Bacteremia · What It Is, Causes, Signs and Symptoms, Treatment, and More

Bacteremia refers to the presence of bacteria in the bloodstream and can be caused by a variety of bacterial organisms including gram-positive (i.e., gram-positive bacteremia) or gram-negative bacteria (i.e., gram-negative bacteremia). In healthy individuals, small amounts of bacteria can be transiently introduced into the blood from wounds and minor medical procedures (e.g., dental procedures). This is usually benign since the body’s host defenses are able to detect and eradicate these organisms. However, immunocompromised individuals with weakened immune systems may not be able to mount a strong immune response, and therefore, even small amounts of bacteria can eventually lead to a severe  bloodstream infection.  

If bacteremia is left untreated, it may progress to sepsis, which is a serious clinical syndrome characterized by life-threatening organ failure caused by a widespread immune response to infection. 

Bacteremia occurs when bacteria enter the bloodstream. Skin and mucosal surfaces, like the respiratory and GI tract, are the first barrier to bacterial invasion. The body’s immune system is then responsible for the detection and clearance of bacteria. When these barriers and host defenses are impaired, large colonies of bacteria are able to grow and colonize exponentially at the site of infection. Eventually, the bacteria originating from the localized site may spread to other parts of the body, including the bloodstream, causing bacteremia.  

Examples of localized bacterial infections include urinary tract infections, skin and soft tissue infections, bone and joint infections, and respiratory infections, like pneumonia.  

Risk factors for bacteremia include immunocompromised states (e.g., history of chronic steroid use, cancer, HIV); older age; medical procedures that involve manipulation of the skin or mucosa (e.g., dental procedures, surgeries); disruption of skin and/or mucosal surfaces (e.g., trauma, burns, ulcers); and indwelling catheters or ports.  

If left untreated, bacteremia may lead to sepsis or septic shock. Sepsis is a potentially life-threatening condition that occurs when an infection spreads through the bloodstream and triggers a widespread immune response. The body's normal immune response to infection can sometimes become overactive, leading to widespread inflammation, which can cause tissue damage and affect organ function. Septic shock happens when low blood pressure due to sepsis does not improve after treatment with intravenous fluids.  

Common signs and symptoms of bacteremia include fever, chills, rigors, and night sweats, as well as increased heart rate and breathing rate. There may also be symptoms related to a specific infection, for example, in a urinary tract infection, individuals may notice foul-smelling urine and experience dysuria, urinary frequency, and/or suprapubic pain. If pneumonia is present, an individual may also experience shortness of breath, cough, and/or sputum production. In the very young, old, and people with a weakened immune system, though, there may not be any symptoms of a specific infection, and the body temperature may be low or normal rather than high. 

If left untreated, bacteremia may progress to sepsis or septic shock. Individuals may have hemodynamic instability including tachycardia, low blood pressure, confusion, and signs of organ failure, such as decreased urine output or high blood lactate. 

Diagnosis of bacteremia begins with a thorough review of symptoms and medical history, including recent medical procedures, history of intravascular device placement, and symptoms of infection. Obtaining vital signs and performing a thorough physical exam helps identify the likelihood of sepsis and the presence of indwelling catheters or devices that could be potential sources of infection. Additionally, other tools, such as the Sequential Organ Failure Assessment, or SOFA, score may help identify those at greatest risk of poor outcomes.  

Initial laboratory tests may include a complete blood count, which may show an elevated or low white blood cell count; two sets of blood cultures assessing aerobic and anaerobic organisms from different sites; a basic metabolic panel to assess for organ dysfunction; and lactate levels. Additional infectious work-up will depend on the site of infection and may include a urinalysis, chest X-ray, sputum culture, or echocardiogram, among others. 

Treatment of bacteremia is aimed at eradicating any potential localized sources of infection and decreasing the bacterial burden via intravenous (IV) antibiotics. Source control is achieved by removing any indwelling catheters, ports, or intravascular devices that could either introduce bacteria into the bloodstream or serve as a nidus for colonization. Broad-spectrum empiric IV antibiotics are often started after blood cultures are drawn as antibiotic therapy should be started as soon as possible.   

A few examples of antibiotics that cover gram-positive organisms include penicillins (e.g., ampicillin, nafcillin) and cephalosporins (e.g., cefazolin) for methicillin-susceptible Staphylococcus aureus (MSSA); glycopeptides (e.g., vancomycin) for methicillin-resistant Staphylococcus aureus (MRSA); and oxazolidinones (e.g., linezolid) for vancomycin-resistant Enterococcus (VRE). For gram-negative organisms, several antibiotic choices are available, including antipseudomonal agents that provide coverage for Pseudomonas aeruginosa, a highly virulent gram-negative bacterium. Examples of such agents include piperacillin-tazobactam, cefepime, and meropenem.  

Once blood cultures and sensitivities return, broad-spectrum antibiotics can be scaled down to an agent that is targeted to the specific organism. Daily blood cultures may be obtained to monitor the clearance of bacteremia. If the bacteremia progresses to sepsis or septic shock, escalation of care to the intensive care unit (ICU) may be warranted. Adjunctive treatment for sepsis includes IV fluids and vasopressor medications (e.g., norepinephrine) to increase blood pressure and restore perfusion to vital organs.   

Treatment duration depends on the local source that led to the bacteremia and the time required to clear the bacteremia. Finally, an infectious disease clinician can also be helpful in providing further guidance on the choice of antibiotic therapy and treatment duration.