To diagnosis a tuberculosis (TB) infection, your healthcare professional will do an exam that includes:

• Listening to you breathe with a stethoscope.

• Checking for swollen lymph nodes.

• Asking you questions about your symptoms.

TB tests

Your healthcare professional will order tests if:

• Tuberculosis is suspected.

• You were likely exposed to a person with active TB disease.

• You have health risks for active TB disease.

Your healthcare team will determine whether a skin test or blood test is the best option.

Skin test

A tiny amount of a substance called tuberculin is injected just below the skin on the inside of one forearm. Within 48 to 72 hours, a healthcare worker will check your arm for swelling at the injection site. The size of the raised skin is used to determine a positive or negative test.

This test is seeing if your immune system reacts, or has made an antibody, to tuberculosis. A positive test indicates you likely have either a latent TB infection or active TB disease. People who had a TB vaccination might get a positive test even if they have no infection.

A negative test means that your body didn't react to the test. It doesn't necessarily mean you don't have an infection.

Blood tests

A sample of blood is sent to a lab. One lab test finds out whether certain immune system cells can "recognize" tuberculosis. A positive test shows that you have either a latent TB infection or active TB disease. Other tests of the blood sample can help determine if you have active disease.

A negative result means you likely do not have a TB infection.

X-ray

A chest X-ray can show irregular patches in the lungs that are typical of active TB disease.

Sputum tests

Your healthcare professional may take a sample of the mucus that comes up when you cough, also called sputum. If you have active TB disease in your lungs or voice box, lab tests can detect the bacteria.

A relatively quick laboratory test can tell if the sputum likely has the TB bacteria. But it may be showing bacteria with similar features.

Another lab test can confirm the presence of TB bacteria. The results often take several weeks. A lab test also can tell if it's a drug-resistant form of the bacteria. This information helps your healthcare professional choose the best treatment.

Other lab tests

Other lab tests that may be ordered include:

• Breath test.

• Procedure to remove sputum from your lungs with a special tube.

• Urine test.

• Test of the fluid around the spine and brain, called cerebrospinal fluid.

To diagnosis a tuberculosis (TB) infection, your healthcare professional will do an exam that includes:

Listening to you breathe with a stethoscope.

Checking for swollen lymph nodes.

Asking you questions about your symptoms.

TB tests

Your healthcare professional will order tests if:

Tuberculosis is suspected.

You were likely exposed to a person with active TB disease.

You have health risks for active TB disease.

Your healthcare team will determine whether a skin test or blood test is the best option.

Skin test

A tiny amount of a substance called tuberculin is injected just below the skin on the inside of one forearm. Within 48 to 72 hours, a healthcare worker will check your arm for swelling at the injection site. The size of the raised skin is used to determine a positive or negative test.

This test is seeing if your immune system reacts, or has made an antibody, to tuberculosis. A positive test indicates you likely have either a latent TB infection or active TB disease. People who had a TB vaccination might get a positive test even if they have no infection.

A negative test means that your body didn't react to the test. It doesn't necessarily mean you don't have an infection.

Blood tests

A sample of blood is sent to a lab. One lab test finds out whether certain immune system cells can "recognize" tuberculosis. A positive test shows that you have either a latent TB infection or active TB disease. Other tests of the blood sample can help determine if you have active disease.

A negative result means you likely do not have a TB infection.

X-ray

A chest X-ray can show irregular patches in the lungs that are typical of active TB disease.

Sputum tests

Your healthcare professional may take a sample of the mucus that comes up when you cough, also called sputum. If you have active TB disease in your lungs or voice box, lab tests can detect the bacteria.

Gram-positive bacteria have a thick peptidoglycan layer in their cell wall and lack an outer membrane. This thick layer retains the crystal violet stain used during Gram staining, so they appear purple under a microscope. They also often contain teichoic acids, which help maintain cell structure and regulate ion passage. These bacteria are generally more sensitive to antibiotics like penicillin and vancomycin because their thick wall is easily targeted.

In contrast, Gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane that contains lipopolysaccharides (LPS), which act as endotoxins. During the Gram staining process, this structure allows the crystal violet stain to be washed out, so Gram-negative bacteria take up the safranin counterstain and appear pink or red. Their outer membrane makes them more resistant to many antibiotics, as it acts as a barrier.

In summary, Gram-positive bacteria are purple, thick-walled, and more antibiotic-sensitive, while Gram-negative bacteria are pink/red, thin-walled with an outer membrane, and often more drug-resistant.

Gram-positive bacteria have a thick peptidoglycan cell wall. Trusted Source that they retain the dye in gram testing, causing them to have a blue color when a person views them under a microscope.

Gram-positive bacteria can be cocci- (spheres) or bacilli- (rod) shaped, or have branching filaments.

