I'm gonna try and start posting again some, depending on my work load. My internship has been going well, the introductory 10 weeks were busy and now I am finishing up my 10-week microbiology rotation. We have rotation 4 days a week and on Wednesdays we have lectures on the full spectrum of lab topics, as well as a test most weeks on a single topic, most recently, antibiotics. In micro, each week is dedicated to one type of culture, such as urine, stool, and respiratory. This last week we covered blood cultures, a very important type of culture as it is used to detect bacteremia and septicemia. Bacteremia is the presence of bacteria in the blood that does not cause a disease and is usually not a threat to a patient's health. Traditionally, the bacteria are present but not actively dividing. The main risk is that, in those with heart valve defects, bacteremia can lead to the seeding of the heart valves, particularly the mitral valve, with bacteria and lead to endocarditis, a serious and potentially fatal condition. Septicemia is the presence of bacteria in the bloodstream that causes disease and is a potential issue for the patient. In this case, the bacteria are usually dividing and thriving in the blood. Septicemia can be caused by a wide variety of conditions, including abscesses, untreated UTIs, and, ironically, endocarditis. The complications of septicemia are numerous and many are potentially fatal.
If not treated and controlled in time, septicemia can lead to multi-organ failure as the bacteria exit the bloodstream and infect the organs of the body. CNS infections can develop as well, as bacteria can cross the blood-brain barrier and enter the CSF. This can lead to encephalitis, meningitis, and the development of brain abscesses, all of which can be rapidly fatal. Another possible consequence is shock. This occurs when the body's immune system responds too strongly and starts damaging the body. One of the main causes of this is tumor necrosis factor alpha (TNF-alpha), a compound released by immune cells in an effort to control and eliminate a localized infection. If it is released systemically, however, it can cause cell death and a precipitous drop in blood pressure, leading to multi-organ involvement and death in a short period of time. Toxins released by the bacteria, especially the endotoxin lipopolysaccharide (LPS), can also cause shock. LPS and TNF-alpha are frequently released in association with septicemia from Gram negative bacteria; in fact only Gram negative bacteria have LPS. Other bacteria have other toxins that can cause serious issues. For example, Gram positive bacteria, especially the staphylococci and streptococci, can cause toxic shock syndrome. The most disastrous and difficult to manage complication of septicemia is disseminated intravascular coagulation (DIC), a tragic condition in which the blood spontaneously clots. As the clotting occurs, platelets and clotting factors are used up. This means that when a small break in a blood vessel occurs - and many do each day, but normally are not an issue - the body is unable to fully heal it, resulting in a hemorrhage. This leads to a complex situation in which both a critical bleed or a disastrous clot are possible, making treatment extremely challenging. Usually, doctors will attempt to quickly resolve the infection to stop the DIC and treat whichever vascular event is a bigger threat to the patient's health - bleeding is treated by plasma or platelet transfusions, clotting by blood thinners. Transfusions of red cells may also be necessary if the patient has lost much blood. Sadly, if DIC is not controlled within about 12 hours, it is usually fatal.
Sepsis can be caused by any number of bacteria, with E. coli, S. aureus, and S. pneumoniae among some of the more common causes. The bacteria can come from an abscess, a UTI, from an infected IV site, from a wound, or from any other event that gives them an opportunity to enter the bloodstream. Sometimes, a blood culture will grow bacteria that are not present in the blood stream of the patient, and for this reason, 2 sets of bottles (one for aerobic bacteria and the other for anaerobic ones) are drawn as this gives a better picture of whether a patient has a blood-borne infection. The vast majority of cases in which bacteria are isolated from a patient without a bacteremia or septicemia are caused by contamination during the process of drawing the blood. Therefore, it is imperative that whoever is collecting the samples makes every effort possible to prevent contamination from occurring.
Blood cultures are collected into bottles that contain a broth to help any bacteria to grow. They should be collected before antibiotic therapy is started, but since broad spectrum antibiotics are usually started as soon as septicemia is suspected, this is not always possible. They are then placed in an incubator. In many automated systems, there is a special compound that is pH sensitive and, as the bacteria grow, it either changes color or emits fluorescence. This is detected by the incubator, and it alerts the laboratory to the presence of a positive blood culture. At that point, the race to identify the bacteria is on. The first step is to Gram stain an aliquot of the broth to determine what type of bacteria is present. This identification guides both subculturing and the initial treatment of the patient, allowing the doctors to focus the antibiotics from broad spectrum to more narrow spectrum ones that are more effective against the infectious agent. In fact, a study indicated that when the time from the culture being flagged as positive to the results of the Gram stain was around an hour, mortality rates were 10%, but when that time was closer to 3 and a half hours, mortality rose to 18%. Once the information gathered from the Gram stain has been relayed to the clinicians, the blood is plated to the correct media, according to whether it an aerobic or anaerobic bottle that was positive and the Gram stain results. Then, once there is growth on the plate or plates, the bacteria is identified, usually by using a panel of biochemical tests and comparing these results to a database of known reaction patterns. The identification is given to the doctor, and in many cases, antibiotic susceptibility testing will be done as well so that the doctor can give their patient the most effective treatment. If the laboratory is able to quickly and accurately identify what bacteria are present in a patient's blood, doctors are usually able to treat them before any serious side effects develop.
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