Evaluation of new Vitek 2 card and disk diffusion method for determining susceptibility of Staphylococcus aureus to oxacillin

Comparative Susceptibility of Pathogenic Methicillin-Resistant and Methicillin-Susceptible Staphylococcus pseudintermedius to Empirical Cotrimazole for Canine Pyoderma

The prevalence of methicillin-resistant Staphylococcus pseudintermedius (MRSP) that causes pyoderma has been gradually shifting, according to many surveillance studies, with annual changes. The empirical cotrimazole regimen remains interesting, but research on cotrimazole susceptibility to MRSP is limited. The objective of this study was to evaluate the susceptibility of cotrimazole to canine pyoderma MRSP isolates. Sixty isolates of S. pseudintermedius were identified as 16 MRSP and 44 methicillin-susceptible S. pseudintermedius (MSSP) using an oxacillin disk diffusion test and VITEK 2 system with VITEK GP card. Using the VITEK 2 system with a VITEK AST-GP81 card, the susceptibility rates of MRSP (15.00%) and MSSP (35.00%) to cotrimazole was observed. The median MIC of cotrimazole on MSSP (median, ≤10; IQR, 10-320) was lower than that of MRSP (median, ≥320; IQR, 10-320) (p = 0.5889, Mann-Whitney test). Percent attainment of PK/PD targets in MRSP (q 12 h, 43.75; q 8 h, 43.75) were lower than that of MSSP (q 12 h, 52.27; q 8 h, 52.27) (p = 0.7710). These findings show the moderately phenotypic cotrimazole susceptibilities of both MRSP and MSSP. Further study is required to develop clinical trials examining the use of cotrimazole in dogs with pyoderma.

Multi-Drug Resistant Staphylococcus aureus Carriage in Abattoir Workers in Busia, Kenya

Abattoir workers have been identified as high-risk for livestock-associated Staphylococcus aureus carriage. This study investigated S. aureus carriage in abattoir workers in Western Kenya. Nasal swabs were collected once from participants between February-November 2012. S. aureus was isolated using bacterial culture and antibiotic susceptibility testing performed using the VITEK 2 instrument and disc diffusion methods. Isolates underwent whole genome sequencing and Multi Locus Sequence Types were derived from these data. S. aureus (n = 126) was isolated from 118/737 (16.0%) participants. Carriage was higher in HIV-positive (24/89, 27.0%) than HIV−negative participants (94/648, 14.5%; p = 0.003). There were 23 sequence types (STs) identified, and half of the isolates were ST152 (34.1%) or ST8 (15.1%). Many isolates carried the Panton-Valentine leucocidin toxin gene (42.9%). Only three isolates were methicillin resistant S. aureus (MRSA) (3/126, 2.4%) and the prevalence of MRSA carriage was 0.4% (3/737). All MRSA were ST88. Isolates from HIV-positive participants (37.0%) were more frequently resistant to sulfamethoxazole/trimethoprim compared to isolates from HIV-negative participants (6.1%; p < 0.001). Similarly, trimethoprim resistance genes were more frequently detected in isolates from HIV-positive (81.5%) compared to HIV-negative participants (60.6%; p = 0.044). S. aureus in abattoir workers were representative of major sequence types in Africa, with a high proportion being toxigenic isolates. HIV-positive individuals were more frequently colonized by antimicrobial resistant S. aureus which may be explained by prophylactic antimicrobial use.

Comparison of PCR and phenotypic methods for the detection of methicillin resistant Staphylococcus aureus

Background and Objectives:

Resistance to methicillin in methicillin resistant strains of Staphylococcus aureus (MRSA) is due to the presence of mec-A gene, which encodes a low affinity penicillin binding protein (PBP)-2a or PBP2. Accurate and rapid identification of MRSA in clinical specimens is essential for timely decision on effective treatment. The aim of the study was to compare three different methods for detection of MRSA namely cefoxitin disc diffusion, CHROM agar MRSA and VITEK-2 susceptibility with PCR which is the gold standard reference method and to find the antibiotic susceptibility pattern of these isolates by VITEK-2.

