Coagulase-Negative Staphylococci (CoNS) are a diverse group of bacteria commonly found on human skin and mucous membranes. While often dismissed as a laboratory contaminant, these organisms can cause genuine Urinary Tract Infections (UTIs). The most common UTI-causing species is Staphylococcus saprophyticus, which frequently infects otherwise healthy individuals. Successfully treating a CoNS UTI requires a tailored approach because these bacteria exhibit complex patterns of antibiotic resistance.
Coagulase-Negative Staphylococci as a Urinary Pathogen
Coagulase-Negative Staphylococci are defined by their inability to produce the enzyme coagulase, distinguishing them from the more aggressive Staphylococcus aureus. Since CoNS naturally colonize the skin, their presence in a urine sample is often difficult to interpret. Physicians must determine if the bacteria represent a true infection or merely contamination introduced during sample collection.
The most recognized CoNS species causing UTIs is Staphylococcus saprophyticus, often presenting as an acute, uncomplicated bladder infection in young, sexually active women. Other CoNS, such as Staphylococcus epidermidis, are more frequently associated with complicated infections involving indwelling urinary catheters. To confirm a true infection, doctors consider clinical symptoms, the presence of white blood cells (pyuria), and high bacterial colony counts.
How Susceptibility Testing Guides Treatment Selection
Choosing the correct antibiotic for a CoNS UTI depends entirely on the results of Culture and Sensitivity (C&S) testing. This procedure identifies the bacterial species and determines which antibiotics can successfully inhibit its growth. This step is important because CoNS have an extensive ability to acquire resistance genes, which can render common antibiotics ineffective.
A significant challenge is the high prevalence of methicillin resistance among many CoNS strains, similar to MRSA. Standard treatment for methicillin-susceptible staph infections will fail against these resistant strains, such as Methicillin-Resistant S. epidermidis (MRSE). The C&S report classifies the organism’s response to each drug using specific codes.
These results are classified into three primary categories: Susceptible (S), Intermediate (I), and Resistant (R). Susceptible means the organism is highly likely to respond to a standard dose of the antibiotic. Intermediate indicates that success is likely only if the drug is concentrated at the infection site, such as in the urine, or if a higher dose is used. Resistant indicates a high probability of treatment failure, meaning that antibiotic should be avoided.
Specific Antibiotic Treatments for CoNS UTIs
Antibiotic selection for a CoNS UTI begins with the susceptibility profile, focusing on agents that achieve high concentrations in the urine. For uncomplicated infections caused by a susceptible strain of S. saprophyticus, first-line oral choices are highly effective. These options include Nitrofurantoin, preferred due to its excellent concentration in the lower urinary tract and low impact on the body’s overall bacterial flora. Treatment with Nitrofurantoin is typically recommended for five days.
Another common first-line agent is Trimethoprim-sulfamethoxazole (TMP-SMX), often used for three to five days, provided local resistance rates are low. If the CoNS strain is resistant to these first-line drugs, or if the infection is complicated, alternative agents must be considered. Fluoroquinolones, such as ciprofloxacin or levofloxacin, are highly active and achieve excellent urinary concentrations. These drugs are generally reserved for more complicated cases due to concerns about promoting widespread resistance and the risk of significant side effects.
For methicillin-sensitive CoNS strains, certain cephalosporins like cephalexin may be an option, though they are sometimes associated with higher recurrence rates. A single-dose option for uncomplicated cystitis is Fosfomycin, useful when patient compliance is a concern. In the rare event of a highly resistant CoNS strain causing a complicated or systemic infection, specialized intravenous antibiotics like Vancomycin may be necessary. Uncomplicated UTIs typically require a short course of treatment (three to seven days), while complicated infections may require a longer course (seven to fourteen days).
Monitoring and Addressing Recurrent Infections
After starting antibiotic treatment, clinical improvement should be noticeable within 48 to 72 hours. Resolution of uncomfortable symptoms, such as burning during urination and frequency, is the primary sign that the treatment has been successful. For most patients with an uncomplicated CoNS UTI, a follow-up urine culture (Test of Cure) is not necessary if symptoms have completely resolved.
If symptoms persist beyond the expected recovery period, it may suggest the initial antibiotic failed due to an undetected resistance pattern or an underlying issue. In such cases, a repeat culture and sensitivity test is performed to guide a switch to a different antibiotic. Recurrent UTIs, defined as multiple episodes within a short period, require a thorough investigation to identify predisposing factors.
These factors might include indwelling devices like catheters or underlying anatomical abnormalities in the urinary tract. In post-menopausal women, the loss of vaginal acidity is a common risk factor that can be addressed with localized estrogen therapy. For frequent recurrences, a physician may prescribe a low-dose antibiotic regimen daily for an extended period, sometimes six months or longer, to prevent re-infection.