Detection of Listeria Species in Fresh Produce Samples from Different Retail Shops in Canterbury, New Zealand

*Corresponding author: Qi Zhu*, Ravi Gooneratne and Malik A. Hussain

Author Affiliations

Department of Wine, Food and Molecular Biosciences, Lincoln University, Lincoln, New Zealand

Citation

Zhu Q, Gooneratne R, Hussain MA. Detection of Listeria species in fresh produce samples from different retail shops in Canterbury, New Zealand. Adv Food Technol Nutr Sci Open J. 2016; 2(3): 96-102. doi: 10.17140/AFTNSOJ- 2-135

Copyright

©2016 Hussain MA. This is an open access article distributed under the Creative Commons Attribution 4.0 International License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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original research

Abstract

Aim: This study investigates the prevalence of Listeria spp. in fresh produce sold in the Canterbury region of New Zealand.
BackgroundL. monocytogenes is a common pathogenic bacterium that can be present and grow on fresh produce. Therefore, systematic risk assessment is needed to avoid a food safety scare.
Results: Microbiological analysis to detect Listeria spp. in fresh produce samples showed that lettuce from the retail shops had the highest level (4.2 log cfu/g) >cucumber (3.2 log cfu/g) >cabbage (2.5 log cfu/g) >carrot (nil). In terms of Listeria prevalence, carrot samples had the lowest Listeria spp. load (<1.05 log cfu/g) and lettuce samples had the highest (>4 log cfu/g). None of the samples showed a positive result for L. monocytogenes presence.
Conclusion: Although the L. monocytogenes was not detected in this study, the potential hazard for contaminating fresh produce by this organism still exist due to high prevalence of Listeria spp.

Introduction

Fresh produce is transported from farm to distribution centres and then to retail shops. It is a general practice that fresh vegetables are washed in water before they are supplied to retailers. After washing, fresh produce commodities are packed and transferred to different shops by different food chains. At any of these steps a careless omission would give Listeria monocytogenes a chance to contaminate fresh vegetables, such as during storage conditions (including temperature, moisture, and oxygen content) and/or sanitation conditions during packing processes and transport. At the end of 2014, a listeriosis outbreak related to caramel apples occurred, which led to 35 people being infected by L. monocytogenes.1 After testing, L. monocytogenes was detected in the apple-packing factory.

Most fresh produce is pre-treated, i.e. by ultraviolet (UV),2 washing3 or chlorine disinfection4 before being sold. However, Listeria spp. and L. monocytogenes were found in fresh produce from retail shops; for example, in pre-packaged mixed vegetable salads,5 parsley6 and lettuces.7

Fresh vegetables are consumed by almost all families. According to a local report,8 in 2013, local consumers spent NZ$ 41.8 million on fresh lettuces and NZ$ 30 million on fresh carrots. New Zealand produced nearly 1.4 million tonnes of vegetables, while 500,000 tonnes were exported and over 900,000 tonnes were consumed locally. Therefore, to understand the safety of fresh produce in retail shops is at least as significant as that in farms. The research described in this article investigated the prevalence of Listeria spp. in selective salad vegetables from different retail shops in the Canterbury region of South Island, New Zealand.

Results and Discussion

Comparsion of Microbial Levels by Different Retail Markets

Market A was an open market that operated each Sunday. Four types of vegetable samples were collected over a 6-week period to compare the total microbial counts and the presence of Listeria spp. Data was statistically analysed using the Tukey test. The total microbial populations in the cabbage, carrot and lettuce were not significantly different and the mean counts were around 7 log cfu/g. The cucumber samples had the lowest microbial count (4.5 log cfu/g). The Listeriaspp. detected in 4 different types of fresh produce samples were significantly different (p<0.05). The highest population of Listeria spp. was detected in lettuce (4.5 log cfu/g), followed by cucumber (about 3 log cfu/g; mean). The Listeria spp. count was the 2nd lowest load in cabbage (1.5 log cfu/g). The Listeria spp. contamination was lowest in carrot with less than 1 log cfu/g (Figure 1).

