Rapid identification and antimicrobial susceptibility testing of positive blood cultures using MALDI-TOF MS and a modification of the standardised disc diffusion test: a pilot study

Tick-tock, beat the clock: comparative analysis of disc diffusion testing with 6-, 10-, and 24-h growth for accelerated antimicrobial susceptibility testing and antimicrobial stewardship

Disc diffusion testing by Kirby-Bauer technique is the most used method for determining antimicrobial susceptibility in microbiological laboratories. The current guidelines by The Clinical and Laboratory Standards Institute (CLSI) 2022 specify using an 18- to 24-h growth for testing by disc diffusion. We aim to determine if using an early growth (6 h and 10 h) would produce comparable results, thus ultimately leading to reduced turnaround time. Six-hour, 10-h, and 24-h growths of 20 quality control strains and 6-h and 24-h growths of 48 clinical samples were used to perform disc diffusion testing using a panel of appropriate antimicrobial agents. Disc diffusion zone sizes were interpreted for all and comparative analyses were performed to determine categorical agreement, minor errors (mE), major errors (ME), and very major errors (VME) according to CLSI guidelines. On comparing with the standard 24 h of incubation, disc diffusion from 6-h and 10-h growths of quality control strains showed 94.38% categorical agreement, 5.10% mE, 0.69% MEs, and no VMEs. Disc diffusion testing for the additional 40 clinical samples yielded a similarly high level of categorical agreement (98.15%) and mE, ME, and VME of 1.29%, 1.22%, and 0% respectively. Disc diffusion testing using early growth is a simple and accurate method for susceptibility testing that can reduce turnaround time and may prove to be critical for timely patient management.

Evaluation of the performance of rapid antibiotic susceptibility test results using the disk diffusion directly from the positive blood culture bottles

PURPOSE

In this study, we aimed to evaluate the compliance of rapid antibiotic susceptibility test (RAST) and conventional laboratory procedures.

METHODS

The RAST was performed directly from the blood cultures of 71 Gram negative bacilli (GNB) and 38 Gram positive cocci (GPC) isolates. The results were evaluated at fourth, sixth and eighth hour. Categorical agreement (CA), very major error (VME), major error (ME) and minor error (mE) were calculated and compared with the results of conventional Vitek-2 system.

RESULTS

Categorical agreement was detected ≥90 in cefotaxime and meropenem at fourth hour in Escherichia coli isolates. An encourage positive CA results were obtained from meropenem, ceftazidime and ciprofloxacin at the fourth hour in Klebsiella pneumoniae isolates. CA was compatible in imipenem, ciprofloxacin, gentamicin, and tobramycin for Pseudomonas aeruginosa at sixth hour. CA was low (<90%) in piperacillin-tazobactam for E. coli and K. pneumoniae, and meropenem in P. aeruginosa isolates. A good CA (≥90) with all tested antibiotics were found at all hours for Acinetobacter baumannii and also very high CA (100%) was detected at sixth and eighth hour in Staphylococcus aureus isolates. CA remained below the standard criteria at fourth hour in vancomycin and high level gentamicin, in addition to imipenem at sixth hours in enterococci isolates. VME and ME were not detected and mE was 12.7% in GNB and 50% in GPC at eighth hour.

CONCLUSIONS

EUCAST RAST at eighth hour will be beneficial in urgent patients due to their high CA rate, easy preparation, inexpensive, and could be performed with the available equipment and personnel.

EUCAST rapid antimicrobial susceptibility testing (RAST) in blood cultures: validation in 55 European laboratories

OBJECTIVES

When bloodstream infections are caused by resistant bacteria, rapid antimicrobial susceptibility testing (RAST) is important for adjustment of therapy. The EUCAST RAST method, directly from positive blood cultures, was validated in a multi-laboratory study in Europe.

METHODS

RAST was performed in 40 laboratories in northern Europe (NE) and 15 in southern Europe (SE) from clinical blood cultures positive for Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus or Streptococcus pneumoniae. Categorical results at 4, 6 and 8 h of incubation were compared with results for EUCAST standard 16-20 h disc diffusion. The method, preliminary breakpoints and the performance of the laboratories were evaluated.

RESULTS

The total number of isolates was 833/318 in NE/SE. The number of zone diameters that could be read (88%, 96% and 99%) and interpreted (70%, 81% and 85%) increased with incubation time (4, 6 and 8 h). The categorical agreement was acceptable, with total error rates in NE/SE of 2.4%/4.9% at 4 h, 1.1%/3.5% at 6 h and 1.1%/3.3% at 8 h. False susceptibility at 4, 6 and 8 h of incubation was below 0.3% and 1.1% in NE and SE, respectively, and the corresponding percentages for false resistance were below 1.9% and 2.8%. After fine-tuning breakpoints, more zones could be interpreted (73%, 89% and 93%), with only marginally affected error rates.

