Diagnosis of bacterial infections

• ALSO KNOWN AS: Bacteria identification, diagnostic testing

Definition

Diagnosis is the process of identifying a disease by its symptoms and identifying the causes of disease. Bacterial diagnosis is the determination of disease caused by bacteria.

Bacteria in the Body

The human body is inhabited by hundreds of different species of bacteria that make up millions of individual bacteria in the body. Most of these microscopic organisms are harmless, and some are beneficial and help the body function at its best.

However, some bacteria, called pathogens, are harmful and cause illness. Pathogens enter the body, usually through the respiratory system. Once inside the body, they multiply, resulting in a bacterial infection. Symptoms of bacterial infection vary according to the type of bacterium and according to where the infection forms. To treat symptoms and cure the illnesses, healthcare providers need first to identify the cause of a disease; they can then diagnose the disease. Pathologists, doctors who specialize in diagnosing diseases, use a number of tests to arrive at a diagnosis of a bacterial infection.

Collecting Specimens

The first steps in diagnostic testing for bacterial infection involve collecting samples, or specimens. Typical test specimens include fluids collected from a person’s blood, throat, lungs, urine, or spine. Pathologists prescribe a four-step process for collecting specimens of suspected pathogens. This process includes the following:

Collect the right specimen. Doctors collect specimens for diagnostic testing based on the person’s symptoms. For example, to test for lung or bronchial diseases such as pneumonia or bronchitis that are caused by bacteria, doctors need to collect secretions from the lungs, called sputum, that the person coughs up into a sterile cup.

Collect the specimen properly. Collecting a specimen from the throat, for example, requires the doctor or nurse to use a throat swab to collect tissue by inserting the swab deep enough into the throat to make the person gag.

Package and label the specimen correctly. Secure packaging prevents contamination of the specimen, and accurate labeling ensures appropriate testing.

Transport and store the specimen carefully. Controlled transport and storage prevent possible harmful bacteria from escaping and endangering public health.

Testing Protocols

Medical professionals test for contagious, or communicable, diseases in a variety of settings. Portable assay machines allow doctors to perform quick, simple tests almost anywhere, including a person’s home. Most tests, however, are done in clinic and hospital laboratories or in independent labs, known as clinical reference labs.

To ensure the highest standards for professionalism and safety, all labs must be certified and licensed by various medical, professional, and government organizations or boards. One of the main certifications is the Clinical Laboratory Improvement Amendments, administered by the Centers for Disease Control and Prevention, an agency of the US Department of Health and Human Services.

Medical professionals who perform the tests must also be properly trained and certified. Pathologists and medical laboratory technologists, also known as technicians or clinical laboratory scientists, receive their specialized training from accredited medical education programs. They also are required to maintain their certification through continuing education and periodic recertification.

Types of Diagnostic Tests

Culturing is the standard method for testing the presence and type of bacteria in the lab. A technologist places the specimen on a special plate—called a cell culture plate, or, more often, a Petri dish—that has been treated with a special substance. This substance, called a culture medium, is a mixture of chemicals, including nutrients, that encourages the bacteria to multiply. If they do not multiply, that is, if no bacteria are present in the specimen, the technologist scores the test “negative.” If they do multiply, the bacteria are then viewed under a microscope or analyzed through chemical testing for the type and numbers. Pathologists can identify specific bacteria based on certain characteristics, such as shape, staining (the color they turn after a certain chemical treatment), and whether they thrive with or without oxygen.

One of the first steps in testing is to determine if the infection is in fact caused by a bacterium. An infection caused by a virus, which is another type of disease-causing microorganism, does not respond to the same treatment as a bacterial infection. (Antibiotics kill or stop the growth of bacteria but have no effect on viruses.) The problem for patients and doctors is that often the symptoms of both types of infections are similar. Bacterial cultures and blood tests help differentiate bacterial infections from viral infections.

The blood test, known as a complete blood count, or CBC, yields a count of white blood cells, red blood cells, and platelets in a patient’s blood. The test can help determine the presence of a bacterial infection. A high white blood cell count indicates a bacterial infection.

Cultures help determine the type of bacterial infection. A blood culture shows if bacteria have entered the bloodstream and reveals such diseases as osteomyelitis (a bone infection) and sepsis (a serious, life-threatening blood infection). A throat culture checks only for Streptococcus, the pathogen that causes strep throat, rheumatic fever, and toxic shock syndrome. A sputum culture detects bacterial infections of the lungs or breathing passages and infections such as pneumonia, tuberculosis, and sinusitis. A urine culture is most commonly done to identify the cause of urinary tract infections. A spinal culture detects the origin of infections of the brain or nervous system, such as meningitis and brain abscess, a swelling of the brain.

Impact

Harmful bacteria that invade the human body cause many dangerous, debilitating, and fatal diseases. Through diagnostic testing for bacterial infections, medical professionals can pinpoint the specific species of bacteria and its numbers. Once pathologists identify the pathogen, doctors devise the proper treatment plans for their patients.

Bibliography

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Professional Guide to Diseases. 9th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2009.

Richardson, Harold, and Fiona Smaill. “Medical Microbiology.” British Medical Journal 317 (1998): 1060.

Seed, Shawna and Rick Ansorge. "Bacterial and Viral Infections." WebMD, 2 June 2024, www.webmd.com/a-to-z-guides/bacterial-and-viral-infections. Accessed 2 Feb. 2025.