Market B was an outlet from a supermarket chain, which had constant suppliers providing fresh vegetables. Four types of vegetables were sampled over the 6-week collection period. Therefore, it was not suitable to compare the microbial levels and Listeria spp. levels for these 4 types of vegetable samples from Market B. The results were similar to as observed

Figure 1: Comparative microbial analysis of fresh produce samples from Market A. Error bars represent standard deviations of the means. Different letters on each bar show that the results have significant differences using the Tukey test, p<0.05.

for Market A, where cabbage, carrot and lettuce belonging to the same group were contaminated and a range between 6 to 7 log cfu/g was recorded. In contrast, the APC value of cucumbers was lower (3 log cfu/g). The level of Listeria spp. in lettuce was the highest mean (4 log cfu/g) among the 4 types of vegetables from Market B. The second highest mean population of Listeria spp. was in cabbage (2.5 log cfu/g) followed by cucumber (2 log cfu/g). The population of Listeriaspp. in carrot was the lowest (1 log cfu/g) (Figure 2).

Market C was an ethnic retail shop that sold a variety of commodities, including fresh produce. Four types of vegetable samples were collected over six weeks to test for total microbial counts and the presence of Listeria spp. There were no significant differences (p>0.05) in the total plate counts of the fresh produce samples sold in Market C (Figure 3). The mean APC values of the fresh produce samples in Market C ranged from 5.5 to 7 log cfu/g. Leafy vegetables (cabbage and lettuce) in Market C had similar Listeria spp. contamination levels when analyzed by the Tukey test. However, the Listeria spp. levels in lettuce (4 log cfu/g) were higher compared to those in cabbage (3 log cfu/g), followed by those in cucumber (3 log cfu/g). Carrot contained the lowest of Listeria spp. population (less than 1 log cfu/g) (Figure 3).

Market D was a fruit and vegetable retail shop outlet where fresh produce was the major commodity sold. Four different types of vegetable samples were collected to evaluate the microbiological quality and the presence of Listeria spp. contamination during 6-weeks of sampling. Statistically significant differences were recorded for total plate counts using the Tukey test. The microbial populations were highest in the cabbage and lettuce samples (7 log cfu/g). The population of total microbes in carrots (6.5 log cfu/g) was slightly lower and significantly different from cabbage and lettuce (7 log cfu/g) (p=0.05). The APC in cucumber were lowest in the samples from Market D (fewer than 5 log cfu/g). In contrast, the levels of Listeria spp. in the cabbage and lettuce were higher than in other types of vegetables. No Listeria spp. were detected in carrot samples from Market D (Figure 4).

Comparsion of Microbial Levels by Fresh Produce Type

The microbial quality (Listeria spp. and APC) of fresh produce was compared between four retail shops. Statistical analysis (Tukey test) of the microbial analyses showed that, the fresh produce samples were not significantly different (p>0.05) among the retail shops selected (Figure 5). The mean Listeria spp. levels in the cabbage samples ranged from 1.5 to 3 log cfu/g (whereas APC values were about 7 log cfu/g). Listeria spp. levels in the

Figure 2: Comparative microbial analysis of fresh produce samples from Market B. Error bars represent standard deviations of the means. Different letters on each bar show that the results have significant differences using the Tukey test, p<0.05.

Figure 3: Comparative microbial analysis of fresh produce samples from Market C. Error bars represent standard deviations of the means. Different letters on each bar show that results have significant differences using the Tukey test, p<0.05.

Figure 4: Comparative microbial analysis of fresh produce samples from Market D. Error bars represent standard deviations of the means. Different letters on each bar show that results have significant differences using the Tukey test, p<0.05.

carrot samples ranged from 0 to 1 log cfu/g and APC values ranged from 6 to 7 log cfu/g. Listeria spp. levels in the cucumber samples ranged from 1.5 to 3.5 log cfu/g (APC levels ranged from 2.5 to 6 log cfu/g). For lettuce samples, Listeria spp. levels were about 4 log cfu/g (and APC levels were about 7 log cfu/g).

Overview of Listeria spp. in Fresh Produce Samples from Retail Shops

In general, the average APC values were similar for cabbage, carrot and lettuce samples collected from the four retail shops (Figure 6A). Similarly the samples from the farms, the APC values were between 6 to 7 log cfu/g. The mean APC of the carrot samples from Market B had the lowest value, which was fewer than 6 log cfu/g. On average, lettuce samples from Market A and Market D had comparatively higher APC (>7 log cfu/g). The cucumber samples had the lowest microbial load, ranging from 2.72 to 5.68 log cfu/g. However, a research report on the microbiological analysis of salad vegetables in markets in India9 reported an average APC of 9 log cfu/g. The Listeria spp. counts for fresh produce from retail shops are presented in Figure 6B. The majority of APC values in cabbage and cucumber were distinguished in the same group using the Tukey test. Carrot samples had the lowest Listeria spp. load (<1.05 log cfu/g) and lettuce samples had the highest Listeria spp. load (4 log cfu/g). There were no significant differences in the same type of fresh produce among the four retail shops. Other research from Dhaka, which evaluated the prevalence of bacteria in salad veg

etables,10 reported much higher Listeria spp. load (6 to 8 log cfu/g) than the results of this study.