CONCLUSIONS

The EUCAST RAST method can be implemented in routine laboratories without major investments. It provides reliable antimicrobial susceptibility testing results for relevant bloodstream infection pathogens after 4-6 h of incubation.

Rapid Detection of Methicillin-Resistant Staphylococcus aureus Directly from Blood for the Diagnosis of Bloodstream Infections: A Mini-Review

Staphylococcus aureus represents a major human pathogen able to cause a number of infections, especially bloodstream infections (BSI). Clinical use of methicillin has led to the emergence of methicillin-resistant S. aureus (MRSA) and MRSA-BSI have been reported to be associated with high morbidity and mortality. Clinical diagnosis of BSI is based on the results from blood culture that, although considered the gold standard method, is time-consuming. For this reason, rapid diagnostic tests to identify the presence of methicillin-susceptible S. aureus (MSSA) and MRSA isolates directly in blood cultures are being used with increasing frequency to rapidly commence targeted antimicrobial therapy, also in the light of antimicrobial stewardship efforts. Here, we review and report the most common rapid non-molecular and molecular methods currently available to detect the presence of MRSA directly from blood.

Rapid identification by MALDI-TOF/MS and antimicrobial disk diffusion susceptibility testing for positive blood cultures after a short incubation on the WASPLab

The objectives of this study were to define the shortest incubation times on the WASPLab for reliable MALDI-TOF/MS-based species identification and for the preparation of a 0.5 McFarland suspension for antimicrobial disk diffusion susceptibility testing using short subcultures growing on solid culture media inoculated by positive blood cultures spiked with a wide range of pathogens associated with bloodstream infections. The 520 clinical strains (20 × 26 different species) included in this study were obtained from a collection of non-consecutive and non-duplicate pathogens identified at Geneva University Hospitals. After 4 h of incubation on the WASPLab, microorganisms' growth allowed accurate identification of 73% (380/520) (95% CI, 69.1-76.7%) of the strains included in this study. The identification rate increased to 85% (440/520) (95% CI, 81.3-87.5%) after 6-h incubation. When excluding Corynebacterium and Candida spp., the microbial growth was sufficient to permit accurate identification of all tested species (100%, 460/460) (95% CI, 99.2-100%) after 8-h incubation. With the exception of Burkholderia cepacia and Haemophilus influenzae, AST by disk diffusion could be performed for Enterobacterales and non-fermenting Gram-negative bacilli after only 4 h of growth in the WASPLab. The preparation of a 0.5 McFarland suspension for Gram-positive bacteria required incubation times ranging between 3 and 8 h according to the bacterial species. Only Corynebacterium spp. required incubation times as long as 16 h. The WASPLab enables rapid pathogen identification as well as swift comprehensive AST from positive blood cultures that can be implemented without additional costs nor hands-on time by defining optimal time points for image acquisition.

Role of the Clinical Microbiology Laboratory in Antimicrobial Stewardship

For adequate antimicrobial stewardship, microbiology needs to move from the laboratory to become physically and verbally amenable to the caregivers of an institution. Herein, we describe the contributions of our microbiology department to the antimicrobial stewardship program of a large teaching hospital as 10 main points ranging from the selection of patients deemed likely to benefit from a fast track approach, to their clinical samples, or the rapid reporting of results via a microbiology hotline, to rapid searches for pathogens and susceptibility testing. These points should serve as guidelines for similar programs designed to decrease the unnecessary use of antimicrobials.

Rapid phenotypic methods to improve the diagnosis of bacterial bloodstream infections: meeting the challenge to reduce the time to result

BACKGROUND

Administration of appropriate antimicrobial therapy is one of the key factors in surviving bloodstream infections. Blood culture is currently the reference standard for diagnosis, but conventional practices have long turnaround times while diagnosis needs to be faster to improve patient care. Phenotypic methods offer an advantage over genotypic methods in that they can identify a wide range of taxa, detect the resistance currently expressed, and resist genetic variability in resistance detection.

AIMS

We aimed to discuss the wide array of phenotypic methods that have recently been developed to substantially reduce the time to result from identification to antibiotic susceptibility testing.

SOURCES

A literature review focusing on rapid phenotypic methods for improving the diagnosis of bloodstream infection was the source.

CONTENT

Rapid phenotypic bacterial identification corresponds to Matrix-assisted laser-desorption/ionization time of flight mass spectrometry (MALDI-TOF), and rapid antimicrobial susceptibility testing methods comprised of numerous different approaches, are considered and critically assessed. Particular attention is also paid to emerging technologies knocking at the door of routine microbiology laboratories. Finally, workflow integration of these methods is considered.

IMPLICATIONS

The broad panel of phenotypic methods currently available enables healthcare institutions to draw up their own individual approach to improve bloodstream infection diagnosis but requires a thorough evaluation of their workflow integration. Clinical microbiology will probably move towards faster methods while maintaining a complex multi-method approach as there is no all-in-one method.