Revised approach for identification and detection of ampicillin and vancomycin resistance in Enterococcus species by using MicroScan panels

Abstract

The frequency of antimicrobial agent-resistant enterococci is increasing, making accurate identification and screening for susceptibility essential. We evaluated the ability of MicroScan Positive Breakpoint Combo Type 6 panels (Dade MicroScan Inc., West Sacramento, Calif.) to identify Enterococcus species and to detect ampicillin and vancomycin resistance. A total of 398 well-characterized Enterococcus isolates from two institutions were inoculated into MicroScan panels, into conventional biochemical assays, and into ampicillin and vancomycin agar dilution media. Resistance was verified by the broth macrodilution method. MicroScan panels accurately detected resistance to ampicillin in 132 of 132 enterococcal isolates, while three isolates for which the MICs were <16 g/ml were classified incorrectly by MicroScan panels as resistant. No beta-lactamaseproducing enterococci were detected. All 64 isolates showing resistance to vancomycin (MICs >32 g/ml) were correctly classified by MicroScan panels. Seven isolates for which the vancomycin MICs were 8 and 16 g/ml were incorrectly classified as susceptible by MicroScan panels, while eight isolates for which the MICs were 4 g/ml were incorrectly labeled as intermediate. Fourteen of these 15 isolates were subsequently identified as motile enterococci. Overall, there were three major errors in susceptibility testing for ampicillin and 15 minor errors for vancomycin. Conventional testing confirmed the identity of 181 Enterococcus faecalis isolates, 157 E. faecium isolates, and 60 isolates of other species; however, 56 of these 60 isolates were misidentified by the MicroScan panels. After recognition of this problem, a reviased approach which included tests for pigment, motility, and sucrose fermentation was devised. In combination with these additional assays, the conventional MicroScan panels accurately identified the 56 originally misidentified isolates. In summary, the ability of MicroScan panels to detect vancomycin and ampicillin resistance in enterococci was confirmed. Our study found that the inability of MicroScan panels to identify enterococci other than E. faecalis and E. faecium can be compensated for by the addition of standard assays.

Change in prevalence and antibiotic resistance of Enterococcus species isolated from blood cultures over an 8-year period

“…To determine whether a shift in the prevalence of Enterococcus species was associated with an increase in antimicrobial resistance, we reviewed all enterococcal isolates from cultures of blood collected from January 1988 through December 1995. Isolates were identified by conventional biochemical assays and assessed for susceptibilities to ampicillin, vancomycin, and high levels of gentamicin and streptomycin by using an agar dilution procedure according to the recommendations of the National Committee for Clinical Laboratory Standards (NCCLS) (3,5,6). Resistance to ampicillin and to vancomycin was verified by the NCCLS broth macrodilution method.…”

Comparison of Simple and Rapid Methods for Identifying Enterococci Intrinsically Resistant to Vancomycin

“…Since occasional isolates of both E. gallinarum and E. casseliflavus that harbor the transmissible vanA gene have been identified (6), however, determination of the level of vancomycin resistance in clinically significant isolates of IVRE may still be warranted. The ability to accurately differentiate IVRE from other VRE, especially E. faecalis and E. faecium, is nonetheless of considerable importance and, unfortunately, has proven to be somewhat problematic for commercial biochemical identification systems (9,11). A number of tests for rapidly and inexpensively identifying IVRE, including motility (7,15), pigment production (7), acidification of methyl-␣-D-glucopyranoside (MGP) (3,5), and susceptibility to efrotomycin (3), have been described in the literature.…”

“…Motility and yellow-pigment production have been the most widely used rapid tests for differentiating IVRE from E. faecalis or E. faecium, and E. gallinarum from E. casseliflavus, and are typically used as supplemental tests when commercial biochemical test systems are employed for enterococcal identification (9,11). The accuracy of these methods has, however, varied considerably in published studies (2,3,9,11,15), hence the interest in alternative approaches to differentiating these organisms. We developed and evaluated an RM assay that, by decreasing the agar concentration by 50%, enables the detec-tion of motility in only 4 h. Only one isolate of the motile enterococcal species examined did not demonstrate motility in our study (in either the RM or conventional motility tests), giving this test the same specificity as MGP acidification.…”

Evaluation of the Revised MicroScan Dried Overnight Gram-Positive Identification Panel To Identify Enterococcus Species

“…The enterococci have emerged as the third most common cause of nosocomial bacteremia (14,16,27). This increase in infections is due in part to an escalation of resistance to antimicrobial therapy, including high-level resistance to aminoglycosides and vancomycin (3,9,13,23,28). Resistance is most frequently associated with Enterococcus faecium, even though other species conferring resistance are appearing (3,15,22,28).…”

“…The (11) was used as a basis for species identification with modifications as outlined in a previous study (13). The reagent for acidification of 1% methyl-␣-D-glucopyranoside (MGP) was prepared and evaluated by the procedure of Devriese et al (8).…”