A VERSATILE MOST PROBABLE NUMBER SYSTEM TO QUANTIFY LACZY-MARKED PSEUDOMONADS PRESENT AT LOW CELL NUMBERS IN THE RHIZOSPHERE

Quantitation of introduced genetically modified micro-organisms (GMMs) in the rhizosphere is a key issue when their release in the environment is planned. An improved most probable number (MPN) system, using a titration plate for incubation of rhizosphere extracts and two microcomputer programmes made recently available, MPNES (Woomer et al. 1990) and MPN 2.80 (Klee 1993) to generate the MPNs, is described. The MPN system was compared with colony counts to assess colonization of sugarbeet roots by an introduced lacZY-modified rhizosphere pseudomonad. The MPNs displayed wider confidence intervals compared with drop-plate counts but allowed the quantitation limit to be lowered to 2.30 log(10) cfu per root system. The MPN system proved useful to quantify GMMs present at low cell numbers in the rhizosphere of sugarbeet.