Common Pathogenic Bacteria Found in Blood

Blood is sterile liquid connective tissue flowing inside blood vessels as a pumping action of the heart. Infection in the blood is called bloodstream infection. It is a serious condition because, through the blood, pathogens can disseminate throughout the body including several internal organs. When untreated, it can even lead to death. Infection may be by bacteria, fungi, viruses, or protozoans.

The presence of bacteria in the blood is called bacteremia. When these bacteria rapidly multiply and release toxins in the blood, then the condition is called septicemia. Bacteria may establish bloodstream infection or disseminate to infect internal organs and tissues. So, in the blood of infected patients, bacteria causing systemic infection together with bloodstream infection can be found. Bacteria are isolated from blood in diseases like endocarditis, typhoid, brucellosis, leptospirosis, plague, TB, hepatitis, meningitis, peritonitis, osteomyelitis, etc. To isolate and identify these bacteria, blood culture, immunological tests, and/or molecular tests are usually done. 

List of Bacteria Isolated From Blood

Gram +ve and Gram -ve Bacteria Found in Blood

Staphylococcus spp. in Blood

• Gram-positive
• catalase-positive
• cocci bacteria
• family Staphylococcaceae
• producing grape-like clusters under a microscope
• normal flora of the skin

Staphylococcus aureus is a commonly isolated species in blood culture. S. aureus bacteremia (SAB) is common and significantly fatal infection. MRSA (methicillin-resistant S. aureus) is more commonly associated with case severity and mortality.    

S. epidermidis, S. saprophyticus, and other coagulase-negative staphylococci (CONS) are also isolated in blood culture, but mostly they are due to contamination during blood collection. Rarely, S. saprophyticus is found to cause bacteremia in patients with chronic and complicated UTIs. 

Streptococcus spp. in Blood

• Gram-positive
• anaerobic and facultative anaerobic
• catalase-negative
• cocci bacteria
• family Streptococcaceae
• arrangement in the form of a chain of spheres.
• normal flora of the upper respiratory tract, oral cavity, GI tract, and skin.

S. pneumoniae is the most common species in this genus isolated in blood culture. It is responsible for severe pneumonia in infants and immunosuppressed patients. 

S. pyogenes is another species causing bacteremia. It is isolated from patients with deep-seated wound infections, rheumatic fever, and post-streptococcal glomerulonephritis.  

S. agalactiae is also reported in some endocarditis. S. mitis, S. parasanguis, S. anginosus, S. salivarius, S. gordonii, S. sanguis, S. sobrinus, and S. vestibularis are the viridans streptococci isolated from immunosuppressed and cancer patients with bacteremia. 

Enterococcus spp. in Blood

• Gram-positive
• facultatively anaerobic
• lactose fermenting
• cocci (diplococci) bacteria
• family Enterococcaceae
• tolerate bile salt concentrations up to 40%
• normal habitants of the lower GI tract

E. faecalis is the main species isolated mostly from hospital-acquired bacteremia and are responsible for the severity of infection. E. faecium is another species responsible for bacteremia. Enterococcus bacteremia cases are increasing rapidly over the last decade. 

E. coli in Blood

• Gram-negative
• rod-shaped
• lactose fermenting
• facultatively anaerobic
• mesophilic coliform bacteria
• Genus Escherichia
• Enterobacteriaceae family

It is the most common Gram-negative bacteria causing bacteremia, especially in the case of pregnancy. It usually enters the bloodstream from infections in other sites; about 50% of the time through UTIs. About 25 – 30% of reported bacteremia is caused by E. coli.

Klebsiella spp. in Blood

• Gram-negative
• rod-shaped
• facultative anaerobic
• capsule-forming coliform
• family Enterobacteriaceae

K. pneumoniae is the most common species in this genus responsible for community-acquired and hospital-acquired bacteremia. It is found in both the primary and secondary bacteremia (like from pneumonia, UTIs, and meningitis).

K. oxytoca and K. variicola are other species commonly associated with bacteremia.   

Salmonella serovars in Blood

• Gram-negative
• rod-shaped
• flagellated
• facultative anaerobic
• Gammaproteobacteria
• family Enterobacteriaceae

Several Typhoidal and non-typhoidal Salmonella spp. are invasive and are associated with bacteremia. Incidence of Salmonella bacteremia varies from 5.2% – 13.7%. 

