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Bacteria can enter the bloodstream in a number of different ways. However, for each major classification of bacteria (gram negative, gram positive, or anaerobic) there are characteristic sources or mechanisms that lead to bacteremia. Causes of bacteremia can additionally be divided into community-acquired or health-care associated.

Gram positive bacteria are an increasingly important cause of bacteremia. Staphyloccocus, streptococcus, and enterococcus species are the most important and most common species of gram-positive bacteria that can enter the bloodstream. These bacteria are normally found on the skin or in the gastrointestinal tract.

Staph aureus is the most common cause of health care associated bacteremia in North and South America and is also an important cause of community-acquired bacteremia. Skin ulceration or wounds, respiratory tract infections, and IV drug use are the most important causes of community-acquired staph aureus bacteremia. In health care settings, intravenous catheters, urinary tract catheters, and surgical procedures are the most common causes of staph aureus bacteremia.

There are many different types of streptococcal species that can cause bacteremia. Group A streptococcus (GAS) typically causes bacteremia from skin and soft tissue infections. Group B streptococcus is an important cause of bacteremia in neonates, often immediately following birth. Viridans streptococci species are normal bacterial flora of the mouth. Viridans strep can cause temporary bacteremia after eating, toothbrushing, or flossing. More severe bacteremia can occur following dental procedures or in patients receiving chemotherapy. Finally, streptococcus bovis is a common cause of bacteremia in patients with colon cancer.

Enterococci are an important cause of health-care associated bacteremia. These bacteria commonly live in the gastrointestinal tract and female genital tract. Intravenous catheters, urinary tract infections and surgical wounds are all risk factors for developing bacteremia from enterococcal species. Resistant enterococci species can cause bacteremia in patients who have had long hospital stays or frequent antibiotic use in the past.

Gram negative bacteria l species are responsible for approximately 24% of all cases of health-care associated bacteremia and 45% of all cases of community-acquired bacteremia. In general, gram negative bacteria enter the bloodstream from infections in the respiratory tract, genitourinary tract, gastrointestinal tract, or hepatobiliary system. Causes of gram negative bacteremia can also be categorized as community-acquired or health-care associated. Gram negative bacteremia occurs more frequently in elderly populations (65 years or older) and is associated with higher morbidity and mortality in this population.

E.Coli is the most common cause of community-acquired bacteremia accounting for approximately 75% of cases. E.Coli bacteremia is usually the result of a urinary tract infection. Other organisms that can cause community-acquired bacteremia include pseudomonas aeruginosa, klebsiella pneumoniae, and proteus mirabilis.

Among health-care associated cases of bacteremia, gram negative organisms are an important cause of bacteremia in the ICU.