Poster Presentation Australian Society for Microbiology Annual Scientific Meeting 2013

Ciprofloxacin disc diffusion testing using 2013 CLSI breakpoints for salmonellae can replace the nalidixic acid screen for ciprofloxacin susceptibility (#246)

Annita Palatinus 1 , Sasha Jaksic 2
  1. Microbiology, Dorevitch Pathology, Melbourne, Victoria, Australia
  2. Clinical Microbiologist, Head of Department, Dorevitch Pathology, Melbourne, Victoria, Australia

Background: Salmonellae are important human pathogens and ciprofloxacin (CIP) is common recommended therapy, if indicated. In the past decade poor response to ciprofloxacin has been identified in infections caused by salmonella isolates which demonstrate decreased CIP susceptibility (DSC) with CIP minimum inhibitory concentration (MIC) of 0.12 – 0.5 mg/L. Most isolates with DCS have a mutation in gyrA and are resistant to nalidixic acid (NA). Thus, disc diffusion (DD) NA testing has been used to screen for DCS, as an alternative to performing more expensive MIC determination. The latest 2013 CLSI guidelines have introduced new CIP DD susceptibility breakpoints for salmonellae that identify isolates with DCS. The aim of this study was therefore to determine if the CIP DD test is more reliable than the traditional NA disc diffusion test for detecting DCS in salmonella isolates at Dorevitch Pathology.
Methods: Salmonellae isolated from routine patient cultures in the period November 2012 – April 2013 were tested in duplicate for NA (ug) and CIP (5ug) susceptibility using CLSI DD method. CIP MICs were determined using Etest strips (bioMeriux).
Results: MIC testing identified 134 (90.5%) salmonella isolates as susceptible to CIP (MIC < 0.06 mg/L). Out of 14 (9.5%) isolates with DCS (MIC 0.12 – 0.5 mg/L), 13 were resistant to NA (<13mm). All 14 DCS isolates tested Intermediate to CIP (21-30mm) by DD. Therefore, in our study NA and CIP had sensitivity of 99.05% and 100% respectively for identifying DCS in salmonellae.
Conclusion: NA DD test is still a reliable screen for DCS in salmonella isolates in our laboratory. However, fluoroquinolone resistance mechanisms are emerging which do not affect NA susceptibility and this screen might need to be replaced with alternative testing methods. Using CIP DD for salmonellae with the new CLSI breakpoints correctly identifies isolates with DCS; hence CIP DD is a suitable replacement for the NA screen.