Insight into Identification of Acinetobacter Species by Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS) in the Clinical Laboratory

Predictive modeling of mortality in carbapenem-resistant Acinetobacter baumannii bloodstream infections using machine learning

Acinetobacter baumannii, a notable drug-resistant bacterium, often induces severe infections in healthcare settings, prompting a deeper exploration of treatment alternatives due to escalating carbapenem resistance. This study meticulously examined clinical, microbiological, and molecular aspects related to in-hospital mortality in patients with carbapenem-resistant A. baumannii (CRAB) bloodstream infections (BSIs). From 292 isolates, 153 cases were scrutinized, reidentified through matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and evaluated for antimicrobial susceptibility and carbapenemase genes via multiplex polymerase chain reaction (PCR). Utilizing supervised machine learning, the study constructed models to predict 14- and 30-day mortality rates, revealing the Naïve Bayes model's superior specificity (0.75) and area under the curve (0.822) for 14-day mortality, and the Random Forest model's impressive recall (0.85) for 30-day mortality. These models delineated eight and nine significant features for 14- and 30-day mortality predictions, respectively, with "septic shock" as a pivotal variable. Additional variables such as neutropenia with neutropenic days prior to sepsis, mechanical ventilator support, chronic kidney disease, and heart failure were also identified as ranking features. However, empirical antibiotic therapy appropriateness and specific microbiological data had minimal predictive efficacy. This research offers foundational data for assessing mortality risks associated with CRAB BSI and underscores the importance of stringent infection control practices in the wake of the scarcity of new effective antibiotics against resistant strains. The advanced models and insights generated in this study serve as significant resources for managing the repercussions of A. baumannii infections, contributing substantially to the clinical understanding and management of such infections in healthcare environments.

Acinetobacter Non-baumannii Species: Occurrence in Infections in Hospitalized Patients, Identification, and Antibiotic Resistance

BACKGROUND

Acinetobacter species other than A. baumannii are becoming increasingly more important as opportunistic pathogens for humans. The primary aim of this study was to assess the prevalence, species distribution, antimicrobial resistance patterns, and carbapenemase gene content of clinical Acinetobacter non-baumannii (Anb) isolates that were collected as part of a sentinel surveillance program of bacterial infections in hospitalized patients. The secondary aim was to evaluate the performance of MALDI-TOF MS systems for the species-level identification of Anb isolates.

METHODS

Clinical bacterial isolates were collected from multiple sites across Russia and Kazakhstan in 2016-2022. Species identification was performed by means of MALDI-TOF MS, with the Autobio and Bruker systems used in parallel. The PCR detection of the species-specific blaOXA-51-like gene was used as a means of differentiating A. baumannii from Anb species, and the partial sequencing of the rpoB gene was used as a reference method for Anb species identification. The susceptibility of isolates to antibiotics (amikacin, cefepime, ciprofloxacin, colistin, gentamicin, imipenem, meropenem, sulbactam, tigecycline, tobramycin, and trimethoprim-sulfamethoxazole) was determined using the broth microdilution method. The presence of the most common in Acinetobacter-acquired carbapenemase genes (blaOXA-23-like, blaOXA-24/40-like, blaOXA-58-like, blaNDM, blaIMP, and blaVIM) was assessed using real-time PCR.

RESULTS

In total, 234 isolates were identified as belonging to 14 Anb species. These comprised 6.2% of Acinetobacter spp. and 0.7% of all bacterial isolates from the observations. Among the Anb species, the most abundant were A. pittii (42.7%), A. nosocomialis (13.7%), the A. calcoaceticus/oleivorans group (9.0%), A. bereziniae (7.7%), and A. geminorum (6.0%). Notably, two environmental species, A. oleivorans and A. courvalinii, were found for the first time in the clinical samples of patients with urinary tract infections. The prevalence of resistance to different antibiotics in Anb species varied from <4% (meropenem and colistin) to 11.2% (gentamicin). Most isolates were susceptible to all antibiotics; however, sporadic isolates of A. bereziniae, A. johnsonii, A. nosocomialis, A. oleivorans, A. pittii, and A. ursingii were resistant to carbapenems. A. bereziniae was more frequently resistant to sulbactam, aminoglycosides, trimethoprim-sulfamethoxazole, and tigecycline than the other species. Four (1.7%) isolates of A. bereziniae, A. johnsonii, A. pittii were found to carry carbapenemase genes (blaOXA-58-like and blaNDM, either alone or in combination). The overall accuracy rates of the species-level identification of Anb isolates with the Autobio and Bruker systems were 80.8% and 88.5%, with misidentifications occurring in 5 and 3 species, respectively.