S. Typhi and Paratyphi, S. choleraesuis, S. Virchow are the common species involved in bacteremia. Non-typhoidal Salmonella is usually associated with malnourished and immunosuppressed individuals. 

Haemophilus spp. in Blood

• Gram-negative
• coccobacilli (pleomorphic)
• aerobic or facultative anaerobic
• Gammaproteobacteria
• family Pasteurellaceae.
• normal flora of upper respiratory tract, oral cavity, lower GI tract, and vagina
• some species are pathogenic

H. influenzae is the most commonly encountered pathogenic species causing meningitis and sepsis. H. parainfluenzae is another species that is associated with many systemic infections including bacteremia.    

Pseudomonas spp. in Blood

• Gram-negative
• rod-shaped
• aerobic
• gammaproteobacteria
• family Pseudomonadaceae
• associated with hospital-acquired infections

P. aeruginosa is the main species involved in Pseudomonas bacteremia and endocarditis. The infection is mainly nosocomial, but community-acquired cases are also numerous. In either case, mortality is high. 

P. luteola, P maltophilia, and P. cepacia are also occasionally reported responsible for bacteremia. 

Neisseria spp. in Blood

• Gram-negative
• aerobic or facultative anaerobic
• cocci (and diplococci)
• Betaproteobacteria
• family Neisseriaceae
• commensal of the nasopharynx
• sometimes found in the mucosal lining of the vaginal tract

N. meningitidis is the most commonly isolated species in blood. They are responsible for meningitis and respiratory tract infections related bacteremia.

N. macacae, N. weaver, N. gonorrhoeae, and N. cinerea are also reported in a few cases of bacteremia.      

Clostridium spp. in Blood

• Gram-positive
• rod-shaped
• anaerobic
• spore-forming bacteria
• family Clostridiaceae
• normal flora of the GI tract and females’ lower reproductive tract

C. perfringens, C. septicum, C. bifermentans are common Clostridial species reported in cases of bacteremia.  

Mycobacterium spp. in Blood

• Gram-positive (Acid-fast bacilli)
• rod-shaped
• aerobic Actinobacteria
• family Mycobacteriaceae
• mycolic acid content in cell-wall making them acid-fast

M. tuberculosis can be isolated in patient with pulmonary and non-pulmonary TB. Besides, there are other commonly encountered species reported in case of bacteremia, viz. M. mucogenicum, M. abscessus, M. fortuitum complex, M. lacticola, M. cosmeticum, M. bacteremicum, M. flavescens, M. immunogenum, M. brumae, M. aurum, M. wolinskyi, M. smegmatis, and M. phocaicum 

Enterobacter spp. in Blood

• Gram-negative
• rod-shaped
• facultative anaerobic
• Gammaproteobacteria
• family Enterobacteriaceae
• associated with nosocomial infection

E. cloacae, E. aerogenes, E. asburiae, E. agglomerans, E. hormaechei, E. sakazakii, E. amnigenus, and E. gergoviae are common species associated in Enterobacter bacteremia. 

Brucella spp. in Blood

• Gram-negative
• facultative coccobacilli
• aerobic and facultative anaerobic
• nonmotile
• Alphaproteobacteria
• family Brucellaceae

B. melitensis, B. abortus, B. suis, and B. canis are Brucella species associated with several systemic infections and bacteremia. 

Yersinia spp. in Blood

• Gram-negative
• coccobacilli
• facultative anaerobic
• Gammaproteobacteria
• family Yersiniaceae.

Y. pestis is a major human pathogen causing plague. It can be isolated from blood of infected patients. Y. enterocolitica and Y. pseudotuberculosis are pathogens causing yersiniosis and can be isolated in blood.

Proteus spp. in Blood

• Gram-negative
• rod-shaped
• aerobic and facultative anaerobic
• motile bacteria
• family Enterobacteriaceae
• swarming colonies.
• responsible for several cases of primary and secondary bacteremia

P. mirabilis is the main species causing Proteus bacteremia. Besides, P. vulgaris is also reported in rare cases. 

Leptospira spp. in Blood

• Gram-negative
• spirochetes
• family Leptospiraceae

It causes invasive deep-seated infection called Leptospirosis. In this infection, the bacteria are found abundantly in the blood of the patient. L. interrogans is the most associated species. 

Citrobacter spp. in Blood

• Gram-negative
• rod-shaped
• non-sporing
• coliform bacteria
• family Enterobacteriaceae

C. freundii, C. diversus, C. braakii are commonly isolated in blood culture. 

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