Materials and Methods:

A Total of 100 non-duplicate S. aureus isolates were collected from different clinical samples among both outpatient and inpatients. Detection of MRSA among these isolates was done by cefoxitin disc diffusion, VITEK-2, CHROM agar MRSA and PCR.

Results:

The sensitivity and specificity of cefoxitin disc diffusion and Vitek was found to be 97.2% and 100%, while that of CHROM agar was found to be 100% and 78.6%. The overall prevalence of MRSA in our study by PCR was 72%.

Conclusion:

Based on the findings in our study, isolates which show cefoxitin zone diameter < 22 mm can be reported as MRSA. However, those isolates which have a zone diameter between 22–24 mm, should ideally be confirmed by PCR.

Identification and characterization of mutations responsible for the β-lactam resistance in oxacillin-susceptible mecA-positive Staphylococcus aureus

Staphylococcus aureus strains that are susceptible to the β-lactam antibiotic oxacillin despite carrying mecA (OS-MRSA) cause serious clinical problems globally because of their ability to easily acquire β-lactam resistance. Understanding the genetic mechanism(s) of acquisition of the resistance is therefore crucial for infection control management. For this purpose, a whole-genome sequencing-based analysis was performed using 43 clinical OS-MRSA strains and 100 mutants with reduced susceptibility to oxacillin (MICs 1.0–256 µg/mL) generated from 26 representative OS-MRSA strains. Genome comparison between the mutants and their respective parent strains identified a total of 141 mutations in 46 genes and 8 intergenic regions. Among them, the mutations are frequently found in genes related to RNA polymerase (rpoBC), purine biosynthesis (guaA, prs, hprT), (p)ppGpp synthesis (rel_Sau), glycolysis (pykA, fbaA, fruB), protein quality control (clpXP, ftsH), and tRNA synthase (lysS, gltX), whereas no mutations existed in mec and bla operons. Whole-genome transcriptional profile of the resistant mutants demonstrated that expression of genes associated with purine biosynthesis, protein quality control, and tRNA synthesis were significantly inhibited similar to the massive transcription downregulation seen in S. aureus during the stringent response, while the levels of mecA expression and PBP2a production were varied. We conclude that a combination effect of mecA upregulation and stringent-like response may play an important role in acquisition of β-lactam resistance in OS-MRSA.

Aptamer-based fluorometric assay for direct identification of methicillin-resistant Staphylococcus aureus from clinical samples

Accurate and rapid identification of methicillin-resistant Staphylococcus aureus (MRSA) is of important clinical significance. In this study, a novel aptamer-based fluorometric assay was developed for detection of MRSA in clinical samples by coupling with immunomagnetic separation. The S. aureus cells in clinical specimens were enriched by magnetic separation. Following lysis by staphylococcal lysin, the PBP2a proteins were released from S. aureus cells and detected by the aptamer-based fluorometric assay. Without lengthy period of bacteria cultivation in the traditional susceptibility testing, this test has an overall testing time of only 2 h with the detection limit of 2.63 × 10³ and 1.38 × 10³ CFU/mL in PBS and spiked nasal swab, respectively. Since it is simple, rapid and sensitive, this method could be used for the detection of MRSA in various clinical samples.

Characterisation of clinical isolates of oxacillin-susceptible mecA-positive Staphylococcus aureus in China from 2009 to 2014

OBJECTIVES

The aim of this study was to investigate the prevalence and molecular characteristics of clinical oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) isolates in China from July 2009 to June 2014.

METHODS

A total of 2068 non-duplicate S. aureus isolates were collected from 21 hospitals. Antimicrobial susceptibility testing was performed by the agar dilution method. All OS-MRSA strains were screened for the presence of the genes mecA, mecC and nuc as well as the Panton-Valentine leukocidin gene (pvl). Staphylococcal cassette chromosome mec (SCCmec) typing, staphylococcal protein A (spa) typing and multilocus sequence typing (MLST) were performed to analyse the isolate genotypes.

