Common Cause of Bacteremia and Sepsis

Pathogens of all four major microbes- bacteria, fungi, viruses, and parasites- may circulate in blood during many diseases.

Staphylococcus aureus, and Streptococcus pnumoniae are the most frequently isolated gram-positive cocci. Escherichia coli, Klebsiella pneumoniae, Salmonella and Pseudomonas aeruginosa are the most common gram-negative rods. Among fungi, Candida and Cryptococcus neoformans are the most common.

Bacteremia

The presence of bacteria in the blood is called bacteremia. Bacteria may reach the bloodstream

• from an existing focus of infection
• from a site with commensal flora
• or by direct inoculation of contaminated material into the vascular system (i.e., trauma).

Based on the duration of the presence of bacteria in the blood, bacteremia  is classified as;

1. Transient (incidental) bacteremia,
2. Continuous bacteremia and
3. Intermittent bacteremia

Transient Bacteremia

Transient bacteremia may occur spontaneously or with minor events like brushing teeth or chewing food. Bacteria may transiently appear while manipulating infected tissues, instrumentation of contaminated surfaces, and surgery on non-sterile sites. These bacteria are often cleared from the blood within minutes by immune cells. Still, if the immune system is overwhelmed or evaded, these organisms may persist in the blood, resulting in the symptoms and signs of septicemia.

Continuous Bacteremia

Bacteria are released into the bloodstream at a fairly constant rate during septic shock, bacterial endocarditis, and other endovascular infections. Continuous bacteremia is also seen during the early stages of infections such as typhoid fever, brucellosis, and leptospirosis.

Intermittent Bacteremia

Bacteria can be found intermittently in the bloodstream of patients with undrained abscesses. Causative agents of pneumonia, meningitis, pyogenic arthritis, and osteomyelitis are often recovered from blood during the early course of these diseases.

Septicemia

Although septicemia (literally sepsis of the blood) implies a more serious clinical condition than bacteremia in practice, many clinicians and microbiologists use the term interchangeably.

Difference between “Bacteremia” and “Septicemia”

Traditionally, the term bacteremia referred to the transitory presence of bacteria in a patient’s blood in the absence of symptoms; the origin of bacteria was usually from a site of commensal colonization. Septicemia meant the presence of bacteria in the blood with clinical signs and symptoms of infection. The difference is largely ignored in new medical literature and publications. 

The presence of fungi in the blood is termed fungemia.

Lists of the most common causes of bacteremia and fungemia are:

Gram-negative organisms associated with bacteremia

1. Escherichia coli  (most common cause)
2. Klebsiella spp.
3. Enterobacter spp.
4. Proteus spp.
5. Salmonella typhi
6. Salmonella spp. other than S. typhi
7. Pseudomonas aeruginosa
8. Neisseria meningitidis
9. Haemophilus influenzae
10. Bacteroides fragilis (anaerobe)
11. Brucella spp.
12. Burkholderia pseudomallei  (in certain areas)

Gram-Positive organisms associated with bacteremia

1. Staphylococcus aureus (most common cause)
2. Staphylococcus epidermidis
3. α-Haemolytic (viridans) Streptococci
4. Streptococcus pneumoniae
5. Enterococcus faecalis (group D)
6. S. pyogenes (group A)
7. S. agalactiae (group B)
8. Listeria monocytogenes
9. Clostridium perfringens
10. Peptostreptococcus spp. (anaerobes)

Fungi 

1. Candida albicans
2. Cryptococcus neoformans etc.

Native valve Endocarditis

1. Viridans streptococci (Streptococcus sanguinis, Streptococcus salivarius, Streptococcus mutans, Streptococcus mitis, etc.)
2. Enterococcus faecalis
3. Enterococcus faecium
4. Streptococcus pyogenes
5. Streptococcus agalactiae
6. Staphylococcus aureus
7. Staphylococcus lugdunensis
8. Coagulase-negative staphylococci
9. Streptococcus pneumoniae
10. Neisseria gonorrhoeae
11. Haemophilus spp.
12. Pseudomonas spp.
13.  Listeria spp.
14. Corynebacterium spp.

Endocarditis in Intravenous drug abuser

1. Staphylococcus aureus
2. Streptococcus spp.
3. Enterococcus spp.
4. Gram-negative rod-shaped bacteria (mostly Pseudomonas spp. and Serratia spp.)
5. Candida spp.
6. Haemophilus spp.
7. Neisseria spp.
8. Anaerobic oral bacteria.

Prosthetic valve Endocarditis

1. Staphylococcus epidermidis
2. Staphylococcus aureus
3. Aerobic Gram-negative rods
4. Fungi (usually Candida spp. and/or Aspergillus spp.)
5. Streptococcus spp.
6. Enterococcus spp.
7. Corynebacterium spp.

References and further readings

• Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition