title: Limits of propidium iodide as a cell viability indicator for environmental bacteria.
abstract: BACKGROUND
  Viability measurements of individual bacteria are applied in various scopes of research and industry using approaches where propidium iodide (PI) serves as dead cell indicator. The reliability of PI uptake as a cell viability indicator for dead (PI permeable) and viable (PI impermeable) bacteria was tested using two soil bacteria, the gram(-) Sphingomonas sp. LB126 and the gram(+) Mycobacterium frederiksbergense LB501T.
  
  
  METHODS
  Bacterial proliferation activities observed viaDAPI and Hoechst 33342 staining were linked to the energy charge and the proportion of dead cells as obtained by diOC(6) (3)-staining and PI-uptake, respectively. Calibration and verification experiments were performed using batch cultures grown on different substrates.
  
  
  RESULTS
  PI uptake depended on the physiological state of the bacterial cells. Unexpectedly, up to 40% of both strains were stained by PI during early exponential growth on glucose when compared to 2-5% of cells in the early stationary phase of growth.
  
  
  CONCLUSIONS
  The results question the utility of PI as a universal indicator for the viability of (environmental) bacteria. It rather appears that in addition to nonviable cells, PI also stains growing cells of Sphingomonas sp. and M. frederiksbergense during a short period of their life cycle.
authors.affiliation: Department of Environmental Microbiology, UFZ, Helmholtz Centre for Environmental Research, Leipzig-Halle, 04318 Leipzig, Germany.
authors.forename: Lei
authors.initials: L
authors.lastname: Shi
authors.forename: Susanne
authors.initials: S
authors.lastname: Günther
authors.forename: Thomas
authors.initials: T
authors.lastname: Hübschmann
authors.forename: Lukas Y
authors.initials: LY
authors.lastname: Wick
authors.forename: Hauke
authors.initials: H
authors.lastname: Harms
authors.forename: Susann
authors.initials: S
authors.lastname: Müller
mesh_terms: D001562:Benzimidazoles / Q000378:metabolism; D002138:Calibration; D002232:Carbocyanines / Q000378:metabolism; D004783:Environmental Microbiology; D004926:Escherichia coli / Q000254:growth & development; D007211:Indoles / Q000378:metabolism; D050296:Microbial Viability; D009161:Mycobacterium / Q000166:cytology* / Q000254:growth & development; D011419:Propidium / Q000032:analysis* / Q000378:metabolism*; D015203:Reproducibility of Results; D020578:Sphingomonas / Q000166:cytology* / Q000254:growth & development; D013194:Staining and Labeling; D013997:Time Factors
chemical_list: D001562:Benzimidazoles; D002232:Carbocyanines; D007211:Indoles; D011419:Propidium; C007293:DAPI; C007392:3,3'-dihexyl-2,2'-oxacarbocyanine; C017807:bisbenzimide ethoxide trihydrochloride