Performance of Phenotypic Tests to detect 1 Penicillinase in Australian Staphylococcus aureus 2 Isolates

17 Recent studies have shown that chromogenic cephalosporin tests are inferior to disc zone edge tests in 18 detecting penicillinase in Staphylococcus aureus isolates resulting in a change to CLSI and EUCAST 19 guidelines in 2012. We sought to confirm these findings using Australian isolates and compare the 20 performance of the CLSI and EUCAST methods which use different disc strengths, penicillin 10 unit 21 (P10) and 1 unit (P1) respectively. Using blaZ PCR as the reference standard the sensitivities of the 22 tests were as follows: Cefinase Disc test 24/38 (63%); P10 disc zone edge test: 34/38 (89%); P10 disc 23 diameter: 25/38 (66%); P1 disc zone edge: 38/38 (100%); P1 disc diameter: 38/38 (100%). We also 24 found that reading of the P10 disc zone edge test was interpreted differently by the clinical laboratory 25 and the study investigators in 11% of instances. Our findings support previous studies showing 26 chromogenic cephalosporin based β-lactamase tests are inferior to disc methods in detecting S. aureus 27 penicillinase. We also conclude that the EUCAST method using the P1 disc has the best performance, 28 particularly because reading of the P1 disc zone diameter closely correlated with penicillinase 29 production and is a less subjective test than zone edge reading. 30 on N ovem er 3, 2017 by gest ht://jcm .sm .rg/ D ow nladed fom