Hemolysis and lecithinase detection. Each strain was streaked onto 5% sheep
blood agar and B. cereus selective agar containing egg yolk and polymyxin B
(Sigma) and incubated at 37°C for 24 to 48 h to detect patterns of hemolysis and
lecithinase production, respectively.
Bacteriocin assays. A colony overlay assay was used to screen for bacteriocinlike
activity (33). All the probiotic strains tested were found to grow well on
Luria-Bertani (LB) medium. For this reason, cultures of the probiotic strains that
were to be tested for bacteriocin-like activity were incubated overnight in LB
medium. Then 5-l portions of the overnight cultures were inoculated as spots
on LB medium plates, which were incubated at 37°C for 24 h before the cells
were killed by exposure to chloroform vapor for 30 min. After exposure to air,
the plates were overlaid with LB medium or brain heart infusion soft agar
(according to the requirements of the indicator strain) that had been inoculated
with an overnight culture of an indicator strain and reincubated. The presence of
zones of growth inhibition around the spots at any of the times examined (5, 8,
24, and 48 h postinoculation) was considered a positive response. Proteinase K
treatment in colony assays of the probiotic strain Enterogermina (B. clausii) was
performed as described by Faye et al., with modifications (9). Portions (80 g
total) of a proteinase K preparation (20-mg ml1 stock solution) were applied as
spots around the producer colonies. The plates were incubated at 37°C for
approximately 2 h before the chloroform treatment and overlay. Control plates
without producer colonies were treated with proteinase K as described above.