Coagulase-negative staphylococci (CoNS) are a broad category of Staphylococcus bacteria that do not produce the enzyme coagulase. These bacteria are frequently found harmlessly on human skin and mucous membranes, forming part of our natural microbial community. However, CoNS can shift from harmless residents to agents of infection under specific conditions.
The Dual Role of Coagulase-Negative Staphylococci
CoNS primarily function as commensal organisms, meaning they normally inhabit the human body without causing harm. They are a significant component of the skin’s natural flora and are common environmental contaminants.
Despite their usual harmless nature, CoNS are opportunistic pathogens. They can cause disease when the body’s natural defenses are weakened or when physical barriers, like the skin, are breached. If these bacteria access normally sterile internal body sites, such as the bloodstream or deep tissues, they can establish an infection.
The distinction between CoNS and Staphylococcus aureus lies in the coagulase enzyme test. Staphylococcus aureus produces coagulase, an enzyme that clots blood plasma and is often associated with more aggressive disease. CoNS, lacking this enzyme, generally cause less aggressive infections, often relying on predisposing factors like foreign materials in the body. Their ability to cause infection depends on the host’s vulnerability and circumstances allowing entry.
Common Infections and Risk Factors
CoNS are a leading cause of infections related to indwelling medical devices, readily adhering to artificial implants and catheters. Examples include central line-associated bloodstream infections (CLABSIs), prosthetic joint infections, and prosthetic heart valve infections.
Catheter-associated urinary tract infections (CAUTIs) can also be caused by CoNS, especially in patients with long-term urinary catheters. Staphylococcus saprophyticus is a notable exception, commonly causing UTIs in otherwise healthy young women, unlike other CoNS species that typically affect compromised individuals.
Several factors increase the risk of CoNS infection. These include prolonged hospitalization, the presence of foreign medical devices (like catheters or prostheses), and weakened immune systems due to conditions such as cancer, HIV, or immunosuppressive medications. Premature infants, with developing immune systems and frequent medical interventions, also face an elevated risk.
Diagnosis and Treatment Challenges
Diagnosing a true CoNS infection presents a significant challenge. Since CoNS are abundant on the skin, they can easily contaminate diagnostic samples like blood cultures. Differentiating between a genuine infection and simple contamination by normal skin flora is difficult, often requiring multiple positive cultures from different sites or over time to confirm an active infection.
Treating CoNS infections involves several hurdles, primarily antibiotic resistance and biofilm formation. Many CoNS strains, especially in healthcare settings, have developed methicillin resistance. This means common first-line antibiotics like oxacillin or flucloxacillin are often ineffective, limiting treatment options.
CoNS are also adept at forming biofilms: complex communities of bacteria encased in a protective matrix. This matrix, composed of extracellular polymeric substances, allows bacteria to adhere strongly to medical device surfaces. Biofilms act as a physical barrier, shielding bacteria from antibiotics and the immune system, making them hundreds to thousands of times more resistant than free-floating bacteria. Effective treatment often requires appropriate antibiotic therapy and physical removal of the infected medical device.