Coagulase-Negative Bacteria
Coagulase-negative staphylococci (CoNS) are the primary group of bacteria that test negative for coagulase production, with Staphylococcus epidermidis being the most common species, followed by S. hominis, S. haemolyticus, S. saprophyticus, and notably S. lugdunensis, which despite being coagulase-negative behaves with virulence similar to S. aureus. 1
Primary Coagulase-Negative Staphylococcal Species
The coagulase-negative staphylococci are differentiated from the more virulent Staphylococcus aureus (which is coagulase-positive) by their inability to produce free coagulase. 2 The major species include:
S. epidermidis: The most frequently isolated CoNS species, accounting for approximately 53% of coagulase-negative staphylococcal isolates and representing 37% of all hospital-acquired bloodstream infections. 1, 3
S. hominis: The second most common species, representing approximately 12% of urinary CoNS isolates. 3
S. haemolyticus: Accounts for approximately 10% of coagulase-negative staphylococcal isolates. 3, 4
S. saprophyticus: Represents about 5% of urinary isolates and is particularly associated with urinary tract infections in young women. 3, 5
S. lugdunensis: A critical exception among CoNS that warrants special attention due to its substantially more virulent behavior, causing aggressive endocarditis with high rates of perivalvular extension and metastatic infection similar to S. aureus. 1, 6
Additional CoNS Species
Other coagulase-negative staphylococcal species documented to cause infections include:
S. capitis: Can cause skin and soft tissue infections, particularly in immunocompromised patients. 7
S. auricularis: Associated with skin and soft tissue infections. 7
S. simulans: Identified in both blood culture isolates and skin infections. 7, 4
S. warneri: Found in blood culture isolates from cancer patients. 4
S. xylosus: Occasionally isolated from blood cultures. 4
Clinical Significance and Epidemiology
CoNS have emerged as the most common cause of hospital-acquired bloodstream infections, increasing from 27% during 1986-1989 to 37% during 1992-1999, surpassing even S. aureus (which accounts for only 12.6% of hospital-acquired BSIs). 1, 8 This dramatic rise reflects the increased use of indwelling medical devices, particularly intravascular catheters. 1, 2
Critical Clinical Caveat: Contamination vs. True Infection
The most important pitfall is that 65-94% of CoNS-positive blood cultures represent contamination rather than true infection. 6 Multiple positive blood cultures from multiple sites are required to distinguish true bacteremia from contamination. 6 A single positive blood culture with negative concurrent cultures likely represents contamination and should not automatically trigger vancomycin therapy. 6
Exception: S. lugdunensis Management
Despite being coagulase-negative, S. lugdunensis should be managed identically to S. aureus due to its high virulence and ability to cause endocarditis and metastatic infections. 1, 6 Many laboratories lack the capability to differentiate S. lugdunensis from other CoNS species, making microbiological identification crucial. 1