Common Pathogenic Bacteria Found in Blood

Bacteria are microscopic organisms that can be found in various environments and habitats. Some bacteria are beneficial for human health, such as those that live in the gut and help with digestion. However, some bacteria can cause infections and diseases when they enter the body through wounds, inhalation, ingestion, or contact with contaminated surfaces.

One of the most serious types of bacterial infections is when bacteria enter the bloodstream and cause bacteremia. Bacteremia is the presence of viable bacteria in the blood, which can trigger an inflammatory response throughout the body known as sepsis. Sepsis can lead to organ failure, shock, and death if not treated promptly and effectively.

Bacteria can enter the blood from different sources, such as:

• Infections in other parts of the body, such as pneumonia, urinary tract infections, skin infections, or stomach infections
• Medical procedures or devices that breach the skin barrier, such as surgery, catheters, intravenous lines, or injections
• Exposure to contaminated food, water, or animals that carry zoonotic pathogens

Bacteria isolated from blood can be classified into different groups based on their morphology, staining properties, biochemical characteristics, and genetic features. One of the most common ways to categorize bacteria is by their Gram stain reaction, which depends on the structure of their cell wall. Gram-positive bacteria have a thick layer of peptidoglycan in their cell wall that retains a purple dye, while Gram-negative bacteria have a thin layer of peptidoglycan and an outer membrane that loses the dye and appears pink.

Some of the most common pathogenic bacteria found in blood are:

• Gram-positive bacteria: Staphylococcus spp., Streptococcus spp., Enterococcus spp., Clostridium spp., Mycobacterium spp.
• Gram-negative bacteria: Escherichia coli, Klebsiella spp., Salmonella serovars, Haemophilus spp., Pseudomonas spp., Neisseria spp., Enterobacter spp., Brucella spp., Yersinia spp., Proteus spp., Leptospira spp., Citrobacter spp.

To isolate and identify these bacteria from blood samples, various methods are used, such as:

• Blood culture: A technique that involves inoculating blood samples into sterile media and incubating them under optimal conditions to allow bacterial growth and detection
• Immunological tests: Tests that use antibodies or antigens to detect specific bacterial components or markers in blood samples
• Molecular tests: Tests that use nucleic acid amplification or hybridization to identify bacterial DNA or RNA sequences in blood samples

The identification of bacteria isolated from blood is crucial for diagnosing the cause and source of infection, determining the antibiotic susceptibility profile, and guiding the appropriate treatment and management of patients with bacteremia and sepsis. In this article, we will review some of the common pathogenic bacteria found in blood and their characteristics, clinical manifestations, and treatment options.

Gram-positive bacteria are bacteria with thick cell walls that stain blue or purple when treated with a chemical dye called Gram stain. They have no outer membrane and a complex cell wall composed of peptidoglycan, polysaccharides, teichoic acids, and proteins. They can be classified into two main shapes: cocci (spherical) and bacilli (rod-shaped).

Some gram-positive bacteria are normal flora of the skin, oral cavity, respiratory tract, or gastrointestinal tract. However, they can also cause infections when they enter the bloodstream through wounds, catheters, surgery, or other means. The most common gram-positive bacteria isolated from blood are:

• Staphylococcus spp. These are gram-positive cocci that form grape-like clusters under a microscope. They are normal flora of the skin and mucous membranes. The most common species that cause bacteremia (bacteria in blood) are Staphylococcus aureus and Staphylococcus epidermidis. S. aureus can cause serious infections such as endocarditis (infection of the heart valves), osteomyelitis (infection of the bone), and septic shock (a life-threatening condition where blood pressure drops and organs fail). Some strains of S. aureus are resistant to many antibiotics, such as methicillin-resistant S. aureus (MRSA). S. epidermidis and other coagulase-negative staphylococci (CONS) are usually less virulent, but they can cause infections in people with weakened immune systems or implanted devices.
• Streptococcus spp. These are gram-positive cocci that form chains or pairs under a microscope. They are normal flora of the upper respiratory tract, oral cavity, gastrointestinal tract, and skin. The most common species that cause bacteremia are Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus agalactiae, and viridans streptococci. S. pneumoniae is responsible for severe pneumonia and meningitis in infants and immunocompromised patients. S. pyogenes can cause wound infections, rheumatic fever, and post-streptococcal glomerulonephritis (a kidney disease). S. agalactiae can cause neonatal sepsis (infection in newborns) and endocarditis. Viridans streptococci are a group of species that can cause dental caries, endocarditis, and bacteremia in immunosuppressed and cancer patients.
• Enterococcus spp. These are gram-positive cocci that form pairs or short chains under a microscope. They are normal flora of the lower gastrointestinal tract and female genital tract. They can tolerate high concentrations of bile salts and other harsh conditions. The most common species that cause bacteremia are Enterococcus faecalis and Enterococcus faecium. They are associated with hospital-acquired infections and can cause endocarditis, urinary tract infections, intra-abdominal infections, and wound infections. Some strains of enterococci are resistant to many antibiotics, such as vancomycin-resistant enterococci (VRE).

