Evaluation of the TPX MRSA assay for the detection of methicillin-resistant Staphylococcus aureus

Biofilm formation of two genetically diverse Staphylococcus aureus isolates under beta-lactam antibiotics

PURPOSE

Our aim was to evaluate the biofilm formation of 2 genetically diverse Staphylococcus aureus isolates, 10379 and 121940, under different concentrations of beta-lactam antibiotics on biomass content and biofilm viability.

METHODS

Biofilm formation and methicillin resistance genes were tested using PCR and multiplex PCR. PCR was combined with bioinformatics analysis to detect multilocal sequence typing (MLST) and SCCmec types, to study the genetical correlation between the tested strains. Then, the crystal violet (CV) test and XTT were used to detect biomass content and biofilm activity. Antibiotic susceptibility was tested using a broth dilution method. According to their specific MIC, different concentrations of beta-lactam antibiotics were used to study its effect on biomass content and biofilm viability.

RESULTS

Strain 10379 carried the icaD, icaBC, and MRSA genes, not the icaA, atl, app, and agr genes, and MLST and SCCmec typing was ST45 and IV, respectively. Strain 121940 carried the icaA, icaD, icaBC, atl, and agr genes, not the aap gene, and MLST and SCCmec typed as ST546 and IV, respectively. This suggested that strains 10379 and 121940 were genotypically very different. Two S. aureus isolates, 10379 and 121940, showed resistance to beta-lactam antibiotics, penicillin, ampicillin, meropenem, streptomycin and kanamycin, some of which promoted the formation of biofilm and biofilm viability at low concentrations.

CONCLUSION

Despite the large differences in the genetic background of S. aureus 10379 and 121940, some sub-inhibitory concentrations of beta-lactam antibiotics are able to promote biomass and biofilm viability of both two isolates.

Why Can't We Just Use PCR? The Role of Genotypic versus Phenotypic Testing for Antimicrobial Resistance Testing

There is a need for phenotypic susceptibility testing that is expeditious and that can be performed directly from clinical specimens. While rapid pathogen identification is important, it is the susceptibility result that is essential for antimicrobial optimization. The options for rapid susceptibility testing are limited, with the majority of commercial tests available offering genotypic resistance detection only. In this article, a laboratorian and a clinician discuss the benefits and limitations of genotypic and phenotypic susceptibility testing and provide examples of how results should be interpreted to maximize the clinical utility.

Rapid EUCAST disc diffusion testing of MDR Escherichia coli and Klebsiella pneumoniae: inhibition zones for extended-spectrum cephalosporins can be reliably read after 6 h of incubation

Objectives

The need for rapid antibiotic susceptibility testing increases with escalating levels of antimicrobial resistance in Enterobacteriaceae. Our objective was to evaluate the accuracy of reading EUCAST disc diffusion, ROSCO ESBL and carbapenemase detection kits and the Mast Carbapenemase Activity Test (CAT-ID) disc, after 6 h of incubation.

Methods

We used a collection of 128 isolates of Escherichia coli and Klebsiella pneumoniae with a wide variety of resistance mechanisms. Inhibition zones read from digital photo images with the BD Kiestra™ Total Lab Automation System after 6 h of incubation were compared with standard reading, after 18 h, of the same Mueller-Hinton agar plates.

Results

For WT isolates, zones were generally smaller at 6 h than at 18 h. Cefotaxime had excellent categorical agreement of 99%, despite the high number of challenge isolates. However, for some other antimicrobials, hetero-resistant subpopulations were commonly invisible at 6 h, which resulted in an unacceptable number of errors when using standard EUCAST breakpoints. Accurate ESBL detection was possible at 6 h for isolates lacking other β-lactamases. Carbapenemase detection was not reliable after 6 h.

Conclusions

Inhibition zone reading at 6 h is an accurate method for susceptibility testing of extended-spectrum cephalosporins for Enterobacteriaceae. For other antimicrobials, 6 h reading can be used for preliminary reports of clearly resistant or susceptible isolates, preferably with application of adjusted breakpoints including an area of uncertainty between susceptible and resistant values.

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.

High-throughput screening of colonization samples for methicillin-resistant Staphylococcus aureus

BACKGROUND

We present here the first application of 2-photon excited fluorescence detection (TPX) technology for the direct screening of clinical colonization samples for methicillin-resistant Staphylococcus aureus (MRSA).

METHODS

A total of 125 samples from 14 patients with previously identified MRSA carriage and 16 controls from low-prevalence settings were examined.

RESULTS

The results were compared to those obtained by both standard phenotypic and molecular methods. In identifying MRSA carriers, i.e. persons with at least 1 MRSA positive colonization sample by standard methods, the sensitivity of the TPX technique was 100%, the specificity 78%, the positive predictive value 75%, and the negative predictive value 100%. The TPX assay sensitivity per colonization sample was 89%, the specificity 93%, the positive predictive value 84%, and the negative predictive value 95%. The median time for a true-positive test result was 3 h and 26 min; negative test results are available after 13 h. The assay capacity was 48 samples per test run.

CONCLUSIONS

The TPX MRSA technique could provide early preliminary results for clinicians, while simultaneously functioning as a selective enrichment step for further conventional testing. Costs and workload associated with hospital infection control can be reduced using this high-throughput, point-of-care compatible methodology.