Single-cell analysis of antimicrobial compound-induced cell death of bacterial cells

Due to the stochasticity of metabolic reactions and cell cycles of bacterial cells, it is necessary to examine the antibacterial activities of antimicrobial compounds (AMCs) such as antibiotics and antimicrobial peptides (AMPs) at the single-cell level. Here, we review recent studies of the bactericidal activities of AMCs at the single-cell level. First, we discuss recent investigations of the interaction of various AMPs with single bacterial cells, as monitored in real time using optical microscopy. This strategy provides information on AMP-induced membrane damage in single cells [e.g. the onset time of damage to the cell membrane (CM) and outer membrane of single cells]. The rate of AMP-induced CM damage is estimated as the fraction of cells with CM damage [Pdamage (t)] at a specific interaction time t. Second, we discuss the use of single-cell analysis of the bactericidal activity of AMCs. The fraction of dead cells after the exposure to AMCs for time t is determined as the fraction of the microcolonies containing only one cell [Psingle (t)]. For some AMPs, the Pdamage (t) and Psingle (t) values are similar, indicating that AMP-induced CM damage is the direct cause of cell death. Third, we discuss single-cell analysis of the processes and mechanisms of antibiotic-induced cell death. For example, fluoroquinolones and aminoglycosides are observed to induce cytoplasmic condensation and cell lysis, leading to cell death. Based on these studies, we provide our perspective on future investigations using single-cell analysis to assess the processes and the mechanisms of the bactericidal activities of AMCs.