Staphylococcus saprophyticus Infection

Etiology

S. saprophyticus is the second most common cause of community-acquired urinary tract infections, after Escherichia coli. In females ages 16 to 25, it causes up to 42% of all infections. Over 40% of all young, sexually active women contain S. saprophyticus as part of their normal genitourinary flora.[3][4][5]

Patients with nosocomial UTIs, the elderly, pregnant patients, and those with urinary catheterization have an increased incidence of S. saprophyticus colonization. Men have a lower incidence of S. saprophyticus infections.

General risk factors for UTIs include the history of recurrent UTIs, female sex, recent sexual intercourse, pregnancy, neurogenic bladder, indwelling catheter, and benign prostatic hypertrophy.[6][7]

S. saprophyticus is also a common culprit involved in polymicrobial UTIs. Polymicrobial infections are more likely to occur in patients that are immunocompromised, elderly, those who have diabetes, have indwelling catheters, HIV, and/or malignancies. Polymicrobial infections are less common in young, healthy, sexually active females.

Epidemiology

In the United States, urinary discomfort is a common complaint in patients seeking medical attention. UTIs are one of the top 10 diagnoses made in emergency departments annually. Nearly half of all women will experience a UTI in their lifetime, and between 5% and 20% of non-hospitalized patients, the infection will be due to S. saprophyticus. Despite highly successful treatment rates, up to 60% of all patients will experience a recurrent UTI within one year.

Pathophysiology

Bacterial colonization of the bladder and ureter epithelium by S. saprophyticus occurs via several different types of adhesins. These include hemagglutinins with autolytic and adhesive properties, as well as surface-associated lipase that forms fimbria-like surface appendages, helping the bacteria to maintain tight adherence to these surfaces.

It is suspected that the high survivability of S. saprophyticus inside the urinary tract is in part due to the adhesins anchored within the cell wall, allowing the organism to effectively adhere and colonize the uroepithelium, together with urease, which contributes to the persistent growth of the infection.

Some strains of S. saprophyticus have the ability to create biofilms, increasing their virulence, especially in patients with catheters. Once biofilms have been produced, antibiotic resistance is exacerbated. In these cases, S. Saprophyticus may be resistant to vancomycin and only effectively treated via linezolid.

History and Physical

The characteristic history of dysuria, urinary frequency, urinary urgency, and suprapubic pain will be common in symptomatic UTI patients. In those patients with pyelonephritis, back or flank pain, nausea, and fever or chills may also be present.

Physical examination may reveal suprapubic tenderness, in 10% to 20% of cases, and should include urine specimen for analysis. However, in most cases of uncomplicated UTI, a physical examination is unremarkable. In complicated cases or pyelonephritis, patients may present with fever, tachycardia, and/or costovertebral angle tenderness.

Evaluation

The diagnosis of S. saprophyticus requires a confirmatory urine culture. A positive culture is indicated by greater than 100,000 colony-forming units per mL, with a sensitivity and specificity of more than 90%.

UTI, in general, may be diagnosed more cost-effectively with a urine dipstick alone. A dipstick that is positive for leukocytes esterase and/or nitrites is the most simplistic method of UTI diagnosis. In cases of negative dipstick results, and high clinical suspicion, a bacterial urine culture should also be obtained.[8][9]

• An adequate urine sample should be obtained from a mid-stream catch or straight catheterization, which more effectively avoids contamination.

Imaging is not necessary for cases of uncomplicated UTIs. If renal pathology, such as pyelonephritis, is suspected a CT scan is the most sensitive modality for demonstrating complications such as hydronephrosis or renal abscess.

Treatment / Management

Treatment with outpatient antibiotics is indicated in symptomatic or complicated UTIs and pyelonephritis. It is important to take into consideration specific local resistance patterns when choosing appropriate antibiotic coverage.[10]

The antibiotic of choice in uncomplicated S. saprophyticus UTIs is nitrofurantoin 100 mg orally twice daily for five days, or for seven days in complicated cases. Trimethoprim-sulfamethoxazole (TMP-SMX) 160 mg/800 mg by mouth twice daily for three days may be given alternatively in uncomplicated cases.

Symptomatic treatment for pain and nausea should also be addressed.  Acute uncomplicated UTIs are unlikely to cause renal injury. Thus NSAIDs are a preferred analgesic. Pyridium may also be given to alleviate associated dysuria. Ondansetron or promethazine are commonly prescribed antiemetics. Most patients will notice symptomatic relief within 36 hours from antibiotic treatment alone.

Patients who are hemodynamically unstable, have associated kidney injury, abscess formation, or emphysematous pyelonephritis, have failed outpatient treatment, have intractable nausea, vomiting, or pain, are unable to tolerate oral intake, or are unable to comply with medical treatment may require admission.

Differential Diagnosis

Other diagnoses include non-S. saprophyticus UTI or cystitis, urethritis, pyelonephritis, or nephrolithiasis.

Prognosis

The majority of S. saprophyticus infections can be adequately treated with antibiotics. However, if left untreated, they may progress to pyelonephritis. Untreated pyelonephritis may lead to further complications, such as renal insufficiency.