Gram-negative

Gram-negative bacteria have an outer membrane. However, they have a thinner peptidoglycan cell wall. This means they do not hold the blue dye used in Gram testing and do not appear blue. Instead, they appear red or pink in color.

Gram-positive and Gram-negative bacteria are two different types of bacteria. One method that scientists use to classify bacteria is Gram staining, which is the most commonly performed laboratory procedure in microbiology.

The term “Gram” refers to a specimen staining method developed by Hans Christian Gram in 1884. It involves staining an organism with crystal violet or methylene blue dye and then observing the organism under a microscope.

Gram-positive

Gram-positive bacteria have a thick peptidoglycan cell wall. This meansTrusted Source that they retain the dye in gram testing, causing them to have a blue color when a person views them under a microscope.

Gram-positive bacteria can be cocci- (spheres) or bacilli- (rod) shaped, or have branching filaments.

Gram-negative

Gram-negative bacteria have an outer membrane. However, they have a thinner peptidoglycan cell wall. This means they do not hold the blue dye used in Gram testing and do not appear blue.

Instead, they appear red or pink in color.

Gram-negative bacteria are among the most significantTrusted Source public health problems in the world. This is due to their high resistance to antibiotics.

Characteristics 

Below are the specific characteristics of Gram-positive and Gram-negative bacteria.

Gram-positive

Gram-positive bacteria have the following characteristics:

• Membranes: Gram-positive bacteria do not have a protective outer membrane.

• Cell wall: They have a thick peptidoglycan cell wall.

• Shape: Gram-positive bacteria are sphere- or rod-shaped, or they have branching filaments.

• Toxins: These bacteria also contain certain toxins that can cause a number of foodborne diseases. Toxins presentTrusted Source in Gram-positive bacteria include:

  • emetic toxin

  • diarrheal enterotoxins

  • neurotoxins

  • enterotoxin

Gram-negative

Gram-negative bacteria have the following characteristics:

• Membrane: Gram-negative bacteria have two membranesTrusted Source — one external and one internal.

• Cell wall: These bacteria also have a thinner peptidoglycan cell wall than Gram-positive bacteria, which sits between their two membranes.

• Shape: They can be spherical-, rod-, or spiral-shaped.

• Toxins: If something disturbs the cell wall of Gram-negative bacteria, they can release endotoxins.

1. Gram Stain Procedure:

   The Gram stain involves applying a primary stain (crystal violet), followed by a mordant (iodine), a decolorizer (alcohol or acetone), and finally a counterstain (usually safranin or fuchsine). 

2. Cell Wall Differences:

1. Gram-positive bacteria possess a thick peptidoglycan layer in their cell walls, which traps the crystal violet-iodine complex, making them stain purple. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which allows the crystal violet-iodine complex to be washed away during decolorization, so they take up the counterstain and appear pink or red. 

3. Other Differentiation Tests:

1. While the Gram stain is the most common, other tests can be used to further differentiate bacteria, including:

   • Catalase test: Distinguishes bacteria based on the presence of the enzyme catalase, which breaks down hydrogen peroxide. 

   • Oxidase test: Identifies bacteria that produce cytochrome c oxidase. 

   • Biochemical tests: Various tests like carbohydrate fermentation, amino acid utilization, and enzyme activity tests can provide more specific information for identification. 

   • KOH string test: A rapid test using potassium hydroxide to differentiate Gram-positive and Gram-negative bacteria. 

   • Real-time PCR: Molecular methods like PCR can be used for rapid and accurate identification of bacterial species, including differentiating Gram-positive and Gram-negative bacteria. 

Tuberculosis (TB) diagnosis involves a multi-step process that may include physical exams, imaging tests like chest X-rays, and laboratory tests to identify the presence of Mycobacterium tuberculosis bacteria. Sputum cultures, blood tests (IGRA), and skin tests are commonly used to confirm the infection

TB Tests

Screening for latent TB infection (LTBI)

Diagnosing active TB in combination with other tests (e.g., chest X-ray, sputum culture)

Required for employment, immigration, or school in some countries.

skin test

• Injection: A small amount of purified protein derivative (PPD), which contains TB antigens, is injected just under the top layer of skin on the inside of the forearm.

• Reaction: If a person has been infected with TB bacteria before, their immune system will recognize the antigens and react by forming a raised, firm bump at the site.

• Timing: The test site must be checked 48 to 72 hours later by a healthcare professional.

Blood tests

A blood test can be used to help diagnose tuberculosis (TB). The most common type of TB blood test is called an Interferon Gamma Release Assay (IGRA). These tests measure the immune system's response to TB bacteria, and they are used to help diagnose both latent TB infection (LTBI) and active TB disease.

x-ray

It helps identify abnormalities in the lungs that are suggestive of active TB disease. While not definitive on its own, a chest X-ray can be a valuable first step in screening for TB, especially in combination with other tests.