Bloodstream infection

Bloodstream infections occur when pathogens enter and multiply in the blood. Finding microbes in the blood—usually through blood cultures—is never normal. This condition is distinct from sepsis, which involves the body’s extreme inflammatory or immune reaction to those pathogens. Bacteria can reach the bloodstream as a serious complication of other infections, such as pneumonia or meningitis.

Gram positive bacteremia

They may also enter during surgery, especially when mucous membranes like those in the gastrointestinal tract are involved. Catheters or other foreign objects placed in arteries or veins—including those used during intravenous drug use—can introduce bacteria as well. Even routine activities like dental procedures or brushing teeth can cause a brief, temporary presence of bacteria in the blood. Having bacteria in the blood can lead to several significant health problems.

Mechanism

The immune system’s response may trigger sepsis and, in severe cases, septic shock. If severe sepsis progresses to septic shock, the risk of death is high, especially without prompt treatment; however, mild sepsis caught early usually responds well to care. Bacteria can also travel through the bloodstream to other parts of the body—a process called hematogenous spread—causing infections like endocarditis or osteomyelitis far from the original site. Treatment involves antibiotics, and in high-risk situations, preventive antibiotics may be given.

Signs and symptoms

A low level of bacteria in the blood is often temporary and cleared quickly by the immune system. When the immune system does react, it can produce sepsis, marked by fever, chills, and low blood pressure. A severe immune response may lead to septic shock and multiple organ dysfunction syndrome, both potentially fatal. Bloodstream infections are classified by the type of microbe causing them.

Causes

Bacteria enter the bloodstream through various routes, and the source often depends on the bacterial group—gram positive, gram negative, or anaerobic. Causes are also divided into healthcare-associated (acquired during medical care) and community-acquired (picked up outside a healthcare facility, often before hospitalization). Gram positive bacteria are becoming a more common cause of bloodstream infections. The most important and frequent species are Staphylococcus, Streptococcus, and Enterococcus, which normally live on the skin or in the gastrointestinal tract.

Staphylococcus aureus is the leading cause of healthcare-associated bacteremia in North and South America and also a major cause of community-acquired cases. In the community, skin ulcers, wounds, respiratory infections, and IV drug use are the main triggers. In hospitals, intravenous catheters, urinary catheters, and surgeries are the usual sources.

Many streptococcal species can cause bacteremia. Group A streptococcus typically enters from skin and soft tissue infections. Group B streptococcus is a common cause in newborns, often right after birth. Viridans streptococci are normal mouth bacteria and can cause temporary bacteremia after eating, brushing, or flossing; more serious infections can follow dental work or chemotherapy.

Streptococcus bovis is frequently linked to colon cancer. Enterococci are a significant cause of healthcare-associated bacteremia. They normally reside in the gastrointestinal tract and female genital tract. Risk factors include intravenous catheters, urinary tract infections, and surgical wounds.

Resistant enterococcal strains can infect patients with long hospital stays or a history of frequent antibiotic use. Gram negative bacteria account for about 24% of healthcare-associated bacteremia and 45% of community-acquired cases. They typically enter the bloodstream from infections in the respiratory, genitourinary, gastrointestinal, or hepatobiliary systems. Gram-negative bacteremia is more common in people aged 65 and older, and it leads to higher illness and death rates in this group.

E. coli is the most frequent cause of community-acquired bacteremia, responsible for about 75% of cases, usually from a urinary tract infection. Other community-acquired organisms include Pseudomonas aeruginosa, Klebsiella pneumoniae, and Proteus mirabilis. In Africa, Salmonella—which mainly causes gastroenteritis in developed countries—is a common cause of bacteremia, especially in children lacking antibodies and in HIV-positive patients of any age. In healthcare settings, gram negative organisms are a major cause of bacteremia in intensive care units.

Catheters in veins, arteries, or the urinary tract can provide entry points. Surgeries on the genitourinary, intestinal, or hepatobiliary tracts also raise the risk. Pseudomonas and Enterobacter species are the most important gram negative causes in the ICU. Several factors increase the likelihood of developing bacteremia.