Bacteria are all around us. They are ubiquitous, microscopic creatures which live amongst us – sometimes silently, and sometimes with deadly effects. In the 17th century the Dutch scientist Antonie van Leeuwenhoek perfected microscopes to a level where we could actually visualize bacteria. Ever since then mankind has been opened up to a world of virtually invisible life. Because we have found bacteria to be potentially very dangerous to our health, doctors and scientists have developed various methods for growing bacteria outside of a living organism. These methods are key to our ability to diagnose disease and develop effective treatments against harmful bacteria.
The first of these methods used to grow bacteria is known as a blood culture. A blood culture is done when a small sample of blood is taken from a potentially infected person. Blood is then placed on a series of special plates, which are incubated in an attempt to grow the bacteria that may be found in the blood. This blood sample is typically taken from a vein, most commonly in the arm. Care must be taken during this blood drawn to ensure that outside contaminants are not exposed to the blood during the draw.
Blood cultures can take anywhere from a couple of days to two weeks for results. A typical blood culture will be kept for up to seven days, although in cases where an infection which is known to be slow-growing is suspected, 14 days can be the standard.
Blood cultures are oftentimes performed when a person is suspected to have an endocarditis, which is an infection involving the heart. Sepsis, which is a generalized infection involving the blood, is also a reason to blood cultures. Meningitis, osteomyelitis, and sometimes pneumonia are other reasons to attempt a blood culture. These specific bacteria which are commonly isolated in a blood culture include Staphylococcus aureus, strep pneumonia, E. coli, and sometimes pseudomonas.
Throat cultures are commonly used to detect a specific type of bacteria which is a common cause of pharyngitis. This type of culture is looking to find Group A beta hemolytic streptococci, specifically a bug known as streptococci pyogenes. This nasty little bacteria is the most common cause of bacterial pharyngitis. If not treated properly and aggressively, bacterial pharyngitis can lead to many long-term problems, such as rheumatic fever or scarlet fever.
Throat cultures are taken by touching a sterile swab, which looks much like a long q-tip, onto the back of the throat where the infection is suspected. This throat swab is then transferred to a special growth plate which contains chemicals that the bacteria are known to grow well with. A typical throat culture for this bacteria will take approximately 24 hours to get results.
Sputum cultures are by far the grossest of the culturing techniques available. They involve taking a sample of the phlegm which you cough up when you have pneumonia, tuberculosis, or a lung abscess. It is important to note that sputum is not the same thing as saliva. Sputum is significantly thicker than saliva, and comes from the lower parts of your lung, not your salivary glans.
A sputum sample is often examined with a special technique known as Gram staining to determine if it is an acceptable sample. Gram staining can often be done in the doctors office, before the sample is sent to a formal laboratory to attempt a culture.
Spinal Fluid Cultures
Spinal fluid cultures are an attempt to grow bacteria from the fluid which is in your spinal column. The primary reason for doing a spinal fluid culture is when you suspect meningitis, which is an infection of the tissues surrounding the spinal column and brain. Bacterial meningitis can be a very severe problem, and needs to be treated quickly and aggressively. Because this can be a medical emergency, these cultures often take priority in a laboratory, as the results can have a great impact on potential treatments. In most cases a doctor will take the spinal fluid sample, and then begin antibiotic treatment before the results are even back. Should the results be negative, the antibiotics can always be discontinued at a later time.
Stool cultures are generally only done if one suspects an infection of the bowels, known as enterocolitis. There are three primary bacteria which are the most common causes of diarrhea in the United States. These include shigella, salmonella, and Campylobacter. Do you remember me mentioning that sputum cultures were the grossest culturing technique? Well I was wrong – stool cultures are. Prior to culture it is often common to take a look at a stool sample under a microscope directly, using staining techniques which can reveal the presence of infective organisms.
Urine cultures are performed when a doctor suspects that you may have an infection in your bladder, or an infection of your kidneys known as pyelonephritis. The overwhelming majority of these infections are caused by a bacteria known as E. coli. In a normal healthy person urine in your bladder is actually sterile and devoid of any bacteria. This does not mean that you should go around drinking it, however it is not normally infectious.
Cultures for bacteria are an important tool that doctors use to diagnose disease. Without these methods, we would be left guessing what specific bugs are causing an infection. Treatments would not be as specific. If you have questions about the various culture techniques and how they are useful to your doctor, feel free to ask your doctor for details.