Laboratory tests for bacterial infections.

Why do we need laboratory tests to bacterial infections?

Several tests are performed in a laboratory to establish or confirm the diagnosis of bacterial skin. infection. Although a complete history and examination of the patient are vital, laboratory tests can help the doctor reach a diagnosis.

the culture of bacterial species with antibiotic susceptibility testing is considered the gold standard laboratory test.

Skin samples for bacterial tests.

Skin samples can be collected in the following ways.

• A dry sterile cotton swab is rubbed on the suspected skin site, for example, blistered or dry skin lesions or pustules.
• A moist swab is taken from a mucous membrane surface, such as inside the mouth.
• Aspiration of fluid/pus from a skin injury using a needle and syringe (a swab is more likely to produce organism)
• A skin biopsy: a small sample of skin removed under local anesthetic.

What is meant by cultivation? bacteria?

Cultivation or growth of bacteria is most often done by brushing the skin swab onto sheep blood agar plates and exposing them to different conditions. The bacteria that grow depend on the medium used to culture the sample, the temperature for incubation, and the amount of oxygen available.

• An obligate aerobe can only grow in the presence of oxygen.
• An obligate anaerobe cannot grow at all in the presence of oxygen.

What is antibiotic susceptibility testing?

Antibiotic susceptibility testing determines which antibiotics inhibit The growth of bacteria that have been cultured. This information allows selection of the most appropriate antibiotic to treat a particular infection. Antibiotic susceptibility tests are used to determine:

• The effectiveness of particular antibiotics against particular bacteria
• Whether the bacteria are resistant to selected antibiotics
• Identify patterns of resistance to bacterial antibiotics.

What is a Gram stain?

A Gram stain uses a series of stains or dyes on a sample, followed by inspection under a light microscope to detect and identify bacteria as Gram-positive or Gram-negative. A Gram stain can be done on the original sample, but is usually done on cultured bacteria after transferring a colony of bacteria from the agar plate to a glass microscope slide.

• A Gram-positive bacterium appears purple due to the crystal violet dye that adheres to the cell wall.
• A Gram-negative bacterium appears red because it is counteracted with a red dye such as saffron.

The Gram stain also identifies the shape and behavior of the bacteria.

• Coconuts are round in shape
• The bacilli are rod-shaped.
• Some bacteria form clumps while others form chains

* Note the palisade/parallelism of cells in the 'Chinese letters' formation

What other tests can be done with bacterial cultures?

Coagulase test

coagulase is a enzyme produced by certain bacteria that convert fibrinogen to fibrin and is seen as clustering of cells in plasma. Coagulase test differentiates positive coagulase Staphylococcus aureus coagulase negative staphylococci.

Catalase test

Catalase is an enzyme that breaks down hydrogen peroxide into hydrogen and oxygen. The bacterial sample is added to a test tube of hydrogen peroxide. The production of bubbles (oxygen) indicates a positive result. The catalase test differentiates catalase-positive staphylococci and micrococci from catalase-negative streptococci.

What blood tests are done in bacterial infections?

Blood tests require a blood sample accessed with a needle from a vein. Examples of those requested for bacterial infection include:

• Complete blood count – a bacterial infection often increases the white blood cell count with neutrophilia
• C-reactive protein (CRP) – this rises above 50 in severe bacterial infections
• Procalcitonin – a marker of generalized septicemia due to bacterial infection
• Serology – tests 10 to 14 days apart to determine the immune response to a particular organism
• Rapid plasma reactivation test (RPR) (and others) – if syphilis is suspected
• Blood culture: if it is high fever > 38°C

Polymerase chain reaction (PCR)

PCR involves isolating and amplifying lengths of bacteria DNA from a sample of skin, blood, or other tissue. The DNA is compared with bacterial DNA from organismsthus identifying the species. It is useful for slow-growing bacteria such as anaerobic bacteria and mycobacteria (tuberculosis and atypical mycobacteria), as they cannot be cultured by standard methods.

Enzyme-linked immunosorbent assay (ELISA)

ELISA can test for specific organisms by detecting bacteria antigen during an infection or antibacterial antibody. Detection of the antibody confirms contact with an organism at some point, but is not necessarily the reason for a current infection.

ELISA is rarely used for the detection of skin bacteria. It can be used to detect:

• Borrelia burgdorferi in Lyme disease
• Treponema pallidum in syphilis