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    difference between gram positive and gram negative bacteria


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    Major Difference Between Gram

    Gram Positive and Gram Negative Bacteria are differentiated according to their Gram staining characteristics and difference in their cell-wall structure.

    BiologyBiology Difference BetweenDifference Between Gram Positive And Gram Negative Bacteria

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    Difference Between Gram-positive and Gram-negative Bacteria

    Table of Contents

    Gram Staining

    Gram-Positive and Gram-Negative Bacteria – Overview

    Difference between Gram-Positive and Gram-Negative Bacteria – Key Points

    Difference between Gram-Positive and Gram-Negative Bacteria – Table

    Bacteria are a large group of minute, unicellular, microscopic organisms, which have been classified as prokaryotic cells, as they lack a true nucleus. These microscopic organisms comprise a simple physical structure, including cell wall, capsule, DNA, pili, flagellum, cytoplasm and ribosomes.

    Bacteria can be gram-positive or gram-negative depending upon the staining methods. Let us have a detailed look at the difference between the two types of bacteria.

    Gram Staining

    This technique was proposed by Christian Gram to distinguish the two types of bacteria based on the difference in their cell wall structures. The gram-positive bacteria retain the crystal violet dye, which is because of their thick layer of peptidoglycan in the cell wall.

    This process distinguishes bacteria by identifying peptidoglycan that is found in the cell wall of the gram-positive bacteria. A very small layer of peptidoglycan is dissolved in gram-negative bacteria when alcohol is added.

    Gram-Positive and Gram-Negative Bacteria – Overview

    The gram-positive bacteria retain the crystal violet colour and stain purple whereas the gram-negative bacteria lose crystal violet and stain red. Thus, the two types of bacteria are distinguished by gram staining.

    Gram-negative bacteria are more resistant to antibodies because their cell wall is impenetrable.

    Gram-positive and gram-negative bacteria are classified based on their ability to hold the gram stain. The gram-negative bacteria are stained by a counterstain such as safranin, and they are de-stained because of the alcohol wash. Hence under a microscope, they are noticeably pink in colour. Gram-positive bacteria, on the other hand, retains the gram stain and show a visible violet colour upon the application of mordant (iodine) and ethanol (alcohol).

    Gram-positive bacteria constitute a cell wall, which is mainly composed of multiple layers of peptidoglycan that forms a rigid and thick structure. Its cell wall additionally has teichoic acids and phosphate.  The teichoic acids present in the gram-positive bacteria are of two types – the lipoteichoic acid and the teichoic wall acid.

    In gram-negative bacteria, the cell wall is made up of an outer membrane and several layers of peptidoglycan. The outer membrane is composed of lipoproteins, phospholipids, and LPS. The peptidoglycan stays intact to lipoproteins of the outer membrane that is located in the fluid-like periplasm between the plasma membrane and the outer membrane. The periplasm is contained with proteins and degrading enzymes which assist in transporting molecules.

    The cell walls of the gram-negative bacteria, unlike the gram-positive, lacks the teichoic acid. Due to the presence of porins, the outer membrane is permeable to nutrition, water, food, iron, etc.

    Difference between Gram-Positive and Gram-Negative Bacteria – Key Points

    The cell wall of gram-positive bacteria is composed of thick layers peptidoglycan.

    The cell wall of gram-negative bacteria is composed of thin layers of peptidoglycan.

    In the gram staining procedure, gram-positive cells retain the purple coloured stain.

    In the gram staining procedure, gram-negative cells do not retain the purple coloured stain.

    Gram-positive bacteria produce exotoxins.

    Gram-negative bacteria produce endotoxins.

    Difference between Gram-Positive and Gram-Negative Bacteria

    Following are the important differences between gram-positive and gram-negative bacteria:

    Difference between Gram-Positive and Gram-Negative Bacteria

    Gram-Positive bacteria Gram-Negative bacteriaCell Wall

    A single-layered, smooth cell wall  A double-layered, wavy cell-wall

    Cell Wall thickness

    The thickness of the cell wall is 20 to 80 nanometres The thickness of the cell wall is 8 to 10 nanometres

    Peptidoglycan Layer

    It is a thick layer/ also can be multilayered It is a thin layer/ often single-layered.