CONCLUSIONS

This study provides important new insights into the methods of identification, occurrence, species distribution, and antibiotic resistance traits of clinical Anb isolates.

Antibiotic Resistance and Genetic Variability of Acinetobacter spp. from Wastewater Treatment Plant in Kokšov-Bakša (Košice, Slovakia)

This study investigated the genetic variability and antibiotic resistance of Acinetobacter community depending on the stage of wastewater treatment in Kokšov-Bakša for the city of Košice (Slovakia). After cultivation, bacterial isolates were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and their sensitivity to ampicillin, kanamycin, tetracycline, chloramphenicol and ciprofloxacin was examined. Acinetobacter spp. and Aeromonas spp. dominated bacterial populations in all wastewater samples. We identified 12 different groups based on protein profiling, 14 genotypes by amplified ribosomal DNA restriction analysis and 11 Acinetobacter species using 16S rDNA sequence analysis within Acinetobacter community, which showed significant variability in their spatial distribution. While Acinetobacter population structure changed during the wastewater treatment, the prevalence of antibiotic-resistant strains did not significantly vary depending on the stage of wastewater treatment. The study highlights the role of a highly genetically diverse Acinetobacter community surviving in wastewater treatment plants as an important environmental reservoir assisting in the further dissemination of antibiotic resistance in aquatic systems.

Acinetobacter nosocomialis Causes as Severe Disease as Acinetobacter baumannii in Northeast Thailand: Underestimated Role of A. nosocomialis in Infection

Infections by Acinetobacter species are recognized as a serious global threat due to causing severe disease and their high levels of antibiotic resistance. Acinetobacter baumannii is the most prevalent pathogen in the genus, but infection by Acinetobacter nosocomialis has been reported widely. Diagnosis of patients with A. baumannii infection is often misdiagnosed with other Acinetobacter species, especially A. nosocomialis. This study investigated whether there were significant differences in clinical outcomes between patients infected with A. baumannii versus A. nosocomialis in Northeast Thailand, and to characterize serological responses to infection with these pathogens. The results show that A. baumannii had higher levels of multidrug resistance. Despite this, clinical outcomes for infection with A. baumannii or A. nosocomialis were similar with mortalities of 33% and 36%, respectively. Both pathogens caused community-acquired infections (A. baumannii 35% and A. nosocomialis 29% of cases). Plasma from uninfected healthy controls contained IgG antibody that recognized both organisms, and infected patients did not show a significantly enhanced antibody response from the first week versus 2 weeks later. Finally, the patterns of antigen recognition for plasma IgG were similar for patients infected with A. baumannii or A. nosocomialis infection, and distinct to the pattern for patients infected with non-Acinetobacter. In conclusion, our data revealed that infection with A. nosocomialis was associated with a similarly high level of mortality as infection with A. baumannii, the high rate of community-acquired infection and antibodies in uninfected individuals suggesting that there is significant community exposure to both pathogens. IMPORTANCE Bacterial infections by Acinetobacter species are global threats due to their severity and high levels of antibiotic resistance. A. baumannii is the most common pathogen in the genus; however, infection by A. nosocomialis has also been widely reported but is thought to be less severe. In this study, we have prospectively investigated 48 reported cases of A. baumannii infection in Northeast Thailand, and characterized the serological responses to infection. We found that 14 (29%) of these infections were actually caused by A. nosocomialis. Furthermore, the incidence of antibiotic resistance among A. nosocomialis strains, APACHE II scores, and mortality for patients infected with A. nosocomialis were much higher than published data. Both A. baumannii and A. nosocomialis had unexpectedly mortality rates of over 30%, and both pathogens caused a high rate of community-acquired infections. Importantly, background antibodies in uninfected individuals suggest significant community exposure to both pathogens in the environment.

