Most probable bacterial composition in frozen vegetables

A commercial “concentrating pipette” was scrutinized from the perspective of sample volume reduction to augment native bacterial quantitation and identification within food samples. Aspects related to the elution protocol were evaluated for their impact on bacteria recovery. The results demonstrated there was no significant effect from different diluents/eluents on the relative recovery rate.

Our test E. coli strain was routinely grown overnight in either Luria–Bertani (LB) broth or BHI and enumerated on solid media (same broth with 2% [w/v] agar) at 30 ◦C. All work with naturally occurring bacteria on pre-thawed frozen vegetables was performed using BHI broth and enumerated on BHI solid media grown at 30 and 37 ◦C. Two temperatures were used for bacterial growth inasmuch as any pathogens present would be more likely to show up from the background at 37 ◦C

Further, results from a complementary technique, dynamic light scattering, imply that recoveries less than unity are likely due to an inability to completely elute the bacterial cells from the capillary filter as opposed to bacterial injury caused by sample processing. When concentrating PBS washes from thawed frozen vegetables and enumerating after culturing at either 30 or 37 ◦C, the recovery rates were only slightly lower than the simplified experimental conditions.

However, in food wash samples, significantly more mass-normalized bacteria were recovered at the lower culture temperature. Nevertheless, the vegetable wash concentration process did not change the most probable isolate composition relative to that of the controls at either culture temperature.

The study demonstrates the potential for the system to serve as an effective tool for bacterial recovery and concentration within water-based food systems such as equipment rinses, process wash water, and other samples coming from food processing plants.

Following the measurement of each experimental sample, the unit was cleaned. The cleaning cycle entailed circulating 125 mL of diluent through the system for 2 min, and then completely draining the rinse water from the system.

All washing was performed in ethanol-rinsed, dried and UVirradiated plastic zip-lock bags after which the food matrix and buffer wash was gently agitated at 80 rpm for approximately 20 min and immediately passed through a 40 μm nylon cell strainer (BD Falcon; Becton Dickinson Biosciences, Bedford, MA) to remove large food particles. The food washes typically had a slightly green hue.

In this work, we measured the performance of a 'concentration pipette' equipped with a 0.2 μm polysulfone hollow fibre filter 'tip' in terms of reduced sample volume to improve quantitative metagenomics studies of indigenous foodborne bacteria. The results showed the relative recovery of bacteria in the E. coli spiking buffer and the perturbation of bacterial composition due to the concentration process.