Live bacterial cells as analytical tools for speciation analysis: Hypothetical or practical? A.J. Aller, M.A. Castro This review tries to show the capability of live bacterial cells in speciation analysis, so it emphasizes speciation analysis of metalloid and organomet- alloid species by combining an uptake process by live bacterial cells with a specific detector. Especially covered in depth is the uptake process, because better understanding of equilibrium and retention mechanisms provides the basis for optimizing the use of bacterial cells. We outline the main param- eters affecting the uptake process and the analytical characteristics. We also cover bacteria-based biosensors that represent a research field of growing interest in using live bacteria. We also highlight some future considerations about the role of recombinant bacterial cells in analytical chemistry. ª 2006 Elsevier Ltd. All rights reserved. Keywords: Accumulation; Bacteria-based biosensors; Live bacterial cells; Recombinant bacteria; Speciation analysis; Uptake process 1. Introduction Speciation analysis is evolving into a routine procedure in analytical chemistry and is becoming essential to solve problems in many fields (e.g., environ- ment, biology, biochemistry, clinical chemistry, nutrition and toxicology), because both adverse and beneficial effects depend on the physical and/or chemical forms of compounds [1–3]. Consequently, obtaining information about each analyte species is of paramount concern, while the possibility of discriminating between ana- lyte species presents analytical chemists with interesting challenges [4–7]. Speciation analysis includes all analyti- cal procedures directed at identifying every chemical species of a sample. How- ever, the amounts of each analyte species can vary over a broad concentration range, depending on the sample type, and full information about all species is impossible to derive in the majority of situations due to the lack of sensitivity exhibited by many of the detectors used. This means that, in practice, speciation studies treat only the main species (i.e. all those species that can be detected using a particular analytical procedure). For speciation studies, the development of the main instrumental separation tech- niques has been covered well in several interesting reviews [8–13] and books [14–16]. Nonetheless, separation methodo- logies based on the use of microorganisms have more recently attracted increasing interest, as shown in several reviews [17–20]. Different whole microorganisms, such as fungi [21], yeast [22–26], algae [27–31], and organelles (erythrocytes) [32,33] have been employed as analytical preconcentrators for speciation purposes. However, bacteria have also attracted great interest in analytical chemistry for the uptake and preconcentration of metal- lic [34–40], metalloid [40] and organo- metallic [41,42] species, as well as in other fields [43–45]. We should pay particular attention to the fact that, even though most current research has been oriented towards the uptake of cation species, anion and organo-metal/loid speciation by bacterial cells has also become of growing interest. Below, we will provide reasons for con- sidering live bacterial cells as analytical tools for speciation analysis, highlighting the way in which they have been used and examining the role they can potentially play in the near future. 2. Looking for an ideal extractant There is no ideal extractant capable for the simultaneous extraction of all chem- ical species present in a sample and A.J. Aller*, M.A. Castro Department of Biochemistry, Area of Analytical Chemistry, Faculty of Biological and Environmental Sciences, University of Leo´n, E-24071 Leo´n, Spain * Corresponding author. Tel.: +34 987 291536; Fax: +34 987 29 18 83; E-mail: dbbjaf@unileon.es Trends in Analytical Chemistry, Vol. 25, No. 9, 2006 Trends 0165-9936/$ - see front matter ª 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2006.03.009 887 0165-9936/$ - see front matter ª 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2006.03.009 887