Quafety 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.
http://www.ingentaconnect.com/content/iafp/jfp/2015/00000078/00000002/art00010