Evaluation of CHROMagar™ LIN-R for the Screening of Linezolid Resistant Staphylococci from Positive Blood Cultures and Nasal Swab Screening Samples

Abstract

The increasing number of nosocomial pathogens with resistances towards last resort antibiotics, like linezolid for gram positive bacteria, leads to a pressing need for screening and, consequently, suitable screening media. Some national guidelines on infection prevention (e.g., in Germany) have already recommended screening for linezolid-resistant bacteria, despite an accurate screening medium that was not available yet. In this study, we analyzed the performance and reliability of the first commercial chromogenic medium, CHOMagar™ LIN-R, for screening of linezolid-resistant gram-positive isolates. Thirty-four pure bacterial cultures, 18 positive blood cultures, and 358 nasal swab screening samples were tested. This medium efficiently detected linezolid-resistant S. epidermidis isolates from pure bacterial cultures and from positive blood cultures with a high sensitivity (100%) and specificity (100%). Among the 358 nasal swab screening samples prospectively tested, 10.9% were cultured with linezolid-resistant isolates (mostly S. epidermidis). Of note, slight growth was observed for 7.5% samples with linezolid-susceptible isolates of S. epidermidis (n = 1), S. aureus (n = 1), Enterococcus faecalis (n = 4), Lactobacillus spp. (n = 3), gram negatives (n = 18). Moreover, few Candida spp. also cultured on this medium (1.4%).

Nasotracheal enterococcal carriage and resistomes: detection of optrA-, poxtA- and cfrD-carrying strains in migratory birds, livestock, pets, and in-contact humans in Spain

“…It is of interest to remark that all the linezolid resistant enterococci were recovered in the ChromAgar LIN agar plates in which isolates were grown as green colonies. Nevertheless, linezolid susceptible isolates were also recovered in this media, as also indicated by other authors [24].…”

“…After overnight incubation, different dilutions of the broth culture were carefully dispensed onto blood agar (BioMerieux) and ChromAgar LIN (CHROMagar™ LIN, Paris, France) plates and incubated for 24 h at 37 °C, for enterococci recovery. The CHROMAgar™ medium has been previously shown to have high sensitivity and specificity on pure linezolid resistant enterococci and staphylococci isolates [ 24 ]. After overnight growth, 2 to 5 different colonies per sample with the morphology of enterococci were randomly selected and identified by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF; Bruker Daltonics, Bremen, Germany) using the standard extraction protocol recommended by Bruker.…”

CHROMAgar™ LIN-R as an efficient screening tool to assess the prevalence of linezolid-resistant enterococci in German hospital patients—a multicentre study approach, 2021–2022

“…Growth of Candida spp., albeit to a lower extent, has also been observed by a recent study assessing the performance of CHROMAgar™ LIN-R from blood cultures and nasal swab screening samples. 22 Although the technical note of CHROMAgar™ LIN-R states that growth of Gram-negative bacteria and yeast is inhibited ( https://www.chromagar.com/wp-content/uploads/2021/11/NT_EXT_119_V1.0-1.pdf ), residual contamination is obviously not avoided. However, LRS and LRE should easily be differentiated due to their typical colony appearance of pink and steel blue colour.…”

“…and Gram-negatives as observed by Girlich et al . 22 In another recent validation report of CHROMAgar™ LIN-R in routine practice, specificity was estimated at 90% due to growth of non-targeted organisms including LSE and linezolid-susceptible staphylococci. 21 Although we would like to stress that comparing our results from rectal samples with those from nasal swabs or referring to accuracy of a test is imperfect, it is known that heavily inoculated samples may cause growth of linezolid-susceptible bacteria (see CHROMAgar™ LIN-R technical note).…”