Microbiological investigation of Raphanus sativus L. grown hydroponically in nutrient solutions contaminated with spoilage and pathogenic bacteria

The survival of eight undesired (spoilage/pathogenic) food related bacteria (Citrobacter freundii PSS60,Enterobacter spp. PSS11, Escherichia coli PSS2, Klebsiella oxytoca PSS82, Serratia grimesii PSS72, Pseudomonasputida PSS21, Stenotrophomonas maltophilia PSS52 and Listeria monocytogenes ATCC 19114T) was investigatedin mineral nutrient solution (MNS) during the crop cycle of radishes (Raphanus sativus L.) cultivated in hydroponicsin a greenhouse. MNSs were microbiologically analyzed weekly by plate count. The evolution of thepure cultures was also evaluated in sterile MNS in test tubes. The inoculated trials contained an initial totalmesophilic count (TMC) ranging between 6.69 and 7.78 Log CFU/mL, while non-sterile and sterile controltrials showed levels of 4.39 and 0.97 Log CFU/mL, respectively. In general, all inoculated trials showed similarlevels of TMC in MNS during the experimentation, even though the levels of the inoculated bacteria decreased.The presence of the inoculums was ascertained by randomly amplified polymorphic DNA (RAPD) analysis appliedon the isolates collected at 7-day intervals. At harvest, MNSs were also analyzed by denaturing gradientgel electrophoresis (DGGE). The last analysis, except P. putida PSS21 in the corresponding trial, did not detectthe other bacteria, but confirmed that pseudomonads were present in un-inoculated MNSs. Despite the highcounts detected (6.44 and 7.24 CFU/g), only C. freundii PSS60, Enterobacter spp. PSS11 and K. oxytoca PSS82were detected in radishes in a living form, suggesting their internalization.