Comparative Study of Different Diagnostic Routine Methods for the Identification of Acinetobacter radioresistens

Recent publications indicate that A. radioresistens can cause infections in humans, even though it is rarely reported in routine diagnostics. However, the fact that it is infrequently detected may be explained by the misidentification of the species by conventional methods. It is also likely that A. radioresistens is not considered clinically relevant and therefore not consistently included in diagnostic results. To elucidate the medical significance of this probably clinically underestimated bacterial species, we created a well-documented reference strain collection of 21 strains collected in routine diagnostics. For further analysis of A. radioresistens, it is essential to know which methods can be used to achieve a trustworthy identification. We, therefore, compared three methods widely used in routine diagnostics (MALDI-TOF MS, VITEK 2, and sequencing of housekeeping genes) in terms of secure and reliable identification of A. radioresistens. As reference methods, whole genome-based approaches were applied. VITEK 2 led to misidentification for four strains. However, MALDI-TOF MS and sequencing of housekeeping genes led to reliable and robust identifications.

Advances in automated techniques to identify Acinetobacter calcoaceticus-Acinetobacter baumannii complex

Acinetobacter species, particularly those within Acinetobacter calcoaceticus-A. baumannii complex (ACB complex), have emerged as clinically relevant pathogens in hospital environments worldwide. Early and quick detection and identification of Acinetobacter infections is challenging, and traditional culture and biochemical methods may not achieve adequate levels of speciation. Moreover, currently available techniques to identify and differentiate closely related Acinetobacter species are insufficient. The objective of this review is to recapitulate the current evolution in phenotypic and automated techniques used to identify the ACB complex. Compared with other automated or semiautomated systems of bacterial identification, matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) demonstrates a high level of Acinetobacter species identification and discrimination, including newly discovered species A. seifertii and A. dijkshoorniae.

Evaluation of Vitek-MS™ and Microflex LT™ commercial systems for identification of Acinetobacter calcoaceticus-baumannii complex

INTRODUCTION

Acinetobacter is a genus that comprises a group of opportunistic pathogens responsible for a variety of nosocomial infections. The Acinetobacter calcoaceticus-Acinetobacter baumannii (Acb) complex includes some species of clinical importance, mainly A. baumannii, A. pittii and A. nosocomialis, which share phenotypic similarities that make it very difficult to distinguish between them using a phenotypic approach. The aim of this study was to evaluate two commercial matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) systems for the identification of different Acinetobacter species, with a special focus among those belonging to the Acb complex.

METHODS

One hundred and fifty-six Acinetobacter spp. clinical strains, identified by amplified ribosomal DNA restriction analysis (ARDRA) and rpoB gene sequencing, were analysed by two different MALDI-TOF systems.

RESULTS

Considering only the 144 strains of the Acb complex evaluated in this study, the Vitek-MS™ and Microflex LT™ systems correctly identified 129 (89.6%) and 143 (99.3%) strains, respectively.

CONCLUSION

After analysing 156 strains belonging to Acinetobacter spp., both Vitek-MS™ and Microflex LT™ proved to be rapid and accurate systems for the identification of Acb complex species showing a good correlation. However, both manufacturers should improve their databases to include new species in them.

The applicability of gene sequencing and MALDI-TOF to identify less common gram-negative rods (Advenella, Castellaniella, Kaistia, Pusillimonas and Sphingobacterium) from environmental isolates

Our aim was to identify less common non-fermenting gram-negative rods during the bioremediation process. Five genera were found: Advenella, Castellaniella, Kaistia, Pusillimonas and Sphingobacterium, for a total of 15 isolates. Therefore, we evaluated the applicability of four methods currently available for bacteria identification: (1) conventional biochemical methods, (2) the VITEK^®-2 system, (3) MALDI-TOF mass spectrometry and (4) 16S rRNA gene sequencing. The biochemical methods and the VITEK^®-2 system were reliable only for the Sphingobacterium isolate and solely at the genus level. Both MALDI-TOF mass spectrometry platforms (Bruker and VITEK^® MS) did not achieve reliable identification results for any of these genera. 16S rRNA gene sequencing identified eight isolates to the species level but not to the subspecies level, when applicable. The remaining seven isolates were reliably identified through 16S rRNA gene sequencing to the genus level only. Our findings suggest that the detection and identification of less common genera (and species) that appeared at certain moments during the bioremediation process can be a challenge to microbiologists considering the most used techniques. In addition, more studies are required to confirm our results.

Evaluation of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for identifying VIM- and SPM-type metallo-β-lactamase-producing Pseudomonas aeruginosa clinical isolates

Background

Metallo-β-lactamase-producing Pseudomonas aeruginosa (MBL-PA) are important causative agents of nosocomial infections and are associated with significant mortality rates, especially in intensive care units. The timely detection and typing of these strains is essential for surveillance, outbreak prevention and antibiotic therapy optimization. In this study, fifteen VIM-type and fifteen SPM-type MBL-PA strains were selected as strains to establish MALDI-TOF MS SuperSpectra.

