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ABSTRACT
Background: Staphylococcus saprophyticus is a common cause of urinary tract infection. Resistance to some of the agents commonly used to treat infections has been found, including resistance to trimethoprim, amoxicillin and meth/ flucloxacillin. However, true resistance prevalence may be under estimated as susceptibility testing is not recommended by the Clinical and Laboratory Standards Institute. In addition, testing and determination of resistance to β-lactams has not been clearly defined and in-house testing has revealed discrepancies between phenotypic methods for the detection of β-lactamase.The study objective was to compare Phoenix nitrocefin penicillinase detection, BBL chromogenic cefinase disk, Phoenix penicillin MIC, and ampicillin 2µg disk diffusion for the detection of β-lactam resistance in S. saprophyticus.
Methods: 111 consecutive S. saprophyticus isolates, collected between May to August 2013 at Canterbury Health Laboratories, New Zealand, were routinely analysed in BD Phoenix, using PMIC/ID-78 panels. Isolate identification was confirmed with MALDI-TOF and novobiocin disk diffusion. All study isolates were then repeated in Phoenix, using AST panel type PMIC-84. Chromogenic cefinase was performed using BBL discs, and ampicillin 2µg disk diffusion was performed according to EUCAST. On three isolates determined to be penicillinase hyper-producers, MICs to penicillin and ampicillin were performed by E-test, and an in-house PCR was used to determine blaZ and mecA status.
Results: 64/111 (57.7%) isolates were classified as β-lactamase positive by Phoenix nitrocefin, with only 28 of these isolates also positive by cefinase disk test. Three isolates were cefinase positive/Phoenix nitrocefin negative. 44/111 isolates were negative by both methods. Evaluation of Phoenix penicillin MIC values revealed three isolates with elevated MICs (≥2mg/L) and by E-test (≥1mg/L). Each of these 3 isolates had ampicillin disk diffusion zones of 14-15mm, which is below the EUCAST epidemiology cut-off limit of <18mm, and ampicillin MICs of 0.5mg/L. In addition, all three isolates were blaZ PCR positive but mecA negative. Resistance to other antimicrobial agents among all of the study isolates was found in trimethoprim (3.6%), trimethoprim/sulfamethoxazole (2.7%), tetracycline (12.6%) and erythromycin (17.1%). All isolates were susceptible to amoxicillin-clavulanate, ciprofloxacin, nitrofurantoin, and vancomycin.
Conclusions: Phoenix nitrocefin test classified 57.7% of S. saprophyticus isolates as β-lactamase positive, which was significantly more than the 27.9% positive by cefinase disk test. Whereas only three of these isolates showed penicillinase hyper-production, with penicillin MIC...