Characterization of bacteria at a single-cell level is of great significance for basic biological studies and public health.However,flow cytometry as an efficient tool for single-cell analysis,is not favor for bacteria characterization due to their small size and minute content of structural molecules.During the past decade,our laboratory has been working on the development of high sensitivity flow cytometry(HSFCM)by employing the strategies for single molecule fluorescence detection in a sheathed flow1,and many studies on the detection and characterization of bacteria at the single-cell level have been carried out.Combining immunofluorescent staining and nucleic acid labeling,rapid and simultaneous quantification of specific pathogenic strain and total bacterial cells was demonstrated2.Detection and quantification of bacterial autofluorescence were achieved3.The analysis of low copy number β-galactosidase was accomplished when combined with fluorogenic substrate4.Meanwhile,rapid screening of antibiotic-resistant bacteria was enabled using a covalent reporter of β-lactamase activity5 and minority population of antibiotic-resistant bacteria in clinical urine samples can be probed upon immunofluorescent staining of β-lactamase6.Recently,the HSFCM was further employed to monitor the spread of antibiotic-resistance in bacteria through plasmid conjugation.