Journal of Applied Microbiology 1997, 83, 17–24 Analysis of conductance responses during depolymerization of pectate by soft rot Erwinia spp. and other pectolytic bacteria isolated from potato tubers B.A. Fraaije, M. Bosveld 1 , R.W. Van den Bulk and F.M. Rombouts 1 DLO-Centre for Plant Breeding and Reproduction Research (CPRO-DLO), Wageningen, and 1 Department of Food Science, Wageningen Agricultural University, Wageningen, The Netherlands 5906/09/96: received 6 September 1996, revised 21 October 1996 and accepted 23 October 1996 B.A. FRAAIJE, M. BOSVELD, R.W. VAN DEN BULK AND F.M. ROMBOUTS. 1997. Different bacteria isolated from potato tubers were screened for their pectolytic properties by examining pitting in polypectate agar, recording conductance responses in polypectate medium and performing potato tuber soft rot tests. For bacteria found positive in conductimetry, the role of polygalacturonase (PG) and pectate lyase (PL) in the generation of conductance changes in a polygalacturonic acid (PGA) medium was further analysed using enzyme activity staining after gel electrophoresis and high- performance anion exchange chromatography. The extent of the conductance changes during depolymerization of PGA was dependent on the amounts of galacturonate monomers and oligomers accumulated in the medium. In comparison with an unidentified saprophyte and a Klebsiella strain, both mainly having PL activity, soft rot Erwinia spp. rapidly produced larger conductance responses, due to a combined action of multiple forms of PG and PL. The responses of Erwinia spp. were initially associated with the accumulation of large amounts of monomers and saturated dimers to heptamers, due to PG activity. Subsequently, as well as monomers and saturated dimers, large amounts of unsaturated dimers were also detected, due to PL activity. The role of PG as an important conductimetric factor was also demonstrated for a pectinase preparation derived from Aspergillus niger. Besides detection, automated conductimetric assays in pectate media may also be useful for monitoring of pectolytic activity in pectinase preparations and for screening of pectolytic activity of micro- organisms under different media and growth conditions. INTRODUCTION erate significant conductance responses in pectate medium, whereas Klebsiella spp. only produced weak conductance Recently, plant pathogenic Erwinia carotovora and Erw. chry- responses. santhemi bacteria have been detected in potato peel extracts by Soft rot Erwinia spp. produce multiple isozymic forms automated conductance measurements in a pectate medium (- 3) of polygalacturonase (PG) and pectate lyase (PL) (Ried (Fraaije et al. 1996). Because of the specificity of the con- and Collmer 1986; McMillan et al. 1994), enabling the bac- ductance responses in pectate medium, the ability of Erwinia teria to split a-1,4-glycosidic linkages in low methoxylated spp. to cause conductance changes seems to be mainly associ- pectins by hydrolysis and b-elimination, respectively. In com- ated with pectolytic activity. However, other pectolytic bac- parison with Erwinia spp., the pectolytic systems of Pseudo- terial strains isolated from potato tubers, such as Pseudomonas, monas, Xanthomonas and Cytophaga spp., which have been Flavobacterium and Xanthomonas spp., were not able to gen- associated with spoilage of vegetables (Liao 1989), and those of the enterobacteria Klebsiella and Yersinia spp. (Bagley and Correspondence to: Dr B.A. Fraaije, DLO-Centre for Plant Breeding and Starr 1979) were much simpler, as only one or two different Reproduction Research (CPRO-DLO), PO Box 16, 6700 AA Wageningen, The Netherlands. forms of PL were detected. © 1997 The Society for Applied Bacteriology