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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. [1] 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.[2] 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.[3]

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[4].  Group B streptococcus is an important cause of bacteremia in neonates, often immediately following birth.[5]  Viridans streptococci species are normal bacterial flora of the mouth.  Viridans strep can cause temporary bacteremia after eating, toothbrushing, or flossing[5].  More severe bacteremia can occur following dental procedures or in patients receiving chemotherapy.[5]  Finally, streptococcus bovis is a common cause of bacteremia in patients with colon cancer.[6]  

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.[7]  Resistant enterococci species can cause bacteremia in patients who have had long hospital stays or frequent antibiotic use in the past.[8]     

Gram negative bacterial species are responsible for approximately 24% of all cases of health-care associated bacteremia and 45% of all cases of community-acquired bacteremia.[9][10]  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.[11]

E.Coli is the most common cause of community-acquired bacteremia accounting for approximately 75% of cases.[12] 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.

  1. ^ Cervera, Carlos; Almela, Manel; Martínez-Martínez, José A.; Moreno, Asunción; Miró, José M. (2009-01-01). "Risk factors and management of Gram-positive bacteraemia". International Journal of Antimicrobial Agents. 34 Suppl 4: S26–30. doi:10.1016/S0924-8579(09)70562-X. ISSN 1872-7913. PMID 19931813.
  2. ^ Biedenbach, Douglas J.; Moet, Gary J.; Jones, Ronald N. (2004-09-01). "Occurrence and antimicrobial resistance pattern comparisons among bloodstream infection isolates from the SENTRY Antimicrobial Surveillance Program (1997-2002)". Diagnostic Microbiology and Infectious Disease. 50 (1): 59–69. doi:10.1016/j.diagmicrobio.2004.05.003. ISSN 0732-8893. PMID 15380279.
  3. ^ Lowy, Franklin D. (1998-08-20). "Staphylococcus aureus Infections". New England Journal of Medicine. 339 (8): 520–532. doi:10.1056/NEJM199808203390806. ISSN 0028-4793. PMID 9709046.
  4. ^ Schwartz, Brian (2016). Current Medical Diagnosis and Treatment 2017. New York: McGraw Hill. pp. chapter 33. ISBN 978-1-25-958511-1.
  5. ^ a b c Wessels, Michael (2015). Harrison's Principles of Internal Medicine. New York: McGraw Hill. pp. Chapter 173. ISBN 978-0-07-180215-4.
  6. ^ Mayer, Robert (2015). Harrison's Principles of Internal Medicine. New York: McGraw Hill. pp. Chapter 110. ISBN 978-0-07-180215-4.
  7. ^ Maki, Dennis (1988). "Enterococcal bacteremia: clinical features, the risk of endocarditis, and management". Medicine. 67: 248.
  8. ^ Kasper, Dennis (2016). Harrison's Manual of Medicine. New York: McGraw Hill. pp. Chapter 87. ISBN 978-0-07-182852-9.
  9. ^ Gaynes, Robert; Edwards, Jonathan R.; National Nosocomial Infections Surveillance System (2005-09-15). "Overview of nosocomial infections caused by gram-negative bacilli". Clinical Infectious Diseases: An Official Publication of the Infectious Diseases Society of America. 41 (6): 848–854. doi:10.1086/432803. ISSN 1537-6591. PMID 16107985.
  10. ^ Diekema, D. J.; Beekmann, S. E.; Chapin, K. C.; Morel, K. A.; Munson, E.; Doern, G. V. (2003-08-01). "Epidemiology and outcome of nosocomial and community-onset bloodstream infection". Journal of Clinical Microbiology. 41 (8): 3655–3660. doi:10.1128/JCM.41.8.3655-3660.2003. ISSN 0095-1137. PMC 179863. PMID 12904371.
  11. ^ High, Kevin (2017). Geriatric Medicine and Gerontology 7th Edition. New York: McGraw Hill. pp. Chapter 125. ISBN 978-0-07-183345-5.
  12. ^ Luzzaro, F.; Viganò, E. F.; Fossati, D.; Grossi, A.; Sala, A.; Sturla, C.; Saudelli, M.; Toniolo, A.; AMCLI Lombardia Hospital Infectious Study Group (2002-12-01). "Prevalence and drug susceptibility of pathogens causing bloodstream infections in northern Italy: a two-year study in 16 hospitals". European Journal of Clinical Microbiology & Infectious Diseases: Official Publication of the European Society of Clinical Microbiology. 21 (12): 849–855. doi:10.1007/s10096-002-0837-7. ISSN 0934-9723. PMID 12525919.
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