Methods

This study was undertaken to evaluate the utility of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) with the VITEK MS plus system in the detection of VIM- and SPM-type MBL-PA isolates. For each species, we increased the reference spectra, and then, a SuperSpectrum was created based on the selection of 39 specific masses. In a second step, we validated the SuperSpectra with the remaining 50 isolates (25 isolates of VIM-type and 25 isolates of SPM-type).

Results

Fifty MBL-PA strains were used as the validation strains, including twenty-five VIM-type and twenty-five SPM-type MBL-PA strains. Complete antimicrobial susceptibility testing and genotypic characterizations were performed for all isolates, which were subsequently identified using the newly created SuperSpectra databases following a previously reported method. The results showed that there was 92% agreement between the MBL profile generated by MALDI-TOF MS and that obtained using gene sequencing analysis methods.

Conclusion

MALDI-TOF MS is a promising, rapid and economical method for detecting VIM- or SPM-type MBL-PA that could be successfully introduced into the routine diagnostic workflow of clinical microbiology laboratories.

An update on the routine application of MALDI-TOF MS in clinical microbiology

Introduction: Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has entered clinical diagnostics and is today a generally accepted and integral part of the workflow for microbial identification. MALDI-TOF MS identification systems received approval from national and international institutions, such as the USA-FDA, and are continuously improved and adopted to other fields like veterinary and industrial microbiology. The question is whether MALDI-TOF MS also has the potential to replace other conventional and molecular techniques operated in routine diagnostic laboratories. Areas covered: We give an overview of new advancements of mass spectral analysis in the context of microbial diagnostics. In particular, the expansion of databases to increase the range of readily identifiable bacteria and fungi, the refined discrimination of species complexes, subspecies, and types, the testing for antibiotic resistance or susceptibility, progress in sample preparation including automation, and applications of other mass spectrometry techniques are discussed. Expert opinion: Although many new approaches of MALDI-TOF MS are still in the stage of proof of principle, it is expectable that MALDI-TOF MS will expand its role in the clinical microbiology laboratory of the future. New databases, instruments and analytical software modules will continue to be developed to further improve diagnostic efficacy.

Evaluation of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for identifying Burkholderia pseudomallei and Burkholderia thailandensis isolates

Since Burkholderia thailandensis is included in the reference spectra of the VITEK MS libraries rather than Burkholderia pseudomallei, B. pseudomallei cannot be correctly identified in the current version of VITEK MS. This study was undertaken to evaluate the utility of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) with the VITEK MS plus system in the detection of B. pseudomallei and B. thailandensis isolates. For each species, we increased the reference spectra, and then, a SuperSpectrum was created based on the selection of 39 specific masses. In a second step, we validated the SuperSpectra with 106 isolates identified by 16S rRNA gene sequencing. The results showed that there was 100% agreement between the validation strains analyzed by MALDI-TOF MS and those evaluated using 16S rRNA gene sequencing analysis methods. Therefore, MALDI-TOF MS is a promising, rapid, and economical method to monitor the outbreaks and spread of B. pseudomallei and B. thailandensis isolates.

Limitations of routine MALDI-TOF mass spectrometric identification of Acinetobacter species and remedial actions

A set of 204 taxonomically well-defined strains belonging to 17 Acinetobacter spp., including 11 recently described species (A. albensis, A. bohemicus, A. colistiniresistens, A. courvalinii. A. dispersus, A. gandensis, A. modestus, A. proteolyticus, A. seifertii, A. variabilis, and A. vivianii) and six species of the so-called haemolytic clade (A. beijerinckii, A. gyllenbergii, A. haemolyticus, A. junii, A. parvus, and A. venetianus), were subjected to MALDI-TOF mass spectrometric profiling. The identification outputs were evaluated using the current version (8.0.0.0) of the commercially available Bruker Daltonics, Biotyper database, which does not contain reference entries for six of the species tested. Up to 29% of the strains were falsely identified as different Acinetobacter spp. present in the Biotyper database, resulting mostly from the close phylogenetic relationship of species of the haemolytic clade. To obtain more reliable identification, extending the commercial database showed only partial improvement, while the use of an alternative MALDI matrix solution (strongly acidified ferulic acid) allowed correct identification of nearly all problematic strains.