Figure 5: Comparative microbial analysis of fresh produce samples from Market A to D. (A) cabbage (B) carrot (C) cucumber (D) lettuce. Error bars represent standard deviations of the means. Different letters on each bar show that results have significant differences using the Tukey test, p>0.05.

 

The results of this study showed that cabbage and lettuce had similar levels of total microbial loads. However, the contamination of Listeria spp. in lettuce was higher than in cabbage. The total microbial load in carrot samples was higher in the cucumber samples while the prevalence of Listeria spp. was higher in the cucumber samples. Overall, all 4 different types of vegetable samples had similar microbiological quality regardless of the market source. However, some variations were observed in the samples when analyzed on a weekly basis. L. monocytogenes was not detected in the fresh produce samples tested. However, ongoing risk assessment activities are required to ensure fresh produce safety.

 

Figure 6: Comparative microbial analysis of samples from four retail shops. A, APC and B, Listeria spp. Error bars represent standard deviations of the means. Different letters on each bar show that results have significant differences using the Tukey test, p<0.05.

monocytogenes, and lettuce and other fruit and vegetable samples were negative for this bacterium.14 In another microbial assessment of fresh produce, L. monocytgenes was found in 3.4% of lettuce samples (≤2 log cfu/g).15 The prevalence of pathogens in fresh produce reported by Rahman and Noor,10 the population of Listeria spp. on fresh produce (cucumber, carrot, lettuce) ranged from 6 log cfu/g to 8 log cfu/g, which was much higher than results reported here. In a previous study, Listeria spp. were commonly detected in cabbage and lettuce samples.16

Sample collection from retail shops did not seem to be influenced by seasonal changes. Therefore, all fresh produce from retail shops were collected over 6-weeks. Microbial quality may be influenced by storage conditions, including temperature, relative humidity (RH), O2 and CO2 contents, storage time and surfaces touched.

A group of scientists completed research on the effect of storage temperature and RH on the growth of L. monocytogenes17 They examined lettuce leaves, parsley leaves and cucumber epidermis samples at 3 temperatures and at two RH values. L. monocytogenes was detected in the cucumber epidermis samples. The results showed that L. monocytogenes preferred higher temperature and RH. In addition, L. monocytogenes had difficultly surviving on intact vegetable surfaces.17

Francis et al18 studied the effect of atmosphere and acid adaptation on growth of L. monocytogenes using intact lettuce leaves to prepare lettuce agar to monitor the growth of L. monocytogenes. Lettuce agar plates with L. monocytogeneswere stored separately under 3 conditions (a. air; b. 10% CO2, 5% O2, 85% N2; and c. 25% CO2, 75% N2) at 8 oC. Microbial levels were tested over the storage period. The results showed that the vitality of the Listeria spp. was improved when samples were stored under 25% CO2 conditions, and the most optimal condition for vegetable packing was in a 5 to 10% CO2 content atmosphere.18

In addition, in a research report on storage temperatures, L. monocytogenes growth in fresh produce at 7 °C and 15 °C for 6 days was compared.19 The result showed that the population of L. monocyogenes increased more at 15 °C over the storage period. Similar findings have been reported on the effect of storage conditions on L. monocytogenes growth at 3 different temperatures (4 °C, 12 °C and 22 °C) for comparison.20 L. monocytogenes survived better at higher temperatures with an increase in cell count of 0.3 log cfu/g in the first 17 h of storage.

 

Conclusion

The results of this study showed that cabbage and lettuce had similar levels of total microbial loads. However, the contamination of Listeria spp. in lettuce was higher than in cabbage. The total microbial load in carrot samples was higher in the cucumber samples while the prevalence of Listeria spp. was higher in the cucumber samples. Overall, all 4 different types of vegetable samples had similar microbiological quality regardless of the market source. However, some variations were observed in the samples when analyzed on a weekly basis. L. monocytogenes was not detected in the fresh produce samples tested. However, ongoing risk assessment activities are required to ensure fresh produce safety.

 

Conflicts of interest

The authors declare that they have no conflicts of interest.