RESULTS

A total of 34 S. aureus isolates were mecA-positive but were susceptible to oxacillin [minimum inhibitory concentration (MIC)≤2mg/L]. All OS-MRSA isolates were resistant to cefoxitin and most were also multiresistant to other antibiotics besides β-lactams. Among the 34 OS-MRSA isolates, nine spa and three SCCmec types were detected and, combined with MLST, ST338/59-t437-SCCmecV (47%; 16/34) was the predominant clone. In addition, 17 strains (50%) carried the pvl gene.

CONCLUSIONS

The most frequent clone of OS-MRSA isolates in China was ST338-t437-SCCmecV. Most of the OS-MRSA isolates were susceptible to the majority of antibacterial agents except macrolides, clindamycin and chloramphenicol.

Disk Diffusion Testing for Detection of Methicillin-Resistant Staphylococci: Does Moxalactam Improve upon Cefoxitin?

Disk diffusion testing is widely used to detect methicillin resistance in staphylococci, and cefoxitin is currently considered the best marker for mecA-mediated methicillin resistance. In low-inoculum diffusion testing (colony suspension at 10⁶ CFU/ml), the addition of moxalactam in combination with cefoxitin has been reported to improve on cefoxitin alone for the detection of methicillin-heteroresistant staphylococci. However, moxalactam is absent from EUCAST and CLSI guidelines, which use high-inoculum diffusion testing (colony suspension at 10⁸ CFU/ml), calling into question the potential interest of including moxalactam in their recommendations. The inhibition zone diameters of cefoxitin and moxalactam, alone and in combination, were evaluated for concordance with mecA and mecC positivity in a large collection of clinical Staphylococcus isolates (611 Staphylococcus aureus, Staphylococcus lugdunensis, and Staphylococcus saprophyticus isolates and 307 coagulase-negative staphylococci other than S. lugdunensis and S. saprophyticus isolates, of which 22% and 53% were mecA-positive, respectively) and in 25 mecC-positive S. aureus isolates using high-inoculum diffusion testing. Receiver operating characteristic, sensitivity, and specificity analyses indicated that the detection of mecA- and mecC-positive and negative isolates did not improve with moxalactam, either alone or in combination with cefoxitin, compared to cefoxitin alone. These findings were similar in both the S. aureus/S. lugdunensis/S. saprophyticus group and in the coagulase-negative staphylococci group. Our results do not support the use of moxalactam as an additional marker of methicillin resistance when testing with high-inoculum disk diffusion.

The use of molecular methods for the detection and identification of methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major pathogen in many hospitals and long-term care facilities as well as in the community. To limit the spread of MRSA, early detection and proper treatment are essential. Because conventional culture as gold standard is time consuming, new techniques such as PCR-based and hybridization assays have emerged for the rapid detection of MRSA. This review will focus on the currently available molecular-based assays and on their utility and performance for detection of S. aureus, of its virulence factors and of the markers for acquired resistance.

mecA-positive methicillin-sensitive Staphylococcus aureus clinical isolates in Zenica-Doboj Canton, Bosnia and Herzegovina

Forty-four mecA-positive and eight mecA-negative Staphylococcus aureus isolates confirmed by PCR were further tested by disc-diffusion (DD) oxacillin and cefoxitin, oxacillin Epsilon (E)-test, and oxacillin and cefoxitin minimal inhibitory concentration (MIC) Strip methicillin-resistant phenotype in S. aureus (MRSA) tests. Among 44 mecA-positive S. aureus isolates, two (4·5%) were detected as MRSA by DD-oxacillin, 17 (38·6%) by DD-cefoxitin test, and seven (15·9%) by the E-test. In the cefoxitin MIC Strip MRSA test, 19 (43·2%) isolates were resistant. In the oxacillin MIC Strip MRSA test, 18 (40·9%) isolates were resistant and 26 (59·1%) were sensitive, i.e. oxacillin-sensitive MRSA (OS-MRSA) (MIC range 0·25-≤0·25 mg/l). Fifteen out of 26 OS-MRSA (57·7%) belonged to spa-CC 355/595, 78% of which belonged to the largest PFGE clone. Some discrepancies between the phenotypic methods for MRSA identification obtained in this study were caused by large proportion of OS-MRSA. Misidentification of OS-MRSA as MSSA might result in an appearance of highly resistant MRSA in patients treated with beta-lactam antibiotics.

