Enumeration of sublethally injured Escherichia coli O157:H7 ATCC 43895 and Escherichia coli strain B-41560 using selective agar overlays versus commercial methods

Enhancing cell resistance for production of mixed microbiological reference materials with Salmonella and coliforms by freeze-drying

The reference material (RM) is a technical requirement for the quality assurance of analytical results and proficiency tests or interlaboratory comparisons. Microbiological RMs are most available in the dehydrated form, mainly by freeze-drying, and maintaining bacterial survival after preparation is a challenge. Thus, obtaining the most resistant cells is essential. Considering that bacteria present cross-response to dehydration after being submitted to an array of stress conditions, this study aimed to evaluate the influence of growth conditions on enterobacteria for the production of mixed microbiological RMs by freeze-drying in skim milk powder. Salmonella enterica serovar Enteritidis, Cronobacter sakazakii, Escherichia coli, and Citrobacter freundii were grown in a minimal medium with 0.5 M NaCl and 0 to 5.0 mM of manganese sulfate (MnSO4) until stationary phase. Salmonella Enteritidis presented an increased resistance to dehydration in the presence of Mn, while C. sakazakii was the most resistant to freeze-drying and further storage for 90 days. Mixed microbiological RMs were produced by freeze-drying and containing Salmonella Enteritidis and coliforms in skim milk powder with 100 mM of trehalose and the Salmonella survival rate was 91.2 to 93.6%. The mixed RM was stable after 30 days at -20 °C, and Salmonella and coliforms were detected by different methods being, the Rambach Agar the best for the bacterial differentiation. The results showed that the culture conditions applied in this study resulted in bacterial cells being more resistant to dehydration, freeze-drying, and stabilization for the production of mixed microbiological RMs more stable and homogeneous.

Evaluation of chlorine dioxide, acidified sodium chlorite and peroxyacetic acid for control of Escherichia coli O157:H7 in beef patties from treated beef trim

The effectiveness of 30 to 400ppm chlorine dioxide (CDO), acidified sodium chlorite (ASC) and peroxyacetic acid (PAA) to control Escherichia coli O157:H7 in beef was examined. Ground beef made from treated meat was vacuum packaged and stored at 4°C for 4d. CDO or ASC concentration by storage time interaction for inactivation of E. coli O157:H7 was significant (P<0.05). Exposure of beef to 200 and 400ppm CDO caused 0.73 and 1.25logcfu/g reduction in the numbers of pathogen, respectively, and an additional 2.08 and 2.25logcfu/g reduction, respectively, occurred at day 4. At 400ppm ASC caused a 0.87logcfu/g reduction and an additional 0.86logcfu/g inactivation at day 4. PAA caused ≤0.8logcfu/g reduction in pathogen numbers at ≤400ppm. Among tested antimicrobials, CDO was most effective and had a positive interaction with cold storage where additional E. coli O157:H7 inactivation occurred.