Coagulase-negative staphylococci, often abbreviated as CoNS, represent a broad category of bacteria within the Staphylococcus genus. This diverse group is characterized by its inability to produce the enzyme coagulase, a feature that distinguishes them from Staphylococcus aureus. There are over 30 different species of CoNS, and they are frequently found inhabiting human skin and mucous membranes.
Commensal Flora and Opportunistic Pathogen
CoNS species exist as commensal flora, a natural component of the human microbiome on skin and mucous membranes. They commonly colonize areas like the armpits, groin, anterior nares, and conjunctiva. This presence is benign, as these bacteria coexist with the host without causing disease.
However, CoNS can transition from harmless residents to opportunistic pathogens under specific conditions. They exploit breakdowns in the body’s natural defenses, particularly in individuals with weakened immune systems. The presence of foreign medical devices significantly increases the risk of infection. These devices include central venous catheters, prosthetic heart valves, and orthopedic implants.
Associated Clinical Infections
CoNS are recognized for causing infections associated with indwelling medical devices, primarily due to their ability to form biofilms. A biofilm is a protective, slimy layer of bacteria that adheres to surfaces, making the bacteria within it highly resistant to antibiotics and the body’s immune responses. This characteristic makes biofilm-related infections challenging to eradicate without removing the device.
Catheter-related bloodstream infections (bacteremia) occur when CoNS colonize intravenous catheters and enter the bloodstream. Prosthetic valve endocarditis, an infection of artificial heart valves, and orthopedic implant infections, affecting prosthetic joints, are other significant device-related issues. Staphylococcus epidermidis is the primary CoNS species implicated in these device-associated infections, though others like Staphylococcus lugdunensis are sometimes involved. CoNS can also cause other infections, such as urinary tract infections, with Staphylococcus saprophyticus being a notable cause, and surgical site infections.
Laboratory Diagnosis and Interpretation
The identification of CoNS often begins with their isolation from clinical samples, such as blood cultures, obtained from patients suspected of having an infection. A challenge for healthcare providers is distinguishing between a true infection and mere contamination of the sample. Because CoNS are ubiquitous on human skin, they can easily be introduced into a blood sample during collection, leading to a false positive result.
To differentiate contamination from genuine infection, clinicians look for specific indicators. The presence of CoNS in multiple blood cultures drawn at different times from separate sites suggests a true infection rather than a single instance of contamination. Additionally, clinical signs of inflammation, such as fever or elevated white blood cell counts, along with the presence of an indwelling medical device, help confirm a diagnosis.
Treatment Considerations and Resistance
If CoNS is determined to be a contaminant rather than the cause of an infection, antibiotic treatment is not necessary. However, if a true infection is diagnosed, appropriate treatment is required. A significant concern in treating CoNS infections is their high rate of antibiotic resistance, particularly to methicillin and related penicillin-class antibiotics. This widespread resistance means many common antibiotics are ineffective against these bacteria.
Due to this resistance profile, vancomycin is the antibiotic of choice for treating serious CoNS infections. For infections involving biofilms on medical devices, antibiotics alone are often insufficient to achieve a cure because the biofilm acts as a protective barrier, preventing adequate antibiotic penetration. In such cases, surgical removal of the infected medical device, such as a catheter or prosthetic joint, is required in addition to antibiotic therapy to resolve the infection.