Prevalence and molecular typing of oxacillin-susceptible mecA-positive Staphylococcus aureus from multiple hospitals in China

Among 1588 non-duplicated Staphylococcus aureus isolates from 10 cities in China, 60 isolates were susceptible to oxacillin (MIC50: 1 μg/mL; MIC90: 2 μg/mL) but were mecA-positive. Twenty-one spa and 5 staphylococcal cassette chromosome mec (SCCmec) types were detected, and combined with multilocus sequence typing method, ST59-t437-SCCmecIV/V was the predominant clone (26.7%, 16/60).

Community-acquired methicillin-resistant Staphylococcus aureus clones circulating in Belgium from 2005 to 2009: changing epidemiology

The present study reports the evolution of the demographic characteristics and the molecular epidemiology of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) in Belgium from 2005 to 2009. Four hundred and ten CA-MRSA isolates were prospectively collected and screened for the presence of Panton-Valentin leucocidin (PVL) and toxic shock syndrome toxin 1 (TSST-1) encoding genes, while clinical information were recorded. PVL- and TSST-1-positive isolates were genotyped by pulsed-field gel electrophoresis (PFGE). Staphylococcal cassette chromosome mec (SCCmec) type, spa type and multilocus sequence type (MLST) were determined on representative isolates. One hundred and fifty-nine (39 %) isolates were PVL-positive. PVL-positive isolates were significantly more frequently isolated from skin or soft tissue than PVL-negative isolates, causing mainly subcutaneous abscesses and furuncles. Patients with PVL-positive CA-MRSA were significantly younger than patients with PVL-negative CA-MRSA. Eighty-seven percent of the PVL-positive isolates belonged to a limited number (n = 7) of PFGE types belonging to sequence types (ST) ST80, ST8, ST30, ST5, ST152, ST338 and a new ST, a single-locus variant of ST1. A temporal evolution of the distribution of these PFGE types was observed, characterised by (1) the dissemination of the ST8-SCCmecIV arcA-positive (USA300) genotype and (2) a genetic diversification. Forty-seven (11 %) strains were TSST-1-positive, of which 65 % clustered into four PFGE types, all belonging to ST5. The epidemiology of CA-MRSA in Belgium is changing, as the rapid diffusion of the USA300 clone seems to occur, together with a clonal diversification.

Evaluation of Clearview Exact PBP2a, a new immunochromatographic assay, for detection of low-level methicillin-resistant Staphylococcus aureus (LL-MRSA)

We evaluated the performance of a new immunochromatographic assay (ICA), the Clearview Exact PBP2a, for rapid detection of penicillin-binding protein 2a (PBP2a) in a challenge set of Staphylococcus aureus strains showing MICs to oxacillin of ≤16 mg/liter. The sensitivity and specificity of the ICA were 96.6% and 100%, respectively.

mecA-positive Staphylococcus aureus with low-level oxacillin MIC in Taiwan

Although the presence of mecA is the genotypic determinant of methicillin-resistant Staphylococcus aureus (MRSA), certain MRSA strains, especially community-associated MRSA (C-MRSA), can display an oxacillin MIC in the Clinical and Laboratory Standards Institute susceptible breakpoint range (≤ 2 μg/ml). Among 91 and 180 isolates thought to be methicillin-susceptible S. aureus (MSSA) with oxacillin MICs of 2 and 1 μg/ml as determined by the Sensititre broth microdilution test initially, 52 (57.1%) and 6 (3.3%), respectively, were mecA positive. These mecA-positive low-oxacillin-MIC isolates belong to the dominant Taiwan C-MRSA clone (clonal complex [CC] 59), 56 of which carried SCCmec type V and were pvl positive, and 43 of which belonged to spa CC t437. All 271 isolates were retested by Sensititre, as well as by Vitek II and disk diffusion (DD). Based on the oxacillin results, the sensitivities of the Sensititre, Vitek II, and DD methods were 48.3% (28/58), 46.6% (27/58), and 89.6% (52/58), respectively. Although cefoxitin was better at detecting these isolates, 12.1, 10.4, and 5.2% of these isolates were still misidentified as MSSA by Sensititre, Vitek II, and DD, respectively. These results highlight the difficulty in the accurate identification of MRSA with borderline oxacillin MICs in the CC59:SCCmec V clone, which likely has contributed to its spread in the health care and community settings. Since this clone has now been detected in other countries, and since other C-MRSA lineages have also been found to have low-level β-lactam resistance, the findings of the present study may be relevant to other regions. Further studies are warranted to determine the extent and clinical impact of such misidentification.

