A Pulsed Electric Field (PEF) bench static system to study bacteria inactivation

Inactivation of Escherichia coli and Staphylococcus aureus by pulsed electric fields increases with higher bacterial population and with agitation of liquid medium

Inactivation of Escherichia coli, E. coli O157:H7, and Staphylococcus aureus in liquid media by pulsed electric fields (PEF) was conducted at varying bacterial populations with and without sample agitation. A laboratory-scale PEF batch unit with a rectangular electric pulse was used, operating under the following conditions: 25 kV/cm (E. coli, E. coli O157:H7) and 30 kV/cm (S. aureus) electric field strengths, 1-μs pulse width, 1-Hz pulse repetition rate, and 20 to 350 pulses for all samples. Not surprisingly, bacterial inactivation (for all three strains) increased with increasing pulse number, achieving the highest reduction at 350 pulses. Log CFU per milliliter microbial inactivation increased commensurately with increasing bacterial population (P < 0.05) but only when samples were treated with more than 200 pulses. For example, when E. coli was treated with 200 pulses at 10(5) CFU/ml, inactivation was only 3.0 Log versus 4.8 Log at the 10(10) inoculation level. When E. coli O157:H7 was treated with 200 pulses at 10(5) CFU/ml, inactivation was only 2.5 Log versus 4.6 Log at the 10(10) inoculation level. When S. aureus was treated with 200 pulses at 10(6) CFU/ml, inactivation was only 2.6 Log versus 4.8 Log at the 10(10) inoculation level. Inactivation of populations was also found to be statistically greater (P < 0.05) when liquid samples were agitated, in comparison to nonagitated samples. Because PEF inactivation activity is influenced by bacterial population and sample agitation, future studies should carefully consider these factors in experimental designs and/or scaled-up industry application.

Size of the pores created by an electric pulse: microsecond vs millisecond pulses

Here, the sizes of the pores created by square-wave electric pulses with the duration of 100 μs and 2 ms are compared for pulses with the amplitudes close to the threshold of electroporation. Experiments were carried out with three types of cells: mouse hepatoma MH-22A cells, Chinese hamster ovary (CHO) cells, and human erythrocytes. In the case of a short pulse (square-wave with the duration of 100 μs or exponential with the time constant of 22 μs), in the large portion (30-60%) of electroporated (permeable to potassium ions) cells, an electric pulse created only the pores, which were smaller than the molecule of bleomycin (molecular mass of 1450 Da, r≈0.8 nm) or sucrose (molecular mass of 342.3 Da, radius-0.44-0.52 nm). In the case of a long 2-ms duration pulse, in almost all cells, which were electroporated, there were the pores larger than the molecules of bleomycin and/or sucrose. Kinetics of pore resealing depended on the pulse duration and was faster after the shorter pulse. After a short 100-μs duration pulse, the disappearance of the pores permeable to bleomycin was completed after 6-7 min at 24-26°C, while after a long 2-ms duration pulse, this process was slower and lasted 15-20 min. Thus, it can be concluded that a short 100-μs duration pulse created smaller pores than the longer 2-ms duration pulse. This could be attributed to the time inadequacy for pores to grow and expand during the pulse, in the case of short pulses.