Coagulase-negative staphylococci (CoNS) represent a diverse group of bacteria, not a single species. The name “coagulase-negative” is derived from a laboratory test that helps classify different types of staphylococci. In this test, bacteria are mixed with plasma, a component of blood; coagulase-positive bacteria produce an enzyme that causes the plasma to clot, while CoNS do not. This group includes many different species, such as Staphylococcus epidermidis, Staphylococcus haemolyticus, and Staphylococcus saprophyticus.
The Role of CoNS on the Human Body
Coagulase-negative staphylococci are a normal and widespread part of the human skin’s microbial community, known as the microbiota. These bacteria colonize the skin and mucous membranes of individuals starting from a few weeks after birth and are considered commensal, meaning they live on the body without causing harm. They are particularly abundant in moist areas like the armpits and groin and can even provide some benefit by inhibiting the growth of more harmful pathogens.
This contrasts with coagulase-positive staphylococci, most notably Staphylococcus aureus, which is more commonly recognized as a cause of infections. While S. aureus is carried by about 30% of people, S. epidermidis, a common CoNS species, is found on the skin of nearly everyone. The primary difference lies in their pathogenic potential; S. aureus possesses more factors that can lead to disease, whereas CoNS typically only cause issues under specific circumstances.
When CoNS Causes Infection
CoNS transition from harmless skin residents to opportunistic pathogens when they gain access to normally sterile sites within the body. This breach often occurs through breaks in the skin or, more commonly, through the insertion of medical devices. Hospitalized patients, individuals with weakened immune systems, and premature newborns are at a higher risk for these types of infections.
A factor in the ability of CoNS to cause infection is the formation of biofilms. A biofilm is a slimy, protective layer that bacteria create on a surface, which shields them from the host’s immune system and antibiotics. This process begins when the bacteria attach to a surface, multiply, and produce substances that help them adhere to each other in multi-layered aggregates. This protective matrix makes infections involving biofilms, especially those on medical implants, particularly difficult to eradicate.
Associated Medical Conditions
The specific type of infection often depends on the species of CoNS and the location within the body. Bloodstream infections, also known as bacteremia or sepsis, are among the most common and serious conditions caused by CoNS, frequently linked to the use of intravascular catheters. If a bloodstream infection becomes severe, it can lead to sepsis, a life-threatening condition characterized by widespread inflammation and potential organ failure.
Infections are frequently associated with foreign bodies. For example, CoNS can infect prosthetic heart valves, leading to a serious condition called endocarditis, which involves inflammation of the heart’s inner lining and can present with symptoms like fever, chills, and a new heart murmur. Other common sites of infection include prosthetic joints, leading to painful inflammation, and cerebrospinal fluid shunts used to treat hydrocephalus. Staphylococcus saprophyticus is a species notable for causing urinary tract infections (UTIs), especially in younger women.
Diagnosis and Treatment
Diagnosing a genuine CoNS infection can be complicated. Because these bacteria are so prevalent on the skin, they can easily contaminate laboratory samples, particularly blood cultures taken via a needle puncture. Because a single positive culture might reflect contamination rather than a true infection, physicians require at least two separate positive blood cultures with the same CoNS species to confirm the bacteria are present in the bloodstream.
Treating CoNS infections is challenging due to widespread antibiotic resistance. Many strains of CoNS are resistant to common antibiotics, including methicillin, which means they are classified as methicillin-resistant CoNS (MR-CoNS), similar to the better-known MRSA. Consequently, treatment often necessitates the use of more powerful intravenous antibiotics, such as vancomycin or linezolid.
The presence of a medical device is a major factor in treatment strategy. In many cases where a CoNS infection is linked to a catheter, implant, or prosthetic, the removal of the contaminated device is necessary for the treatment to be successful. The combination of targeted antibiotic therapy and, when applicable, device removal is the standard approach to managing these opportunistic infections.