Methicillin-resistant Staphylococcus aureus expressing low-level methicillin resistance may not be detected by the VITEK2® system

Low-level methicillin-resistant Staphylococcus aureus may be difficult to detect with the VITEK® 2 system (VK2). Here, we suggest that S. aureus exhibiting VK2-oxacillin MIC of 1 or 2 mg/L and a negative cefoxitin screen should be tested for the presence of mecA or its gene product.

Natural biopolymer for preservation of microorganisms during sampling and storage

Stability of microbial cultures during sampling and storage is a vital issue in various fields of medicine, biotechnology, food science, and forensics. We have developed a unique bacterial preservation process involving a non-toxic, water-soluble acacia gum polymer that eliminates the need for refrigerated storage of samples. The main goal of this study is to characterize the efficacy of acacia gum polymer for preservation of pathogenic bacteria (Bacillus anthracis and methicillin-resistant Staphylococcus aureus-MRSA) on different materials, used for swabbing and filtration: cotton, wool, polyester, rayon, charcoal cloth, and Whatman paper. Acacia gum polymer used for preservation of two pathogens has been shown to significantly protect bacteria during dehydration and storage in all tested samples at the range of temperatures (5-45°C for MRSA and 40-90°C for B. anthracis). Our results showed higher recovery as well as higher viability during the storage of both bacteria in all materials with acacia gum. Addition of acacia gum polymer to swabbing materials or filters will increase efficacy of sample collection and identification of pathogenic bacteria from locations such as hospitals or the environment. Proposed approach can also be used for long-term storage of culture collections, since acacia gum contributes to viability and stability of bacterial cultures.

Expert systems in clinical microbiology

This review aims to discuss expert systems in general and how they may be used in medicine as a whole and clinical microbiology in particular (with the aid of interpretive reading). It considers rule-based systems, pattern-based systems, and data mining and introduces neural nets. A variety of noncommercial systems is described, and the central role played by the EUCAST is stressed. The need for expert rules in the environment of reset EUCAST breakpoints is also questioned. Commercial automated systems with on-board expert systems are considered, with emphasis being placed on the "big three": Vitek 2, BD Phoenix, and MicroScan. By necessity and in places, the review becomes a general review of automated system performances for the detection of specific resistance mechanisms rather than focusing solely on expert systems. Published performance evaluations of each system are drawn together and commented on critically.

Performance of a New MicroScan WalkAway PC30 panel and disk diffusion method for detection of oxacillin resistance in Staphylococcus spp

The performance of the MicroScan WalkAway PC30 panel for detection of oxacillin resistance was evaluated by use of a collection of 420 staphylococcus isolates. The addition of a cefoxitin test (4 mg/liter) to the oxacillin MIC determination increased its raw performance for Staphylococcus aureus; additional data were required for coagulase-negative staphylococci.

