Assessment of Listeria species in fresh produce grown and sold in Canterbury, New Zealand

Abstract

Listeria monocytogenes is a member of the genus, Listeria, which is widely distributed in agricultural environments such as soil, manure and water. It is a pathogenic bacterium that causes disease including gastroenteritis, infections of the blood stream and/or the central nervous system and causes miscarriage in pregnant women. Listeria outbreaks have been regularly linked to fresh produce since the first outbreak in Boston, USA in 1979. Fresh produce, including lettuces, cabbages and carrots, are commonly used by most families as a part of their daily meals. Organic and conventional farms are the main source of fresh produce in most countries. Several investigations have focused on L. monocytogenes in fresh produce. However, research reports to compare the L. monocytogenes prevalence in fresh produce grown at organic farms vs inorganic farms are very few and there have been none from New Zealand. This masters research project aimed to investigate the prevalence of Listeria spp. (L. monocytogenes, in particular) in fresh produce grown and sold in the Canterbury region of New Zealand. In order to generate new information and to improve the understanding about the current situation of Listeria spp. and L. monocytogenes in fresh produce grown in farms, 98 samples, including 78 fresh vegetable samples, twelve soil samples and eight water samples, were collected from three different types of farms (conventional farm [A], crop-livestock rotation farm [B], and a compost farm [C]), for microbiological analysis. A range of fresh produce, particularly cabbage, spinach, pakchoi and parsley, were collected from farms, depending on the seasonal availability. Fresh produce (cabbage, carrot, cucumber and lettuce) sold in four different retail shops were also assessed over a period of six weeks. A combination of detection methods such as aerobic plate counts (APC), Listeria spp. counts using a selective agar, rapid detection kits, and a 3MTM molecular detection system were used in this study. Microbiological (Gram staining) and biochemical (carbohydrate utilisation) methods were also used to further characterise positive Listeria spp. samples. All soil samples from farms tested positive for the presence of Listeria whereas all water samples were negative. In terms of microbiological quality, all three farm types had similar APC levels. Significant differences were recorded for Listeria spp. contamination of soil and water with levels in the conventional farm > crop-livestock integration farm > compost farm. Listeria contamination of cabbage samples from Farm B (2.53±0.76 log cfu/g) was lower than in samples from Farm C (4.44±0.14 log cfu/g). Additionally, the number of colonies observed in spinach samples (5.13±0.32 log cfu/g, Farm C and 3.77±0.61 log cfu/g, Farm B) is comparatively more than parsley (4.51±0.09 log cfu/g, Farm C and 3.98±0.72 log cfu/g, Farm B) from these two farm types. The levels of Listeria spp. in cabbage and parsley were not significantly different when analysed by the Tukey test. The lowest population of Listeria spp. was recorded in the carrot samples, which had a mean of less than 1 log cfu/g. Listeria in pakchoi from Farm A (3.27±0.79 log cfu/g) > from Farm C (2.60±0.49 log cfu/g). Overall, samples from the compost farm had the highest Listeria counts and samples from the conventional farm had the least. Microbiological analysis of the 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). Carbohydrate utilitsation and 3MTM molecular detection system (MDS) analysis revealed that all Listeria positive samples were negative for L. monocytogenes. In conclusion, this is the first research undertaken for the isolation and confirmation of Listeria spp. from soil and fresh produce in Canterbury region, New Zealand. The results showed that Listeria spp. are ubiquitous in the agricultural environment. Listeria spp. contamination of fresh produce was much less than those reported in the literature for meat and dairy products. Although the L. monocytogenes was not detected in this research, the potential hazards for contaminating fresh produce by this organism still exist. L. monocytogenes is a common pathogenic bacteria that can be present and grow on fresh produce. Therefore, a systematic monitoring plan is needed to avoid an outbreak.