Staphylococcus aureus is both a commensal colonizing organism of the skin and upper respiratory tract and a human pathogen, responsible for causing a wide range of infections ranging from fairly benign skin and soft tissue infections to life-threatening conditions such as bacteremia and endocarditis. S. aureus is an uncommon cause of urinary tract infection in the general population, accounting for 0.4%-4% of positive urine cultures.1-3
S. aureus bacteriuria (SABU) is more common among patients with indwelling urinary tract catheters or prior urinary tract instrumentation4 and may only reflect urinary tract colonization in asymptomatic individuals. However, when isolated from urine without an obvious urinary focus, SABU can be a marker of hematogenous seeding of the urinary tract, with an estimated prevalence of 7-16% in patients with S. aureus blood stream infection (SABSI).5-7 S. aureus blood stream infection from a urinary tract source (SABSI-UTS) is an infrequent complication of S. aureus urinary tract infection, accounting for only 3-6% of patients with SABSI.8,9 Interpreting the frequency of SABSI-UTS has been challenging given difficulties in attributing an episode of SABSI to a urinary tract source and variation in defining this syndrome.
Grillo and colleagues conducted a multicentered retrospective cohort study of adult patients hospitalized with SABSI to determine characteristics and 30-day mortality of patients with SABSI-UTS.10 In this study, investigators defined SABSI as the presence of at least one positive blood culture obtained in a patient with signs and symptoms of infection. SABSI-UTS was diagnosed in patients with SABSI and the presence of urinary tract signs/symptoms, the lack of plausible extra-urinary source of infection, and a urinary culture growing ≥ 105 colony-forming units/mL.
Of 4181 episodes of SABSI, investigators identified 132 (3.16%) cases of SABSI-UTS, similar to estimates of prior studies.8,9 Of patients with SABSI-UTS, 116 (87.9%) were men, median age was 70 years, and median Charlson comorbidity score was 5 points. 104 (78.7%) of infections were classified as being nosocomial or health care-associated while 28 (21.2%) of infections were community-acquired. Indwelling urinary catheters were present in 94 (71.2%) patients and urinary manipulation (e.g., catheter change, urinary surgical intervention) occurred in 85 (64.4%) of patients prior to onset of SABSI, with a median time from manipulation to bacteremia of 9 days.
Susceptibility of S. aureus strains and 30-day patient mortality, defined as death due to any cause within 30 days after onset of bacteremia, were compared between patients with SABSI-UTS versus patients with SABSI from another source. The investigators found that SABSI-UTS was more often caused by methicillin-resistant Staphylococcus aureus (MRSA) as compared to SABSI from a non-urinary tract source (40.9% vs 17.5%, P <.001). 30-day mortality was significantly lower in patients with SABSI-UTS as compared to patients with SABSI from a non-urinary tract source (14.4% vs 23.8%, P = .02). Patients with SABSI-UTI caused by MRSA received adequate empiric antibiotic therapy less frequently as compared to those with methicillin-susceptible strains (40.3% vs 79.6%, P <.001), however there was no significant difference in mortality between groups(18.5% (10 of 54) mortality in MRSA vs. 11.5% (9 of 78) mortality in MSSA, P = .261). A difference in mortality between groups may not have been detected due to small sample size and small number of deaths.
On multivariate analysis, patients who were dependent for activities of daily living (adjusted odds ratio [AOR], 3.9; 95% CI, 1.2-13.8) or had persistent bacteremia, defined as bacteremia duration ≥3 days after appropriate antimicrobial therapy, (AOR, 7.9; 95% CI,1.6-39.5) had increased odds of 30-day mortality, after adjusting for age > 70 years and Charlson comorbidity score >5 points.
The findings from this study have important implications for patient care. Although rare, clinicians should recognize that S. aureus blood stream infection can be directly attributable to the urinary tract. Recent urinary catheterization and/or urinary tract manipulation can be risk factors for development of S. aureus urinary tract infection and subsequent blood stream infection. As MRSA was the cause in 40% of SABI-UTI, clinicians should consider empiric MRSA coverage for patients with possible S. aureus blood stream infection from the urinary tract. Finally, this study highlights another reason for the importance of reducing use of urinary catheterization and manipulation to only individuals with a clear indication for this.
Polly van den Berg, MD is currently a second year infectious diseases fellow at Beth Israel Deaconess Medical Center in Boston, MA. Her clinical interests include infection control and antimicrobial stewardship.
Highlighted Study: Grillo S, Cuervo G, Carratalà J, et al. Characteristics and Outcomes of Staphylococcus aureus Bloodstream Infection Originating From the Urinary Tract: A Multicenter Cohort Study. Open Forum Infect Dis. 2020;7(7):ofaa216.