    Teichoic acids

    Presence of teichoic acids Absence of teichoic acids

    Outer membrane

    The outer membrane is absent The outer membrane is present (mostly)


    Absent Occurs in Outer Membrane


    It is more prominent. It is less prominent.


    Cocci or spore-forming rods Non-spore forming rods.

    Flagella Structure

    Source : byjus.com

    Gram Positive vs Gram Negative

    Being able to differentiate bacterial species is important for a host of reasons. Whilst molecular techniques can determine the specific species, even without getting into the molecular nitty gritty, there are phenotypic differences between groups of bacteria that can be used to differentiate them. One such useful classification – if a bacterium is Gram positive or Gram negative - is based on the structure of bacterial cell walls.


    Gram Positive vs Gram Negative

    Published: August 21, 2019

    Last Updated: June 6, 2022

    Karen Steward PhD Read time: 5 minutes

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    Being able to differentiate bacterial species is important for a host of reasons, from diagnosing infection or checking food safety, to identifying which species it is that gives a cheese it’s fantastic character. Bacterial species, and even specific strains can be differentiated using a number of molecular techniques such as PCR, quantitative PCR, genome sequencing and mass spectrometry. But even without getting into the molecular nitty gritty, there are phenotypic differences between groups of bacteria that can be used to differentiate them. This includes characteristics like their shape (bacilli vs cocci for example), growth in particular nutrients and preference for high or low oxygen environments. Depending on the characteristic being studied, bacterial species may be broken down into broad groups, but taken together this information can narrow the possible identities greatly. One such useful classification – if a bacterium is Gram positive or Gram negative - is based on the structure of bacterial cell walls.

    Gram positive bacteria

    Gram positive bacteria Gram negative bacteria

    Distinctive purple appearance after gram staining

    Pale reddish color after gram staining

    Bacteria include all staphylococci, all streptococci and some listeria species

    Bacteria include enterobacter species, salmonella species and pseudomonas species

    Thick peptidoglycan layer

    Thin peptidoglycan layer

    No outer lipid membrane

    Outer lipid membrane present

    No O-specific side chains present

    O-specific side chains present

    Teichoic and lipoteichoic acids present Teichoic and lipoteichoic acids not present

    Difference in structure of Gram positive vs Gram negative bacteria  

    The diagram below illustrates the differences in the structure of Gram positive and Gram negative bacteria. The two key features that lead to the differing visualization properties of Gram positive and Gram negative species are the thickness of the peptidoglycan layer and presence or absence of the outer lipid membrane. This is because the wall structure affects the cell’s ability to retain the crystal violet stain used in the Gram staining procedure which can then be visualized under a light microscope.

    Gram positive bacteria have a thick peptidoglycan layer and no outer lipid membrane whilst Gram negative bacteria have a thin peptidoglycan layer and have an outer lipid membrane.

    As Gram positive bacteria lack an outer lipid membrane, when correctly referring to their structure rather than staining properties, are termed monoderms. The outer lipid membrane possessed by Gram negative bacteria means that, when referring to their physical structure, they are termed diderms.

    The Gram staining technique was developed in 1884 by Danish bacteriologist Hans Christian Gram.1 Whilst a Gram stain will not tell you the specific species you are looking at, it can be a quick way to narrow down greatly the list of potential candidates and direct follow-up testing where necessary.

    Gram positive vs Gram negative stain

    Gram stain procedure - Preparing a sample

    1. Label a clean glass microscope slide with your sample identification. Ensure you use a pencil as many inks are removed by the reagents used in the staining procedure.2. If preparing your slide from a liquid bacterial culture:

    Dab a small drop culture onto the slide using a sterile loop. Gently smear the droplet in a circular motion into an area of approximately 1 cm diameter. For very dense cultures it may be necessary to pre-dilute your culture to ensure individual bacterial cells can be seen under a microscope following staining.

    If the source material is from a bacterial plate:

    Resuspend a loop of colony material in sterile phosphate buffered saline (PBS) and then proceed as for a liquid culture.

    3. Once the smear has air dried, pass the smeared slide through a flame two or three times.

    Gram stain procedure - Gram staining a sample

    1. Gently flood the smear with crystal violet and leave for 1 minute. Tilt the slide slightly and gently rinse with tap water or distilled water.2. Gently flood the smear with Gram’s iodine and leave for 1 minute. Tilt the slide slightly and gently rinse with tap water or distilled water. The smear will now appear purple.3. Decolorize the smear using 95% ethyl alcohol or acetone. Tilt the slide slightly and apply the alcohol drop by drop until the alcohol runs almost clear (5-10 seconds). Immediately rinse with water to avoid over-decolorizing.

