Inoculation density is affecting growth conditions of Listeria monocytogenes on fresh cut lettuce

Abstract

Listeria monocytogenes is a particular risk for the ready-to-eat food sector because of its ability to grow in various environmental conditions. In the literature, growth and survival of L. monocytogenes on food is tested using inoculation densities ranging from less than 10 to over 10 CFU g. Inoculation densities on food have been rarely tested as a factor for growth. In this study, inoculation densities from 10 to 10 of L. monocytogenes were tested on iceberg lettuce (Lactuca sativa) in modified atmospheres and air in model packages at 4 and 8 °C to identify any potential inoculation density effects. On days 0, 2, 5 and 7, L. monocytogenes was extracted from the lettuce surface and enumerated via selective media. The resulting growth curves identified a significant inoculation density effect at 4 and 8 °C with significantly higher amounts of growth (1-2 logs) when lettuce was inoculated at 10 CFU g as opposed to 10 and 10 CFU g. In contrast, the use of different atmospheres had limited influence on growth of L. monocytogenes. In conclusion, greater emphasis on inoculation density of L. monocytogenes should be taken in inoculation experiments when confirmation of growth or the efficacies of growth inhibiting treatments are tested on ready-to-eat food such as lettuce.

Growth Potential of Listeria monocytogenes on Refrigerated Spinach and Rocket Leaves in Modified Atmosphere Packaging

“…This was repeated for the necessary number of bags prior to inoculation (quadruplicates for five sampling dates, eight bags as non-inoculated controls). Using the previously prepared L. monocytogenes dilutions, 100 µL of L. monocytogenes suspension (representing 10 or 100 cfu g −1 of food product) was distributed uniformly over the 20 g of leafy vegetable product within the polypropene bags (eight control bags were treated with 100 µL sterile PBS [14]). Each packaging containing 20 g sample of vegetable product was then atmospherically treated (8 kPa CO 2 , 4 kPa O 2 , 88 kPa N 2 ) using a vacuum packer (Multivac, Dublin, Ireland).…”

“…Storage temperatures were checked daily. Environmental conditions were identical to previous growth studies with iceberg lettuce [14,25].…”

“…Aliquots of 100 µL were then plated on Chromocult Listeria selective agar (ALOA) containing Listeria selective supplement (both Merck, Darmstadt, Germany). The plates were incubated at 37 • C for 24-48 h. Colony forming units (cfu) on days 0, 2, 5, 7, and 9 were transformed into log 10 cfu g −1 , mean values and standard deviations were determined and plotted, areas under the curve were determined [14], and median values were used to calculate growth potentials. Maximum growth rates were calculated as outlined in Appendix A.…”

“…Storage period and temperature are important factors influencing the growth and survival of L. monocytogenes in foods such as RTE salads [13]. However, recent challenge studies also identified that inoculation densities for testing affect the outcomes of challenge studies as lower initial inoculation densities (100 cfu g −1 ) may lead to greater growth potentials during shelf life [14]. Despite the possible underestimation of growth potential, assessing growth potentials at high inoculation densities (i.e., 10 5 cfu g −1 ) in RTE food remains popular [15].…”

“…At the outset of the present study, our hypothesis was that the growth potential of L. monocytogenes on spinach and rocket is similar to that of iceberg lettuce. As the growth potential of L. monocytogenes on lettuce is well established [14], the consumption of raw spinach and rocket in salads could pose a potential risk for human infection if it is contaminated with low levels of L. monocytogenes.…”