Pathogens of all four major groups of microbes- bacteria, fungi, viruses and parasites may be found circulating in blood during the course of many diseases.
Presence of bacteria in the blood is called Bacteremia. Bacteria may reach to 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).
On the basis of duration of presence of bacteria in the blood, bacteremia is classified as;
- Transient bacteremia,
- Continuous bacteremia and
- Intermittent bacteremia.
These bacteria are often cleared from the blood within minutes by immune cells but if the immune system is overwhelmed or evaded organisms may persist in the blood, resulting in the symptoms and signs of septicaemia. Although septicaemia (literally sepsis of the blood) implies more serious clinical condition than bacteremia (literally presence of bacteria in the blood), in practice many clinicians and microbiologist used the term interchangeably.
Difference between “Bacteremima” and “Septicaemia”
Traditionally the term bacteremia referred to the transitory presence of bacteria in the blood of a patient in the absence of symptoms; the origin of bacteria was usually from a site of commensal colonization. The term septicaemia meant the presence of bacteria in the blood with clinical signs and symptoms of infection. Their origin was from a focus of infection from which they entered the circulation.
The difference is largely ignored in new medical literature and publications. The presence of fungi in blood is termed as fungemia.
Lists of most common cause of bacteremia and fungemia are:
Gram negative organisms associated with bacteremia
- Escherichia coli (Most common cause)
- Klebsiella spp.
- Enterobacter spp.
- Proteus spp.
- Salmonella typhi
- Salmonella spp. other than S. typhi
- Pseudomonas aeruginosa
- Neisseria meningitidis
- Haemophilus inﬂuenzae
- Bacteroides fragilis (anaerobe)
- Brucella spp.
- Burkholderia (Pseudomonas) pseudomallei (in certain areas)
Gram Positive organisms associated with bacteremia
- Staphylococcus aureus (Most common cause)
- Staphylococcus epidermidis
- α-Haemolytic (viridans) Streptococci
- Streptococcus pneumoniae
- E. faecalis (group D)
- S. pyogenes (group A)
- S. agalactiae (group B)
- Listeria monocytogenes
- Clostridium perfringens
- Peptostreptococcus spp. (anaerobes)