    Source : www.technologynetworks.com


    Find out the differences between gram-positive bacillus and gram-negative bacillus and how they may affect health.

    A to Z Guides Reference


    Difference Between Gram-Positive and Gram-Negative Bacillus

    Medically Reviewed by Dan Brennan, MD on May 16, 2021

    There are many ways of classifying bacteria. One method is based on the cell membrane. In 1884, a bacteriologist named Christian Gram created a test that could determine if a bacterium had a thick, mesh-like membrane called peptidoglycan. Bacteria with thick peptidoglycan are called gram positive. If the peptidoglycan layer is thin, it's classified as gram negative.

    Characteristics of Gram-Positive Bacilli

    A gram-positive bacillus doesn't have an outer cell wall beyond the peptidoglycan membrane. This makes it more absorbent. Its peptidoglycan layer is much thicker than the peptidoglycan layer on gram-negative bacilli.  Gram-positive bacilli are shaped like rods. They can also be characterized based on whether they form spores and whether they need oxygen to survive.

    Types of Gram-Positive Bacilli

    Resident flora. Most gram-positive bacilli live harmlessly on your body without causing problems. These are called resident flora. The can be found in the following places on your body:

    Skin Nose Mouth Throat Vagina

    Some types of gram-positive bacilli are pathogenic, which means they cause diseases in people. Some of the major gram-positive bacteria that cause diseases include:

    Anthrax. This is a potentially fatal infection that usually affects the skin or lungs but can rarely infect the gastrointestinal tract as well. Anthrax is a spore-forming type of bacilli. It has the potential to be used as a biological weapon because its spores can be spread in the air and be inhaled. The spores can live for decades and are not easily killed.Diphtheria. This is a contagious infection of the upper respiratory tract. Vaccination can prevent this infection. It's rare in developed countries.Enterococcal Infections. These infections can be caused by bacteria known as enterococci. There are over 17 different species. They live in your intestines, where they don't normally cause problems. If they move to another part of your body, such as your blood, heart valves, or skin, they can cause serious infections.Erysipelothricosis. This is a skin infection that usually happens when you get a scrape or puncture wound while handling infected animals. The main symptom is a hard, purplish-red rash.Listeriosis. This is an infection caused by eating contaminated food. It causes fever, chills, nausea, vomiting, and diarrhea.

    Treatment of Gram-Positive Bacilli

    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 you absolutely need them. Infection control standards need to be followed closely to prevent the development and spread of antibiotic-resistant infections.

    Gram-Negative Bacteria

    Gram-negative bacteria have a hard, protective outer shell. Their peptidoglycan layer is much thinner than that of gram-positive bacilli. Gram-negative bacteria are harder to kill because of their harder cell wall. When their cell wall is disturbed, gram-negative bacteria release endotoxins that can make your symptoms worse.

    Gram-negative bacteria can cause many serious infections, including:

    Cholera, a serious intestinal infection

    E. coli

    Plague, an infection of the lymph nodes and lungs

    Cat-scratch disease

    Campylobacter, an infection that usually affects the digestive tract

    Legionnaire's disease, a lung infection

    Salmonella, a digestive infection caused by contaminated food

    Klebsiella, a urinary or respiratory tract infection that usually occurs in long-term care

    Pseudomonas, a group of infections that affect different parts of the body

    Tularemia, an infection that's transmitted by wild animals

    Typhoid fever, an infection that is more common in developing countries

    Risks for Developing Gram-Negative Bacterial Infections

    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

    How Are Gram-Negative Bacteria Treated?

    Gram-negative bacteria have high resistance to antibiotics. They are one of the most serious public health issues in the world. Gram-negative bacteria have the ability to cause a lot of diseases in humans. They can reach almost all of the organ systems. Your doctor may need to try several antibiotics to beat the infection. Older antibiotics may work better.

    Preventing and Controlling Antibiotic Resistance

    Antibiotic resistance is made worse by the overuse of antibiotics. You can take the following steps to help control and prevent antibiotic resistance:

    Only use antibiotics prescribed by your healthcare provider.

    Source : www.webmd.com

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