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 intense inflammatory or immune reaction to those pathogens.

Bacteria can reach the bloodstream as a serious complication of other infections, such as pneumonia or meningitis. They can 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—are another common route. Even routine activities like dental procedures or brushing teeth can cause brief, temporary bacteremia.

Once in the blood, bacteria can trigger immune responses that lead to sepsis and septic shock. Severe sepsis and septic shock carry high death rates, particularly if treatment is delayed, though early intervention usually resolves mild sepsis successfully. 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 relies on antibiotics, and in high-risk situations, antibiotic prophylaxis may be given to prevent infection.

**Signs and symptoms** Low-grade bacteremia is often temporary and cleared by the immune system without causing symptoms. More commonly, bacteremia triggers sepsis, marked by fever, chills, and low blood pressure. Severe immune responses can escalate to septic shock and multiple organ dysfunction syndrome, which can be fatal.

**Types** Bloodstream infections are classified by the type of microbe causing them.

**Causes** Bacteria enter the bloodstream through various routes, and the source often correlates with the bacterial type—gram-positive, gram-negative, or anaerobic. Causes are also divided into healthcare-associated (acquired during medical care) and community-acquired (contracted outside healthcare settings, often before hospitalization).

**Gram-positive bacteremia** Gram-positive bacteria are an increasingly common cause of bacteremia. 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 a major cause of community-acquired cases. Community-acquired *S. aureus* bacteremia often stems from skin ulcers or wounds, respiratory infections, or IV drug use. In hospitals, intravenous catheters, urinary catheters, and surgeries are the usual triggers.

Many streptococcal species cause bacteremia. Group A streptococcus typically enters through skin and soft tissue infections. Group B streptococcus is a key cause in newborns, often right after birth. Viridans streptococci, normal mouth flora, can cause temporary bacteremia after eating, brushing, or flossing; more serious cases follow dental procedures or chemotherapy. *Streptococcus bovis* is frequently linked to colon cancer patients.

Enterococci are a significant cause of healthcare-associated bacteremia. These bacteria normally inhabit 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 bacteremia** Gram-negative bacteria cause about 24% of healthcare-associated bacteremia and 45% of community-acquired cases. They typically enter the bloodstream from respiratory, genitourinary, gastrointestinal, or hepatobiliary infections. This type of bacteremia is more common in people aged 65 and older, with higher illness and death rates in that group. *E. coli* is the most frequent cause of community-acquired bacteremia, accounting for roughly 75% of cases, usually from a urinary tract infection. Other community-acquired culprits include *Pseudomonas aeruginosa*, *Klebsiella pneumoniae*, and *Proteus mirabilis*. *Salmonella*, though mainly causing gastroenteritis in developed countries, is a common cause of bacteremia in Africa, especially in children lacking antibodies and in HIV-positive individuals of all ages.

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, as can surgeries on the genitourinary, intestinal, or hepatobiliary tracts. *Pseudomonas* and *Enterobacter* species are the most important gram-negative causes in the ICU.

**Risk factors for bacteremia** Several factors increase the likelihood of developing bacteremia.