Staphylococcus saprophyticus: Which beta-lactam?

BACKGROUND: Staphylococcus saprophyticus is resistant to the drugs most often used for the empirical treatment of urinary tract infections (UTI). The adequacy of antimicrobial treatments prescribed for UTI due to S. saprophyticus is not usually questioned. This study described the epidemiology of such infections and assessed the susceptibility of S. saprophyticus to ceftriaxone and amoxicillin-clavulanic acid.

METHODS: Methicillin-susceptible S. saprophyticus (MSSS) isolated from clinical samples between November 2014 and July 2016 were included. Clinical data were recorded. The minimum inhibitory concentrations (MICs) of amoxicillin-clavulanic acid and ceftriaxone were measured for these MSSS strains and for 17 randomly selected methicillin-susceptible Staphylococcus aureus (MSSA) strains.

RESULTS: Of the S. saprophyticus isolates from urine, 59.5% were associated with a diagnosis of cystitis and 33.3% with pyelonephritis. Sixty percent of S. saprophyticus cystitis cases and 25% of pyelonephritis cases were given an inappropriate antibiotic regimen. The MICs of ceftriaxone ranged from 4 to >32μg/ml for MSSS, and from 1.5 to 4μg/ml for MSSA.

CONCLUSIONS: Many UTIs were treated with an empirical antibiotic therapy that was ineffective for S. saprophyticus, revealing that S. saprophyticus is an aetiology that is insufficiently considered in UTI. High MICs for ceftriaxone in MSSS were observed, which raises questions about the use of this antibiotic in UTIs due to S. saprophyticus.