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Antibiotic resistance is one of the greatest threats to human health.Bacterial resistance to β-lactam antibiotics has resulted in a large number of therapeutic failures.The production of a series of β-lactamases which could hydrolyze the four-member lactam ring of the antibiotic is the major cause of antibiotic resistance in bacteria.Therefore,rapid detection of resistant bacteria is important in accurate disease diagnosis and treatment.It has been reported that patients could be infected with both the susceptible and resistant bacteria simultaneously.The presence of minority populations of resistant bacterial cells may not be detected by the traditional culture-based phenotyping method,even though they might become dominant after inappropriate treatment.In the present study,a laboratory-built high sensitivity flow cytometer (HSFCM)1,2 was used to detect β-lactamase in individual bacterial cells.Two approaches were developed for resistant bacteria labeling either by using a fluorogenic reporter of β-lactamase activity (LBRL1) or by β-lactamase antibody.Both the light scattering and green fluorescence generated from the stained bacteria were detected as they passed through the interrogating laser beam individually.By using E.coli JM109 transformed with plasmid pUC19 (encodes β-lactamase) as a model of antibiotics resistant strain,minority populations (0.5 %) of the resistant bacteria can be detected in a mixture with non-resistant strains.It is believed that HSFCM provides a powerful tool for the reliable detection of minority populations of antibiotic resistant bacterial cells.