Plasmonic Colloidosome-Coupled MALDI-TOF MS for Bacterial Heteroresistance Study at Single-Cell Level

Antimicrobial resistance (AMR) is a long-term public health challenge worldwide, and it is increasingly recognized to be a heteroresistance phenomenon in an isogenic bacterial population. When the minority population of resistant bacteria with strong AMR is not handled in time, such a subpopulation can be enriched leading to the further development of bacterial AMR. However, conventional AMR studies based on ensemble-averaged data from a large population fail to characterize the bacterial heterogeneity. In this work, we develop a method using plasmonic colloidosomes and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) to study single bacterial cell AMR. The plasmonic colloidosomes act simultaneously as bacteria containers and sample spots for MALDI-TOF MS detection. Cells of β-lactamase-producing Escherichia coli (E. coli) are trapped in colloidosome containers (∼200 μm in diameter) in the presence of antibiotic drug ampicillin (AMP). Benefiting from the fast reaction kinetics in microcompartments, the hydrolysis product of AMP by bacteria can be detected by MALDI-TOF MS within 40 min. The colloidosomes as MALDI sample spots also benefit sensitive detection and accurate quantification of AMP and its hydrolysis product. It was found that even an isogenic population could consist of a mixture of bacteria that have different resistance degrees to antibiotics. Taking the β-lactamase-producing E. coli as an example, 20% of the bacterial individuals have relatively strong activity in hydrolyzing AMP. It is expected that the colloidosome-based platform would reveal a prospective application in full characterization of single bacterial cell AMR.