Correlation between the VITEK2 system and cefoxitin disk diffusion for the daily detection of oxacillin resistance in a large number of clinical Staphylococcus aureus isolates

The aim of the present study was to compare the performance of the new VITEK2 AST-P551 card with the cefoxitin disk diffusion method for the daily detection of methicillin resistance with a high number of Staphylococcus aureus clinical isolates. Detection of the PBP2a protein or mecA gene was performed for each discordant case. Seventy (3.3%) isolates out of 2,107 clinical strains showed discordant results, two very major errors, four major errors and 64 minor errors. Fifty-nine (84%) discordant results were resolved, with a final overall agreement of 99.5%. Eleven (0.5%) strains remained discordant (minor error [mE]). Four of 370 MRSA strains were misclassified as susceptible in daily practice by the cefoxitin disk diffusion method. All of these strains were resistant to aminoglycosides and/or fluoroquinolones. The VITEK2 system is highly reliable for methicillin resistance detection at the routine level. Oxacillin-susceptible classified clinical strains with associated resistance patterns required attention.

BD Phoenix and Vitek 2 detection of mecA-mediated resistance in Staphylococcus aureus with cefoxitin

The BD Phoenix (BD Diagnostics, Sparks, MD) and Vitek 2 (bioMérieux, Durham, NC) automated susceptibility testing systems have implemented the use of cefoxitin to enhance the detection of methicillin (meticillin)-resistant Staphylococcus aureus (MRSA). To assess the impact of this change, 620 clinically significant S. aureus isolates were tested in parallel on Phoenix PMIC/ID-102 panels and Vitek 2 AST-GP66 cards. The results for oxacillin and cefoxitin generated by the automated systems were compared to those generated by two reference methods: mecA gene detection and MICs of oxacillin previously determined by broth microdilution according to CLSI guidelines. Testing of isolates with discordant results was repeated to attain a majority or consensus final result. There was 100% final agreement between the results of the two reference methods. For the 448 MRSA and 172 methicillin-susceptible S. aureus isolates tested, the rates of categorical agreement of the results obtained with the automated systems with those obtained by the reference methods were 99.8% for the Phoenix panels and 99.7% for the Vitek 2 cards. A single very major error occurred on each instrument (0.2%) with different MRSA isolates. The only major error was attributed to the Vitek 2 system overcalling oxacillin resistance. In 16 instances (9 on the Phoenix system, 7 on the Vitek 2 system), an oxacillin MIC in the susceptible range was correctly changed to resistant by the expert system on the basis of the cefoxitin result. The inclusion of cefoxitin in the Phoenix and Vitek 2 panels has optimized the detection of MRSA by both systems.

Performance of MicroScan WalkAway and Vitek 2 for detection of oxacillin resistance in a set of methicillin-resistant Staphylococcus aureus isolates with diverse genetic backgrounds

Of 104 genotypically diverse methicillin-resistant Staphylococcus aureus (MRSA) isolates tested with the MicroScan WalkAway (Pos MIC 24 panel) and Vitek 2 (AST-P549 card) systems, 7 and 6 isolates, respectively, showed an oxacillin MIC of < or =2mg/liter. Most of these MRSA isolates were community acquired. However, if the cefoxitin screen of AST-P549 was also considered, MRSA detection failed for only one isolate.

Comparison of three chromogenic media and enrichment broth media for the detection of methicillin-resistant Staphylococcus aureus from mucocutaneous screening specimens : Comparison of MRSA chromogenic media

This study compares the performance of three chromogenic culture agar plates, chromID MRSA, MRSA-Screen and MRSA-Select, by challenging with a collection of Staphylococcus aureus strains and screening samples obtained from hospitalised patients. All chromogenic media showed excellent sensitivity (>95%) and specificity after 18 h on the methicillin-resistant Staphylococcus aureus (MRSA) collection strains, but the specificity of MRSA-Screen decreased markedly after 42 h. Sixty-eight of 1,002 screening specimens yielded MRSA on at least one medium. The sensitivity of all media to detecting MRSA after 18 h was <50%, but this increased to 75% (chromID MRSA), 81% (MRSA-Screen) and 72% (MRSA-Select) after 42 h and 85% after enrichment and plating on the same media. The specificity at 18 h was excellent, but was significantly lower for MRSA-Screen after 42 h and enrichment. In conclusion, all media showed equivalent sensitivities after 18 h of incubation and performed better when enriched before inoculation. MRSA-Screen was more sensitive but less specific than the two other media after 42 h of incubation.