Comparison of agar-based media for primary isolation of glycopeptide-resistant enterococci

Evaluation of a new chromogenic medium, ChromID VRE, for detection of vancomycin-resistant Enterococci in stool samples and rectal swabs

We compared ChromID VRE medium with Enterococcosel containing vancomycin for the detection of vancomycin-resistant Enterococcus in 1,007 specimens. ChromID VRE in combination with Gram straining provided a higher specificity than Enterococcosel, irrespective of the incubation time and enrichment.

[VCA agar (bioMérieux) for selective isolation of vancomycin-resistant Enterococci (VRE) from fecal specimens]

Screening for Vancomycin Resistant Enterococci (VRE) is recommended for preventing nosocomial infections with VRE. The aim of this study was to assess the performance of VCA3 agar (bioMérieux) in VRE isolation from fecal specimens. 220 specimens were cultured on VCA3 agar, which contains vancomycin and in parallel, on CAP agar (Oxoid), which is vancomycin-free. 36 vancomycin resistant enterococci were isolated: 24 isolates of Enterococcus faecium expressed a high-level resistance to vancomycin and 12 isolates of E. gallinarum/casseliflavus exhibited resistance at low-level. The sensitivity of VCA3 appeared greater than that of CAP for VRE isolation: 92% (22/24) vs 79% (19/24) for E. faecium (NS, P>0.05) ; 83% (10/12) vs 50% (6/12) for E. gallinarum/casselliflavus (NS, P>0.05). As expected, initial cultures of multiple gram positive organisms were far more frequent on CAP agar than on VCA3 agar. The isolation rate of vancomycin susceptible gram positive strains was impressively lower on VCA3 medium than on CAP medium. VCA3 agar avoided therefore additional subcultures, useless identification and susceptibility tests. In conclusion, VCA3 medium could be useful for the direct, rapid and selective isolation of VRE from fecal specimens.

Detection of glycopeptide-resistant enterococci in routine diagnostic faeces specimens

Faeces received in a diagnostic laboratory were screened for glycopeptide-resistant enterococci (GRE) on modified Lewisham medium, with and without enrichment in Enterococcosel broth. Colonization by GRE was detected in 102/838 patients (12.2%). In 74 (73%) of colonized patients GRE were detected by both methods and in 28 (27%) they were detected only after enrichment. The carriage rate in hospitalized patients was 32% (93/289) compared with 2.3% (11/425) in the community (GP patients and food-handlers). Carriage of GRE increased with age. Clostridium difficile isolation was associated with GRE colonization, odds ratio 6.76 (P<0.001). Fifty-nine percent (60/102) of the GRE had the VanA phenotype and 41% (42/102) had the VanB phenotype. In the community VanA predominated (91%), whereas 64% (57/89) of the isolates from hospitalised patients were of the VanB phenotype.

Determinants of response to antibiotic therapy

The in vivo situation is far more complex than that of the standard in vitro susceptibility test yet in vitro tests have stood the test of time and are often good predictors of clinical outcome. Nevertheless, our understanding of the pharmacodynamics of antibiotic microbe interaction are giving us new insights into how to improve our performance and interpretation of these tests. These factors include consideration of inoculum effect, antibiotic interactions, cidal effects and the area under the inhibitory time curve (AUIC). There are however other variables which it is difficult to incorporate into in vitro tests, especially the immune status of the patient, which can be crucial to outcome. While the immune system can be boosted in certain instances, e.g. by growth factors or immunoglobulin infusions, our ability to modify the immune response to infection has been frustrated. Understanding the interaction of antibiotics with the immune system and the consequences of the differing actions of the various antibiotic classes on the immune response is another door that is opening for the future.