One System for All: Is Mass Spectrometry a Future Alternative for Conventional Antibiotic Susceptibility Testing?

The two main pillars of clinical microbiological diagnostics are the identification of potentially pathogenic microorganisms from patient samples and the testing for antibiotic susceptibility (AST) to allow efficient treatment with active antimicrobial agents. While routine microbial species identification is increasingly performed with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), routine AST still largely relies on conventional and molecular techniques such as broth microdilution or disk and gradient diffusion tests, PCR and automated variants thereof. However, shortly after the introduction of MALDI-TOF MS based routine identification, first attempts to perform AST on the same instruments were reported. Today, a number of different approaches to perform AST with MALDI-TOF MS and other MS techniques have been proposed, some restricted to particular microbial taxa and resistance mechanisms while others being more generic. Further, while some of the methods are in a stage of proof of principles, others are already commercialized. In this review we discuss the different principal approaches of mass spectrometry based AST and evaluate the advantages and disadvantages compared to conventional and molecular techniques. At present, the possibility that MS will soon become a routine tool for AST seems unlikely – still, the same was true for routine microbial identification a mere 15 years ago.

Proteomics-based discrimination of differentially expressed proteins in antibiotic-sensitive and antibiotic-resistant Salmonella Typhimurium, Klebsiella pneumoniae, and Staphylococcus aureus

This study was designed to compare the differentially expressed proteins between antibiotic-sensitive and antibiotic-resistant Salmonella Typhimurium, Klebsiella pneumonia, and Staphylococcus aureus. The susceptibilities of wild-type (WT), ciprofloxacin (CIP) and/or oxacillin (OXA)-induced, and clinically isolated resistant (CCARM) S. Typhimurium (ST^WT, ST^CIP, and ST^CCARM), K. pneumoniae (KP^WT, KP^CIP, and KP^CCARM), and S. aureus (SA^WT, SA^CIP, SA^OXA, and SA^CCARM) to antibiotics were determined using broth microdilution assay. ST^CIP was highly resistant to piperacillin (MIC > 512 μg/ml), KP^CIP was resistant to chloramphenicol (128 μg/ml) and norfloxacin (16 μg/ml), SA^CIP was resistant to fluoroquinolones (32 μg/ml), and SA^OXA was resistant to ceftriaxone (32 μg/ml). The protein profiles of antibiotic-sensitive and antibiotic-resistant strains were determined using 2-DE analysis followed by LC-MS/MS. The commonly expressed proteins of ST^WT-ST^CIP, ST^WT-ST^CCARM, KP^WT-KP^CIP, KP^WT-KP^CCARM, SA^WT-SA^CIP, SA^WT-SA^OXA, and SA^WT-SA^CCARM were 763, 677, 677, 469, 261, 259, and 226, respectively. The unique protein spots were observed 57 (6.5%), 80 (11.5%), and 68 (13.9%), respectively, for ST^CCARM, KP^CCARM, and SA^CCARM. The highly up-regulated protein, PrsA (10-fold), was observed in ST^CIP resistant to ciprofloxacin (128-fold), levofloxacin (32-fold), norfloxacin (64-fold), and piperacillin (> 16-fold). The up-regulated proteins (YadC, FimA, and RplB) in KP^CIP resistant to chloramphenicol (> 32-fold), ciprofloxacin (32-fold), levofloxacin (6-fold), norfloxacin (128-fold), and sparfloxacin (64-fold). AcrB and RpoB were up-regulated in SA^CCARM resistant to multiple antibiotics. The differentially expressed proteins were related to the antibiotic resistance of ST^WT, ST^CIP, ST^CCARM, KP^WT, KP^CIP, KP^CCARM, SA^WT, SA^CIP, SA^OXA, and SA^CCARM. The resistance-associated proteins could be useful biomarkers for detecting antibiotic-resistant pathogens.

Antibiotic combinations for controlling colistin-resistant Enterobacter cloacae

Enterobacter cloacae is a Gram-negative bacterium associated with high morbidity and mortality in intensive care patients due to its resistance to multiple antibiotics. Currently, therapy against multi-resistant bacteria consists of using colistin, in spite of its toxic effects at higher concentrations. In this context, colistin-resistant E. cloacae strains were challenged with lower levels of colistin combined with other antibiotics to reduce colistin-associated side effects. Colistin-resistant E. cloacae (ATCC 49141) strains were generated by serial propagation in subinhibitory colistin concentrations. After this, three colistin-resistant and three nonresistant replicates were isolated. The identity of all the strains was confirmed by MALDI-TOF MS, VITEK 2 and MicroScan analysis. Furthermore, cross-resistance to other antibiotics was checked by disk diffusion and automated systems. The synergistic effects of the combined use of colistin and chloramphenicol were observed via the broth microdilution checkerboard method. First, data here reported showed that all strains presented intrinsic resistance to penicillin, cephalosporin (except fourth generation), monobactam, and some associations of penicillin and β-lactamase inhibitors. Moreover, a chloramphenicol and colistin combination was capable of inhibiting the induced colistin-resistant strains as well as two colistin-resistant clinical strains. Furthermore, no cytotoxic effect was observed by using such concentrations. In summary, the data reported here showed for the first time the possible therapeutic use of colistin-chloramphenicol for infections caused by colistin-resistant E. cloacae.

Understanding bacterial resistance to antimicrobial peptides: From the surface to deep inside

Resistant bacterial infections are a major health problem in many parts of the world. The major commercial antibiotic classes often fail to combat common bacteria. Although antimicrobial peptides are able to control bacterial infections by interfering with microbial metabolism and physiological processes in several ways, a large number of cases of resistance to antibiotic peptide classes have also been reported. To gain a better understanding of the resistance process various technologies have been applied. Here we discuss multiple strategies by which bacteria could develop enhanced antimicrobial peptide resistance, focusing on sub-cellular regions from the surface to deep inside, evaluating bacterial membranes, cell walls and cytoplasmic metabolism. Moreover, some high-throughput methods for antimicrobial resistance detection and discrimination are also examined. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.