Content Writer-SME | Updated On - Nov 19, 2024
Bacteria are microscopic unicellular organisms that lack a true nucleus and membrane-bound organelles. These microorganisms are composed of a single DNA, flagella, ribosomes, pilli, cell wall, and cytoplasm. Gram positive and gram negative are the two categories of bacteria.
There are few parameters that are used to differentiate between gram negative and gram positive bacteria such as colour, appearance, and composition. Gram staining is a multi-step process which is done to identify the type of bacteria. A gram-positive bacteria would retain a violet colour while a gram-negative bacteria doesn’t.
Gram Positive and Negative Bacteria
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Gram positive and negative bacteria are the two categories of bacteria that can be differentiated by gram staining. Gram staining is a common staining method which was introduced by Hans Christian Gram in 1882. He proposed this method to identify the bacteria based on their cell wall structures. This process is usually carried out using a primary stain called crystal violet.
- The cell wall of bacteria is composed of a layer called peptidoglycan.
- The Gram positive organisms have a thick peptidoglycan layer which retains the violet colour.
- Gram negative organisms can not retain the stain because of its thin peptidoglycan layer.
Gram Staining
There are four major steps in gram staining done for differentiating gram positive and gram negative bacteria.
- Staining with primary stain (crystal violet)
- Staining with mordant (Gram’s iodine)
- Decolourization using ethanol or acetone
- Counterstaining with safranin
Procedure
- The first step of gram staining involves staining or dying of cells with crystal voilet dye. The step is followed by addition of iodine solution, forming a complex.
- Ethanol or acetone is added to the sample decolorizer, which generally dehydrates the peptidoglycan layer and retains the violet color in the cell wall in Gram-positive bacteria.
- The decolorized gram-negative cells are stained red with safranin and observed under the microscope.
Characteristics of Gram Positive Bacteria
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These are the bacteria that give a positive result in the Gram staining method. It is a method used to classify the bacteria depending on their cell wall.
- Gram positive bacteria have thick cell walls consisting of multiple layers of peptidoglycan and it retains the violet stain in the gram staining method.
- Upon the application of mordant stain which is iodine and ethanol, these bacteria show a violet color.
- Gram-positive cell wall are thick and forms a rigid and thick structure. The cell additionally consists of phosphate and teichoic acids.
- Teichoic acids are of two types- Lipoteichoic Acid and Teichoic Wall Acid.
- Examples of gram positive bacteria are Staphylococcus aureus, Bacillus cereus, Clostridium botulinum, etc.
Characteristics of Gram Negative Bacteria
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Gram negative bacteria are those that do not give a positive result in the Gram staining method. It does not hold onto the crystal violet color dye during the Gram staining process rather it holds onto a pale pink color when examined under a microscope.
- The outer membrane of this bacteria is made up of phospholipids and lipoproteins.
- They appear pink because the safranin dye used is washed with ethanol.
- The outer membrane of gram-negative bacteria is permeable due to the presence of porins.
- Nutrients, food, and other components pass through these porins.
- Examples of gram negative bacteria are Pseudomonas, Salmonella, and Escherichia.
Difference between Gram Positive and Negative Bacteria
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Gram positive and gram negative bacteria are distinguished on the basis of several parameters like cell wall structure, presence of outer membrace, teichoic acids, flagellar structure, etc. The table below shows the difference between the two:
| Basis of Comparison | Gram Positive Bacteria | Gram Negative Bacteria |
|---|---|---|
| Gram Staining | Retains crystal violet stain to obtain a purple colour | Does not retain crystal violet stain and is decolorized to obtain pink or red stain. |
| Cell Wall Structure | Smooth | Wavy |
| Cell wall Thickness | 20-30 nm thick | 2-3 nm thick |
| Teichoic Acids | Present in many | Absent |
| Outer membrane | Absent | Present |
| Lipopolysaccharide Content (LPS) | None | High |
| Mesosomes | More notable | Less notable |
| Toxin produced | Exotoxins | Exotoxins and Endotoxins |
| Peptidoglycan Layer | Multilayered Thick layer | Single-layered Thin layer |
| Porins | Absent | It occurs in the outer membrane |
| Lipoprotein Content | Low | High |
| Flagellar Structure | 2 rings (basal body) | 4 rings (basal body) |
| Susceptible to antibiotics | More susceptible to antibiotics | Less susceptible to antibiotics |
| Lipid Content | Low | 20 to 30% |
| Example | Bacillus, Staphylococcus, Enterococcus, Clostridium, Streptococcus. | Pseudomonas, Proteus, Salmonella, Helicobacter, Escherichia. |
In conclusion, gram negative bacteria are considered more harmful than gram positive bacteria due to the presence of a hard shell of peptidoglycan layer in gram negative bacteria. The tough outer layer makes the bacterial cell wall more resistant to antibiotics.
Things to remember
- Gram-positive and gram-negative bacteria are identified based on their ability to retain the gram stain.
- Safranin is used to stain the decolorized gram-negative bacteria into pink or red.
- Anthrax bacterium is one of the examples of gram-positive bacteria.
