PREADAPTED INOCULA FOR LIMITING THE RISK OF ERRORS IN BIODEGRADABILITY TESTS

Reducing the time for biodegradability tests to 28 days poses a proble m when the inoculum contains few biodegraders, as a biodegradable xeno biotic must give a positive result within this time. The influence of initial concentration (X(0), number of cells liter(-1)) on the lag tim e (hours) of para-nitrophenol biodegradability tests was examined usin g different concentrations of adapted Pseudomonas putida with para-nit rophenol as the sole carbon and energy source. Lag time decreased as b acterial density increased according to the expression y = 63.5 - 5.7( log(10)X(0)). The addition of river water to the P. putida concentrati ons shortened the lag time-bacterial density relationship and lag time filled the expression y = 52.4 - 5.1(log(10)X(0)). The addition of ri ver water also increased the rate of para-nitrophenol biodegradation f rom 1.29 mgC (liters x hr)(-1) to 2.11 mgC (liters x hr)(-1). An exami nation of the effect of the initial adapted P. putida density, express ed as total cell, cultivable bacteria, or biodegraders, was also made on the para-nitrophenol biodegradability test outcome, The model-relat ed cell density and the probability of test response give very similar k constants (k(T) = 0.56 x 10(-3) liter total cells(-1); k(V) = 0.11 x 10(-3) liter CFU-1, k(MPN) = 0.16 x 10(-3) liter MPN(-1)). Compariso ns with nonadapted natural mixed culture (activated sludge, river wate r) indicate that the biodegradability test responses were the same as with adapted cells when the nonadapted cell concentrations were at lea st 5 x 10(10) total cells liter(-1). As this high cell concentration l ed to carbon contamination, adapting mixed inocula before the test to increase the number of biodegraders appears to be the best solution. B efore biodegradability tests, cell density can be adjusted using techn iques which are not specific to biodegraders, and only 10(5) total ada pted cells liter(-1) are needed for a 99.9% chance of a positive respo nse in para-nitrophenol biodegradability tests. (C) 1996 Academic Pres s, Inc.