91E-15

UVC effects were determined after different exposure times on the growth response of selected bacteria (Escherichia coli, Listeria innocua), yeast (Saccharomyces cerevisiae, Zygosaccharomyces bailii, Debaryomyces hansenii), and mold spores (Aspergillus flavus, Penicillium digitatum) individually inoculated on tripticase soy or potato dextrose agars.

Bacterial and yeast inocula were prepared by growing cells in appropriate broth, incubated at 35 or 27ºC, for 24 or 48 h, respectively. Mold spores were recovered from inoculated slants by washing the culture surface with sterile water. Agar plates were prepared for every microorganism and 20 replicates (5 m L each) were inoculated. Inoculated (more than 5.0 x 10³ CFU or spores/mL) plates were placed at 10 cm from the UV-lamp (30 W, 254.3 nm), and exposed from 0.5 to 10 min. UVC-doses varied from 0.15 to more than 150 J cm^-2. Control and UVC-treated plates were incubated up to120 h and observed for colony formation.

Depending on microorganism type a minimal UV-dose that inhibit microbial growth was found, increasing UVC-dose confirmed microbial inactivation (no growth). A. flavus was the most resistant organism to UVC. P. digitatum spores were inactivated with 22.5 J cm^-2. Yeast species were sensitive to UVC, and less than 10.0 J cm^-2 were needed to inactivate them. E. coli was inactivated with 90.0 J cm^-2 while 135.0 J cm^-2 were needed to inactivate L. innocua.

Our results encourage further studies to elucidate the effect of UVC as surface disinfectant and demonstrate a wide spectrum microbial inactivation effect at UV-doses between 135 and 150 J cm^-2.