Gram-positive bacteria can be treated with different types of antibiotics depending on their species and resistance patterns. Some examples of antibiotics used for gram-positive bacteremia are penicillins, cephalosporins, vancomycin, daptomycin, linezolid, and tigecycline.

Gram-negative bacteria are a group of bacteria that have a thin cell wall and do not retain the purple color of the Gram stain. They are often more resistant to antibiotics than gram-positive bacteria and can cause serious infections in various body sites. Some of the common gram-negative bacteria and the infections they cause include:

• E. coli – This is the most common gram-negative bacteria causing bacteremia, especially in pregnant women. It usually enters the bloodstream from infections in other sites, such as urinary tract infections, intra-abdominal infections, or surgical site infections.
• Klebsiella spp. – These bacteria can cause community-acquired or hospital-acquired bacteremia, often associated with pneumonia, urinary tract infections, or wound infections. They can also produce extended-spectrum beta-lactamases (ESBLs), which make them resistant to many antibiotics.
• Salmonella serovars – These bacteria can cause typhoidal or non-typhoidal salmonellosis, which are invasive infections that can spread to the bloodstream. They are usually acquired through contaminated food or water or contact with animals. They are more common in malnourished and immunosuppressed individuals.
• Haemophilus spp. – These bacteria are normal flora of the upper respiratory tract, oral cavity, lower gastrointestinal tract, and vagina, but some species can cause bacteremia and sepsis. The most common species is H. influenzae, which can cause meningitis and respiratory tract infections.
• Pseudomonas spp. – These bacteria are associated with hospital-acquired infections, especially in patients with indwelling devices, burns, cystic fibrosis, or neutropenia. The main species is P. aeruginosa, which can cause bacteremia and endocarditis. These bacteria are often resistant to multiple antibiotics and have high mortality rates.
• Neisseria spp. – These bacteria are commensal of the nasopharynx and sometimes found in the mucosal lining of the vaginal tract, but some species can cause invasive infections and bacteremia. The most common species is N. meningitidis, which can cause meningitis and septic shock. Another species is N. gonorrhoeae, which can cause gonorrhea, a sexually transmitted disease.

The table below summarizes some of the characteristics and examples of gram-negative bacteria in blood.

Gram-negative bacteremia is a serious condition that requires prompt diagnosis and treatment with appropriate antibiotics. The choice of antibiotics depends on the type of bacteria, the source of infection, the patient`s risk factors, and the local resistance patterns.

Besides the Gram-positive and Gram-negative bacteria, there are some other bacteria that can be isolated from blood. These bacteria may cause bacteremia, septicemia, or sepsis in certain conditions. Some of these bacteria are:

• Clostridium spp. These are Gram-positive, rod-shaped, anaerobic, spore-forming bacteria that are 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. These are Gram-positive (Acid-fast bacilli), rod-shaped, aerobic bacteria that have a high mycolic acid content in their cell-wall making them acid-fast. M. tuberculosis can be isolated in patients with pulmonary and non-pulmonary TB. Besides, there are other commonly encountered species reported in case of bacteremia, such as 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. These are Gram-negative, rod-shaped, facultative anaerobic bacteria that belong to the family Enterobacteriaceae and are associated with nosocomial infections. E. cloacae, E. aerogenes, E. asburiae, E. agglomerans, E. hormaechei, E. sakazakii, E. amnigenus, and E. gergoviae are common species associated with Enterobacter bacteremia.
• Brucella spp. These are Gram-negative, facultative coccobacilli that are aerobic and facultative anaerobic and belong to the family Brucellaceae. B. melitensis, B. abortus, B. suis, and B. canis are Brucella species associated with several systemic infections and bacteremia.
• Yersinia spp. These are Gram-negative coccobacilli that are facultative anaerobic and belong to the family Yersiniaceae. Y. pestis is a major human pathogen causing plague and 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. These are Gram-negative rod-shaped aerobic and facultative anaerobic motile bacteria that belong to the family Enterobacteriaceae and form swarming colonies. They are 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. These are Gram-negative spirochetes that belong to the family Leptospiraceae. They cause an 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. These are Gram-negative rod-shaped non-sporing coliform bacteria that belong to the family Enterobacteriaceae. C. freundii, C. diversus, C. braakii are commonly isolated in blood culture.

These are some of the other bacteria that can be found in blood and cause serious infections.

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