Inactivation of Escherichia coli by Ultrasound Combined with Nisin

Abstract

The aim of this study was to investigate the inactivation of nonpathogenic Escherichia coli in nutrient broth and milk through the use of either ultrasound (US) alone or US combined with nisin (US + nisin) treatments. The E. coli cells were treated at 0 to 55°C, 242.04 to 968.16 W/cm for 0 to 15 min. The results showed that the inactivation of E. coli by US and US + nisin increased when the temperature, US power density, and treatment time were increased. The inactivation kinetics of E. coli in nutrient broth by US and US + nisin both conformed to linear models. The largest reductions of 2.89 and 2.93 log cycles by US and US + nisin, respectively, were achieved at 968.16 W/cm and at 25°C for 15 min. The suspension media of the E. coli cells influenced the inactivation effect of US, while the growth phases of E. coli cells did not affect their resistance to US. Under all experiment conditions of this study, the differences between US and US + nisin in their respective inactivation effects on E. coli were not obvious. The results suggested that nisin had either no effect at all or a weak synergistic effect with US and that the E. coli cells were inactivated mainly by US, thus indicating that the inactivation of E. coli by US is an "all or nothing" event.

The effect of ultrasound treatment in combination with nisin on the inactivation of Listeria innocua and Escherichia coli

“…3 c). Very few studies have investigated the combination of nisin with ultrasound [11] , [17] , [67] , [68] : none of these studies identify on a microscopic level the combined effects of both treatments. This is the most effective treatment in this system, despite the fact that nisin alone does not affect E. coli cells ( Fig.…”

“…The mechanism for microbial inactivation by ultrasound is generally accepted to be as a consequence of ultrasound transient cavitation (for example, [6] , [7] , [8] , [9] ). More specifically, microorganism inactivation can be attributed to cell membrane damage from extreme temperature hot-spots or high pressures from bubble collapse, which can also further enhance damage by reactive species [10] , [11] . High shear forces on the cell membrane as a result of microstreaming can also cause damage [12] , [13] , [14] .…”

“…The main inactivation pathway by nisin results from pore formation in the cell wall following the binding of nisin to Lipid II in Gram positive bacteria. Nisin is not effective against Gram negative bacteria, due to the different structure of the cell wall [11] , [21] . We propose that ultrasound combined with nisin treatments may result in activity of nisin against Gram negative species, as ultrasound could disrupt the cell wall and allow for nisin penetration.…”

“…The order in which treatments are applied could impact their efficacy and the microbial response, depending on the target in the cell i.e. a treatment which damages the outer membrane followed by one which acts on internal components of the cell could result in enhanced inactivation [11] , [23] . However, both ultrasound and nisin treatments, alone or combined, like other minimal processing approaches, are milder than classical decontamination methods and may therefore present a mild, sublethal stress allowing for post-treatment survival and potential antimicrobial resistance (AMR) development [5] , [24] , [25] , [26] , [27] , [28] .…”

“…The system viscosity also varies significantly between these studies, which could impact the treatment efficacy. Furthermore, to the authors’ best knowledge, very few studies investigate the combination of ultrasound and nisin for microbial inactivation, with the available studies conducted for different experimental set-ups, systems, frequencies, powers and species [11] , [17] , [67] , [68] . As such, a fundamental systematic study of the effect of system viscosity on microbial inactivation by ultrasound, at a range of frequencies and in combination with natural antimicrobials i.e., nisin, is lacking.…”