Nisin-Based Organic Acid Inactivation of Salmonella on Grape Tomatoes: Efficacy of Treatment with Bioluminescence ATP Assay

ABSTRACT The antimicrobial activity of a new nisin-based organic acid sanitizer (NOAS), developed in our laboratory, was tested against viable aerobic mesophilic bacteria and Salmonella populations inoculated on produce surfaces. The activity of NOAS was compared with 200 ppm of chlorinated wash water and a bioluminescence ATP technique to determine the efficacy of treatments compared with plate count methods. The activity of the 10% final concentration of NOAS against viable populations of 109 CFU/mL Salmonella in phosphate-buffered saline (PBS), sterile deionized distilled water, and buffered peptone water was tested in vitro and on grape tomatoes inoculated with Salmonella at 2.5 log CFU/g. A similar batch of inoculated tomatoes were treated with 200 ppm of total available chlorinated water. All treatments for inactivation of viable Salmonella in vitro was tested up to 30 min and 5 min for the attached populations on tomatoes. Inactivation of viable Salmonella at 109 log CFU/mL by 10% the NOAS solution averaged >107 log CFU/mL in PBS, sterile deionized distilled water, and buffered peptone water. Similarly, Salmonella bacteria inactivated on tomato surfaces by the NOAS solution was significantly (P < 0.05) greater than numbers on chlorinated washed tomatoes, and surviving bacterial populations on NOAS and chlorine-treated tomatoes were <1 and 4 CFU/g, respectively. A significant linear correlation coefficient (r2 = 0.99) between the bioluminescence ATP assay and aerobic plate counts of inoculated and untreated grape tomatoes were recorded but not with NOAS and chlorine-treated tomatoes, as bacterial populations were less than the minimum baseline for determination. Also, the results indicated that the NOAS solution is a better alternative antimicrobial wash solution than 200 ppm of chlorinated water. HIGHLIGHTS

Specific Growth Rate Determines the Sensitivity ofEscherichia colito Lactic Acid Stress: Implications for Predictive Microbiology

This study tested the hypothesis that sensitivity ofEscherichia colito lactic acid at concentrations relevant for fermented sausages (pH 4.6, 150 mM lactic acid,aw=0.92, temperature = 20 or 27°C) increases with increasing growth rate. ForE. colistrain 683 cultured in TSB in chemostat or batch, subsequent inactivation rates when exposed to lactic acid stress increased with increasing growth rate at harvest. A linear relationship between growth rate at harvest and inactivation rate was found to describe both batch and chemostat cultures. The maximum difference in T90, the estimated times for a one-log reduction, was 10 hours between bacteria harvested during the first 3 hours of batch culture, that is, at different growth rates. A 10-hour difference in T90would correspond to measuring inactivation at 33°C or 45°C instead of 37°C based on relationships between temperature and inactivation. At similar harvest growth rates, inactivation rates were lower for bacteria cultured at 37°C than at 15–20°C. As demonstrated forE. coli683, culture conditions leading to variable growth rates may contribute to variable lactic acid inactivation rates. Findings emphasize the use and reporting of standardised culture conditions and can have implications for the interpretation of data when developing inactivation models.

Acid inactivation of prions: efficient at elevated temperature or high acid concentration

Scrapie prion rods isolated from hamster and non-infectious aggregates of the corresponding recombinant protein rPrP(90–231) were incubated with hydrochloric acid. The amount of PrP and of infectivity that survived incubation in HCl at varying times, acid concentrations and temperatures was quantified by Western blot densitometry and bioassays, respectively. Prion rods and rPrP aggregates showed similar HCl hydrolysis kinetics of PrP, indicating structural homology. For 1 M HCl and 25 °C, the rate of PrP hydrolysis follows first-order kinetics at 0·014 h−1; the rate of infectivity inactivation is 0·54 h−1. Hydrolysis for 1 h at 25 °C was only slightly proportional to HCl concentration up to 5 M, but complete loss of infectivity and PrP reduction to <2 % was observed at 8 M HCl. The temperature dependence of unhydrolysed PrP, as well as infectivity at 1 M HCl for 1 h, showed a slight decrease up to 45 °C, but a sigmoidal decrease by several orders of magnitude at higher temperatures. The slow hydrolysis of PrP and inactivation of infectivity by acid treatment at room temperature are attributed to solvent inaccessibility of prion rods and rPrP aggregates, respectively. The more effective hydrolysis and inactivation at temperatures above 45 °C are interpreted as thermally induced disaggregation with an activation energy of 50–60 kJ mol−1. Most importantly, infectivity was always inactivated faster or to a higher extent than PrP was hydrolysed at several incubation times, HCl concentrations and temperatures.