conditions.

Diseases

Although some gram-positive bacterial species are harmless, some pathogenic species cause severe infections in humans.

Causative pathogen	Disease
Staphylococcus aureus	Multiple infections like endocarditis, bacteremia, furuncles, carbuncles, cellulitis, scalded skin syndrome, osteomyelitis, septic arthritis, meningitis, pulmonary infections.
Streptococcus pneumoniae	ear infections, sepsis, pneumonia.
Streptococcus pyogenes	scarlet fever, sepsis, rheumatic fever, streptococcal toxic shock syndrome.
Clostridium tetani	Tetanus
Clostridium perfringens	Gas gangrene
Clostridium difficile	Diarrhoea, colitis
Clostridium botulinum	Botulism
Bacillus anthracis	Anthrax
Listeria monocytogenes	Sepsis, neonatal meningitis, Listeriosis

Conclusion

Bacteria are ubiquitous in nature and are not visible to the naked eye. Staining techniques impart colour to the bacterial cells and allow their visualisation. Gram-positive bacteria are those bacteria that appear in violet colour when stained with Gram’s staining technique. The gram-positive bacterial cells are either cocci shaped or rod-shaped. They contain a thick peptidoglycan cell wall with teichoic acids. This thick cell wall retains crystal violet stain even on decolourisation in the step-wise grams staining technique. The thick and unique peptidoglycan layer imparts rigidity to the bacteria cells. They are both beneficial as well as harmful to humans. They are used at the industrial level to produce various metabolites, amino acids, enzymes etc. However, some pathogenic species can cause severe infections in humans.

FAQs

Q1. What are bacterial endospores?

Ans. Bacteria produce highly resistant and dormant endospores to protect themselves in unfavourable conditions. In conditions of nutrient deprival, desiccation, chemical changes, or exposure to ultraviolet radiation, Gram-positive bacteria produce dormant endospores.

Q2. How long can bacterial endospores survive?

Ans. Bacterial endospores can survive for even centuries and protect the bacterial genetic material. Endospores can break dormancy and enter into an active stage upon the advent of favourable external conditions.

Q3. Who invented Gram’s staining?

Ans. Gram’s staining was invented by Hans Christian Gram in 1884. It was invented while Christian Gram was studying the bacterial cocci of lung tissue sections from patients who died of pneumonia.

Q4. What are biological indicators in sterilisation procedures?

Ans. Biological indicators of sterilisation procedures contain spores of microorganisms in the vials. They are used to monitor the efficiency of sterilisers. Biological indicators contain microbial endospores of known heat resistance and the sterilisation procedure should be capable of killing the spores that are further checked with a growth test. For example, Geobacillus stearothermophillus is a gram-positive bacterium whose spores are used in monitoring the efficiency of autoclaves.

Q5. What is toxic shock syndrome?

Ans. Toxic shock syndrome is a fatal condition caused when bacteria enter the human body and produce toxins that have adverse effects. Although it is rare, the effects are severe and can be fatal too.

Q6. What is anthrax?

Ans. Anthrax is caused by Bacillus anthracis, a gram-positive, spore-producing bacterium. It basically infects animals however can infect humans upon inhalation of bacterial spores or also through direct contact with the infected animal, its body parts or even the fur extracted from an infected animal. The symptoms range from pink rashes to opened-up skin which can be fatal due to shock and breathing problems.