- These bacteria have a thick peptidoglycan layer whereas, a thin peptidoglycan layer is found in gram-negative bacteria.
- Gram-positive microorganisms have an outer covering of glycoprotein called glycocalyx.
Previous Year Questions
- The movement of cilia and flagella is due to the presence of...[JKCET 2013]
- Plasma membrane helps in :...[JKCET 2005]
- Which of the following structure is bounded by single membrane?...[JKCET 2005]
- 70 S ribosomes occur in….[AMUEEE 2014]
- Rough endoplasmic reticulum is actively involved in...[JKCET 2014]
- Perinuclear space is present in...[JKCET 2014]
- Chloroplast lacks….[COMEDK UGET 2005]
- Colour of flower petals is due to...[CUCET 2012]
- Streaming of the cytoplasms within some living cell is known as...[CUCET 2011]
- Example of an oxidative organelle is...[CUCET 2011]
- Lampbrush chromosomes are...[COMEDK UGET 2005]
- Interconnected flattened sacs of endoplasmic reticulum are called…
- Lysosomes, the suicidal bags were first observed by…
- Red cell count is carried out by...[WBJEE 2010]
- What is mitoplast ?..
- What is the structural element of cell wall?
- Which of the following components helps in building up of plasma membrane…
- Which one of the following is an exception to cell theory?.
- Enzyme found functional in lysosome is…
- Teichoic acids function as….
Sample Questions
Ques. What are the infections that are caused by Gram positive bacteria? (2 marks)
Ans. The following are the infections caused by gram-positive bacteria. These infections are commonly found in elderly people and pregnant women.
- Urinary Tract Infections.
- Pneumococcal infections.
- Impetigo
- Gastroenteritis
Ques. Mention a few examples of Gram negative bacteria. (2 marks)
Ans. Pseudomonas, Proteus, Salmonella, Helicobacter, Klebsiella, and Escherichia are some examples of Gram-Negative Bacteria.
Ques. Which nonspore-forming gram-positive rods show a negative result for the catalase test? (2 marks)
Ans. Catalase test is a type of procedure carried out to identify which organism is capable of producing catalase enzyme.
- Lactobacillus is the non-spore forming gram-positive rods showing negative results for the catalase test.
- These rods are strictly fermentative organisms that do not show a positive catalase result.
Ans. Endotoxin is the toxic constituent present in Gram-Negative Bacteria. It is a lipopolysaccharide found in the outer membrane of such bacteria.
- They are responsible for producing fever and failure of organs.
- Examples of endotoxins are E.coli and Shigella.
Ques. Among gram-positive and gram-negative bacteria which one is more dangerous? (2 marks)
Ans. Gram-negative bacteria are considered more harmful than gram-positive bacteria. Their outer membrane is kind of hidden by a slime layer and it even hides the antigens which are present in cells.
Ques. What is the evolutionary advantage of gram-positive bacteria? (2 marks)
Ans. It lacks an outer membrane and that’s the reason they are more susceptible to damage from antibiotics. Gram-bacteria have a much thicker wall which is 20-80mn in comparison to 7-8nm which makes them less susceptible to lysis by osmotic pressure.
Ques. How to identify Gram-positive and Gram-negative bacteria after staining? (2 marks)
Ans. We can identify them by the following conditions:
- When a gram stain appears purple under the microscope, the bacteria are gram-positive.
- The bacteria will appear pink or red if the it is a gram-negative bacteria.
Ques. What diseases are caused by gram-positive bacteria in the human body? (2 marks)
Ans. Most gram-positive bacilli are caused by gram-positive bacteria these are:
- Food poisoning
- Respiratory infections
- Diptheria
- Anthrax
- Listeriosis
Ques. How to treat infections due to Gram-positive bacteria? (2 marks)
Ans. Gram-positive bacilli infections are treated with antibiotics. Penicillin, cloxacillin, and erythromycin treat over 90% of gram-positive bacteria.
However, antibiotic resistance is becoming a serious problem with gram-positive infections. New drugs are being developed to help with this problem. Antibiotics should only be used when needed. Infection control standards need to be followed closely to prevent the development and spread of antibiotic-resistant infections.
Ques. What are some of the risks of developing Gram-negative bacteria infections? (2 marks)
Ans. Gram-negative infections are most common in hospitals. Your risk increases the longer you stay in the hospital. Other things that increase your risk of developing a gram-negative infection include:
- Undergoing dialysis for kidney disease
- Being on a ventilator
- Having a weak immune system
- Recently having surgery
- Having war wounds
- Using a catheter
Ques. How to prevent and control the spread of infection caused by gram-negative bacteria? (2 marks)
Ans. Antibiotic resistance is made worse by the overuse of antibiotics. The following are some of the practices that help prevent the spread of infections.
- Only use antibiotics prescribed by doctors.
- Strictly follow the Doctor’s advice about antibiotic use.
- Don't use or share any kind of articles with an infected person.
- Wash your hands.
- Keep your vaccines up to date.
- Avoid contact with people who are sick when you can.
- Prepare your food safely.
- Keep the environment clean.
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