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QUAFETY Scientists determine the prevalence and the levels of bacterial pathogens in rocket and cucumberQuafety Scientists from the University of Athens (Greece) and University of Milano (Italy) have determined the prevalence and levels of Listeria monocytogenes and Escherichia coli O157:H7 in rocket and cucumber samples by deterministic (estimation of a single value) and stochastic (estimation of a range of values) approaches. In parallel, the chromogenic media commonly used for the recovery of these microorganisms were evaluated and compared, and the efficiency of an enzyme-linked immune-sorbent assay (ELISA)-based protocol was validated.
The study shows that:
1. in rocket, prevalence of both L. monocytogenes and E. coli O157:H7 was estimated at 7% (7 of 100 samples);
2. in cucumber, prevalence was 6% (6 of 100 samples) and 3% (3 of 100 samples) for L. monocytogenes and E. coli O157:H7, respectively.
The levels derived from the presence-absence data using Bayesian modelling were estimated at 0.12 CFU/25 g (0.06 to 0.20) and 0.09 CFU/25 g (0.04 to 0.170) for L. monocytogenes in rocket and cucumber samples, respectively. The corresponding values for E. coli O157:H7 were 0.59 CFU/25 g (0.43 to 0.78) and 1.78 CFU/25 g (1.38 to 2.24), respectively. The sensitivity and specificity of the culture media differed for rocket and cucumber samples.
Scientists conclude that the ELISA technique had a high level of cross-reactivity. Parallel testing with at least two culture media was required to achieve a reliable result for L. monocytogenes or E. coli O157:H7 prevalence in rocket and cucumber samples.
Source: Hadjilouka A., Mantzourani K.-S., Katsarou A., Cavaiuolo M., Ferrante A., Paramithiotis S., Mataragas M., Drosinos E.H., "Estimation of Listeria monocytogenes and Escherichia coli O157:H7 prevalence and levels in naturally contaminated rocket and cucumber samples by deterministic and stochastic approaches", February 2015, Journal of Food Protection, Vol. 78 (2), Pages 311-322.
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