Coagulase-Negative Staphylococci (CoNS) are a large group of bacteria commonly found on human skin and mucous membranes. For a long time, these organisms were dismissed as harmless contaminants in clinical samples. They are now recognized as significant opportunistic pathogens, especially in hospital settings and in urinary tract infections (UTIs). Understanding the role of CoNS in UTIs, their identification methods, and their high propensity for antibiotic resistance is a growing focus in clinical microbiology.
Understanding Coagulase-Negative Staphylococci
The name Coagulase-Negative Staphylococci refers to their lack of the enzyme coagulase, a protein that causes blood plasma to clot. This characteristic is the primary way they are differentiated from the more virulent species, Staphylococcus aureus, which is coagulase-positive. CoNS are generally considered commensal bacteria, forming a natural part of the human microbial flora. Over 40 different species exist, with Staphylococcus epidermidis being the most common inhabitant of human skin. Species like S. epidermidis and Staphylococcus saprophyticus can exploit breaks in the body’s defenses to cause infection.
CoNS Role in Urinary Tract Infections
CoNS are responsible for a notable percentage of urinary tract infections. Staphylococcus saprophyticus is the species of CoNS most frequently implicated in UTIs. This uropathogen is the second most common cause of community-acquired UTIs in young women, following Escherichia coli. S. saprophyticus can account for 10% to 20% of all UTI cases in this demographic. The bacteria initiates infection by using specialized cell wall-anchored adhesins to stick firmly to the cells lining the urinary tract. This strong adherence allows the organism to resist the flushing action of urine. The organism’s ability to produce urease, an enzyme that breaks down urea into ammonia, also contributes to its survival.
Laboratory Identification and Clinical Interpretation
The laboratory identification of CoNS in a urine sample is complicated by their status as common skin flora. Since CoNS can easily contaminate a collected urine specimen, a key step is to differentiate a true infection from simple contamination. Initial analysis typically involves culturing the sample and examining the colony morphology on an agar plate.
The critical step for speciation involves biochemical testing, most commonly the novobiocin susceptibility test. This test is designed to distinguish the pathogenic S. saprophyticus from the frequently isolated contaminant, S. epidermidis. The test involves placing a novobiocin-impregnated disk onto an inoculated agar plate. S. saprophyticus is resistant to novobiocin, resulting in a small or absent zone of inhibition around the disk.
In contrast, S. epidermidis is sensitive to novobiocin, showing a large, clear zone where bacterial growth is inhibited. Clinicians interpret the lab results by considering the species identification alongside the colony count and the patient’s symptoms. A true infection is generally indicated by a high colony count of a pathogenic species like S. saprophyticus in a symptomatic patient, while a low count of a sensitive species like S. epidermidis is often dismissed as a skin contaminant.
Antibiotic Resistance Profiles and Treatment Guidance
Antibiotic resistance is a significant concern for CoNS, making antibiotic susceptibility testing (AST) necessary for guiding treatment. A major resistance pattern is methicillin resistance, leading to Methicillin-Resistant Coagulase-Negative Staphylococci (MRCoNS). The prevalence of MRCoNS is high, with estimates suggesting that over 70% of CoNS isolates worldwide are resistant to methicillin and related beta-lactam antibiotics.
This resistance is primarily mediated by the presence of the mecA gene. The mecA gene codes for an altered penicillin-binding protein known as PBP2a. This modified protein reduces the affinity of the bacterial cell wall for beta-lactam antibiotics, rendering drugs like penicillin and cephalosporins ineffective.
Because of this widespread resistance, standard empirical treatments often used for common UTIs may fail when the cause is a resistant CoNS. The AST results provide the resistance profile, allowing clinicians to select an appropriate non-beta-lactam antibiotic. For confirmed MRCoNS infections, treatment often relies on agents such as vancomycin or linezolid, which remain effective against these highly resistant organisms.