2176. A New Rapid Test for Detection of The Cefazolin Inoculum Effect (CIE) in Methicillin-Susceptible Staphylococcus aureus (MSSA)

BACKGROUND: Most MSSA harbor one of the four different variants of β-lactamase (BlaZ) (A, B, C and D). The CIE is defined as an MIC >16 μg/mL when a high inoculum (10(7) CFU/mL) is used and depends on the presence of BlaZ. The presence of the CIE has been associated with therapeutic failure in invasive MSSA infections. In some countries of South America, the prevalence of CIE is high, ranging from 36% to 51% (Colombia and Argentina, respectively). Type A BlaZ is most often associated with the CIE due to its high affinity for cefazolin. Here, we developed a rapid test based on the premise that the extracellular form of BlaZ is responsible for the CIE. We aimed to identify invasive MSSA that exhibit the CIE and validate the test in two cohorts of isolates from patients in Colombia and Argentina METHODS: 152 MSSA clinical isolates were collected from Colombia (n = 71) and Argentina (n = 81). We determined MIC at standard and high inoculum. We developed a test using induction of BlaZ with ampicillin (150 μg/mL) for 20 minutes and, using the supernatant for incubation with nitrocefin for 30 min. A change in color from yellow to red was considered positive. MSSA TX0117 (BlaZ +, with the CIE), ATCC 29213 (BlaZ-negative) and ATCC 25923 (BlaZ + lacking the CIE) were used as controls. BlaZ typing of all Argentinian isolates was available by sequencing RESULTS: A high proportion (43%) of MSSA exhibited the CIE (34% and 52% of Colombian and Argentinian isolates, respectively) by MIC. The rapid test identified 76% of isolates exhibiting the CIE and correctly ruled out all isolates lacking the CIE (sensitivity 80%, specificity 100%). Furthermore, the rapid test detected all isolates with the CIE that harbored Type A BlaZ from Argentina. Conversely, the test failed to identify the CIE in Argentinian isolates that produce type B and C BlaZ. The sensitivity and specificity of the rapid test for the Colombian isolates whose BlaZ type was unknown were 89% and 100%, respectively. CONCLUSION: A rapid test of less than 2 h can readily identify MSSA isolates exhibiting the CIE. For isolates carrying type A BlaZ, which is highly associated with the CIE, the test had a sensitivity and specificity of 100%. Rapid identification of MSSA with the CIE may have important therapeutic consequences in deep-seated infections DISCLOSURES: All authors: No reported disclosures.