Performance of the Vitek 2 system software version 5.03 in the bacterial identification and antimicrobial susceptibility test: evaluation study of clinical and reference strains of Gram-positive cocci

Abstract

Introduction. The genera Enterococcus, Staphylococcus and Streptococcus are recognized as important Gram-positive human pathogens. The aim of this study was to evaluate the performance of Vitek 2 in identifying Gram-positive cocci and their antimicrobial susceptibilities. Methods. One hundred four isolates were analyzed to determine the accuracy of the automated system for identifying the bacteria and their susceptibility to oxacillin and vancomycin. Results. The system correctly identifi ed 77.9% and 97.1% of the isolates at the species and genus levels, respectively. Additionally, 81.8% of the Vitek 2 results agreed with the known antimicrobial susceptibility profi les. Conclusion. Vitek 2 correctly identifi ed the commonly isolated strains; however, the limitations of the method may lead to ambiguous fi ndings.Keywords: Gram-positive cocci. Vitek 2 automated system.Gram-positive cocci are widely distributed as part of the normal fl ora in humans; however, some species are recognized as major human pathogens and cause a large variety of infections worldwide. These microorganisms are frequently isolated from bloodstream infections, skin and soft tissue infections, sepsis, urinary tract infections and lower respiratory tract infections 1 .Automated bacterial identifi cation in the clinical laboratory provides a rapid and reliable diagnosis for most pathogens involved in infectious diseases. A previous study demonstrated the satisfactory performances of the automated methodologies, resulting in their use in routine practice with a highly acceptable level of identification accuracy; additionally, automated identification enabled the interpretation of antimicrobial susceptibility tests for the correct treatment of patients 2 .The aim of the present study was to evaluate the performance of the Vitek 2 automated system in the identifi cation of bacteria and antimicrobial susceptibilities of Gram-positive cocci isolates recovered from clinical samples and reference strains.The study was performed at Laboratório de Cocos GramPositivos (LCGP) of the Universidade Federal de Ciências da Saúde de Porto Alegre (UFCSPA) and Laboratório Qualità, Rio Grande do Sul, Brazil. The isolates included in the present study were selected from the strain collection belonging LCGP. A total of 104 isolates of Gram-positive cocci were analyzed, including 29 reference strains selected from the American Type Culture Collection (ATCC) and 75 clinical strains isolated from different patients; these strains included Staphylococcus coagulase-negative (n=36), Enterococcus spp. (n=33) and Staphylococcus aureus (n=6). All of these strains have been previously characterized by the LCGP with regard to their virulence factors using molecular methods and susceptibility profi les and were identifi ed at the species level using conventional reference methods 3,4 . For the identifi cation of staphylococci, the following characteristics were tested: catalase; colony morphology and pigmentation; Gram stain; hemolysis; susceptibility to novobiocin; p...

Antimicrobial susceptibility profile and research of mec A and erm genes in coagulase-negative staphylococci isolated from platelet concentrates bags

“…Studies by other authors confirm our findings when these two methods are compared (VITEK® 2 and DD test -considered reference methods), reporting the occurrence of much higher error (VME), higher error (ME), and lowest error (MiE) with the VME prevalence profile (Sapino et al, 2012;Paim, Cantarelli, D'Azevedo, 2014), in susceptibility assessed by the automated method.…”

Comparison of methods for the identification of microorganisms isolated from blood cultures

“…Agreement was 91.7 % (11/12) for the genus Enterococcus , similar to the rates reported by Nakasone et al [ 25 ] and Jin et al [ 37 ]. Lower efficiencies of 77.8, 83.1, 87.5, 72 and 77.8 % correct identifications of strains of this genus have been observed by Ligozzi et al [ 33 ], d’Azevedo et al [ 38 ], Moore et al [ 39 ] and Paim et al [ 42 ], respectively. In the case of Staphylococcus , a higher rate of incorrect identifications was observed for CoNS isolates (91.9 %), in agreement with Ligozzi et al [ 33 ] (86 %), Funke and Funke-Kissling [ 34 ] (93.7 %), Kim et al [ 40 ] (87.5 %), Delmas et al [ 41 ] (78.8 %), and Paim et al [ 42 ] (72.9 %).…”

“…Although classical identification methods are still considered the gold standard, these methods are slow, time consuming and prone to subjective interpretations. On the other hand, the VITEK ® 2 system reduces the time necessary for identification and permits the standardization of inter- and intra-laboratory results, the storage of results, issuing rapid epidemiological reports, and simultaneous identification and antimicrobial susceptibility testing; however, the system is poorly efficient in identifying certain species of Gram-positive cocci [ 42 ].…”

Factors Affecting Biodegradation of the Organophosphorus Insecticide Diazinon by Bacterial Mono-Culture of Bacillus Sefensis 7, Isolated From the Rhizosphere of Date Palm Tree

“…The efficiency of this system to correctly identify Staphylococcus spp. and other bacterial genera has been proven previously (Chatzigeorgiou et al, 2011;Paim et al, 2014). Nonetheless, discordance between Vitek II and 16S rRNA gene sequencing for bacterial identification has been reported.…”