Stress-induced antibiotic susceptibility testing on a chip

Optimization of Stress-Based Microfluidic Testing for Methicillin Resistance in Staphylococcusaureus Strains

The rapid evolution of antibiotic resistance in bacterial pathogens is driving the development of innovative, rapid antibiotic susceptibility testing (AST) tools as a way to provide more targeted and timely antibiotic treatment. We have previously presented a stress-based microfluidic method for the rapid determination of antibiotic susceptibility in methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA). In this method, stress is used to potentiate the action of antibiotics, and cell death is measured as a proxy for susceptibility. The method allows antibiotic susceptibility to be determined within an hour from the start of the antibiotic introduction. However, the relatively low dynamic range of the signal (2–10% cell response) even with high antibiotic concentrations (10–50 µg/mL) left room for the method’s optimization. We have conducted studies in which we varied the flow patterns, the media composition, and the antibiotic concentration to increase the cell death response and concordantly decrease the required antibiotic concentration down to 1–3 µg/mL, in accordance with the Clinical and Laboratory Standards Institute’s (CLSI) guidelines for AST breakpoint concentrations.

Rapid phenotypic stress-based microfluidic antibiotic susceptibility testing of Gram-negative clinical isolates

Bacteremia is a life-threatening condition for which antibiotics must be prescribed within hours of clinical diagnosis. Since the current gold standard for bacteremia diagnosis is based on conventional methods developed in the mid-1800s-growth on agar or in broth-identification and susceptibility profiling for both Gram-positive and Gram-negative bacterial species requires at least 48-72 h. Recent advancements in accelerated phenotypic antibiotic susceptibility testing have centered on the microscopic growth analysis of small bacterial populations. These approaches are still inherently limited by the bacterial growth rate. Our approach is fundamentally different. By applying environmental stress to bacteria in a microfluidic platform, we can correctly assign antibiotic susceptibility profiles of clinically relevant Gram-negative bacteria within two hours of antibiotic introduction rather than 8-24 h. The substantial expansion to include a number of clinical isolates of important Gram-negative species-Enterobacter cloacae, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa-reported here underscores the broad utility of our approach, complementing the method's proven utility for Gram-positive bacteria. We also demonstrate that the platform is compatible with antibiotics that have varying mechanisms of action-meropenem, gentamicin, and ceftazidime-highlighting the versatility of this platform.

Rapid antibiotic sensitivity testing in microwell arrays

The widespread bacterial resistance to a broad range of antibiotics necessitates rapid antibiotic susceptibility testing before effective treatment could start in the clinic. Among resistant bacteria, Staphylococcus aureus is one of the most important, and Methicillin-resistant (MRSA) strains are a common cause of life threatening infections. However, standard susceptibility testing for S. aureus is time consuming and thus the start of effective antibiotic treatment is often delayed. To circumvent the limitations of current susceptibility testing systems, we designed an assay that enables measurements of bacterial growth with higher spatial and temporal resolution than standard techniques. The assay consists of arrays of microwells that confine small number of bacteria in small spaces, where their growth is monitored with high precision. These devices enabled us to investigate the effect of different antibiotics on S. aureus growth. We measured the Minimal Inhibitory Concentration (MIC) in less than 3 hours. In addition to being significantly faster than the 48 hours needed for traditional microbiological methods, the assay is also capable of differentiating the specific effects of different antibiotic classes on S. aureus growth. Overall, this assay has the potential to become a rapid, sensitive, and robust tool for use in hospitals and laboratories to assess antibiotic sensitivity.

Microfluidic advances in phenotypic antibiotic susceptibility testing

A strong natural selection for microbial antibiotic resistance has resulted from the extensive use and misuse of antibiotics. Though multiple factors are responsible for this crisis, the most significant factor - widespread prescription of broad-spectrum antibiotics - is largely driven by the fact that the standard process for determining antibiotic susceptibility includes a 1-2-day culture period, resulting in 48-72 h from patient sample to final determination. Clearly, disruptive approaches, rather than small incremental gains, are needed to address this issue. The field of microfluidics promises several advantages over existing macro-scale methods, including: faster assays, increased multiplexing, smaller volumes, increased portability for potential point-of-care use, higher sensitivity, and rapid detection methods. This Perspective will cover the advances made in the field of microfluidic, phenotypic antibiotic susceptibility testing (AST) over the past two years. Sections are organized based on the functionality of the chip - from simple microscopy platforms, to gradient generators, to antibody-based capture devices. Microfluidic AST methods that monitor growth as well as those that are not based on growth are presented. Finally, we will give our perspective on the major hurdles still facing the field, including the need for rapid sample preparation and affordable detection technologies.