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Psychrotrophic bacterial pathogens in minimally processed lettuce

As discussed in the May issue of Food Safety & Hygiene, changing lifestyle patterns and the increased consumption of healthier foods has increased the demand for freshly prepared, minimally processed salads An example is minimally processed, cut and packaged lettuce. This is a product that is exposed to a range of conditions during growth, harvest, distribution and processing and these conditions may increase the potential for microbial contamination. Some of the potential contaminants are psychrotrophic bacterial pathogens (such as Listeria monocytogenes, Yersinia enterocolitica and Aeromonas hydrophila).

These pathogens can cause a range of disease manifestations in humans. The diseases caused by L. monocytogenes infection vary from a non-specific flu-like illness to sepsis or meningitis, which can be fatal. Those most at risk, particularly for the more severe disease manifestations, include the elderly, foetuses during the third trimester of pregnancy, and the immunocompromised. For Y. enterocolitica, illness usually presents as a self-limiting enterocolitis; some long-term complications such as arthritis have been reported. While the role of Aeromonas as a cause of foodborne disease is debatable, Aeromonas species (Aer. hydrophila, Aer. caviae, Aer. veronii biovars sobria and veronii, Aer. jandaii, Aer. schubertii, Aer. trotu) can cause gastrointestinal and wound infections.

Each of the pathogens noted above are capable of growing at the chill temperatures that the minimally processed food industry relies upon to maintain product quality post-harvest. This coupled with the high moisture content, the absence of a terminal lethality treatment, the potential for temperature abuse of these commodities, and the use of packaging regimes to extend shelf-life, may provide the condition and the time for the survival and growth of psychrotrophic pathogens.

A survey of Australian products

In Australia, there is limited microbiological data on minimally processed, cut and packaged lettuce. For this reason, Food Science Australia in collaboration with the CRC for International Food Manufacture and Packaging Science conducted a survey of this commodity for the occurrence of potentially pathogenic psychrotrophic bacterial species.

A total of 120 packaged, trimmed and cut lettuce samples were purchased from retail supermarkets or provided by a salad production facility over an eight month period (October-May). All samples were tested for total aerobic plate counts and for the presence of potentially pathogenic species belonging to the genera of Listeria, Aeromonas and Yersinia.

As reported in Letters in Microbiology 30 2000 456-60 most lettuce samples (76%) contained between 105-107 colony forming units (cfu) g-1 aerobic bacteria. The aerobic plate counts were not influenced by the time of testing. For example, counts on factory samples, which were tested within 24 hours of processing, ranged from 103-107 cfu g-1. All samples tested appeared organoleptically acceptable as determined by subjective visual and odour inspection, even those samples with apparent high counts.

Potentially pathogenic psychrotrophic bacterial species were detected in lettuce samples. L. monocytogenes was isolated from 3 samples, A. hydrophila or A. caviae from 66 samples, and Y. enterocolitica from 71 samples. Comparable frequencies of isolation have been reported in similar studies undertaken by other research groups in France, England and Germany.

Determining the pathogenic potential

The predominance of Y. enterocolitica and Aeromonas spp. in packaged lettuce suggested that the product provided an environment that favoured their survival. It was possible to determine the pathogenic potential of the Y. enterocolitica isolates by screening for an array of biochemical, serological and genetic traits. The Y. enterocolitica isolates lacked many of the phenotypic and genetic markers associated with virulence in primary pathogenic strains. As the roles of the reputed virulence factors of Aeromonas spp. in human infection are uncertain, the pathogenic potential of the Aeromonas isolates in lettuce remains unclear.

Of the psychrotrophic bacterial pathogens detected in lettuce during the survey, L. monocytogenes has presented the greatest concern to both the food industry and regulatory agencies since the early 1980s. Although L. monocytogenes was detected at a much lower frequency than Y. enterocolitica and Aeromonas spp. in the packaged lettuce, its presence required further investigation.

Can Listeria grow in the product?

Given the ubiquitous distribution of L. monocytogenes, it will be difficult to eliminate it from a product like minimally processed lettuce. In order to assess more fully the risks associated with its presence in lettuce, we studied its growth potential in packaged lettuce using bacterial challenge testing.

We found that at 4°C, increases in the number of L. monocytogenes were less than 10-fold after storage for 7 days and were less than 100-fold after 14 days storage. The oxygen content of the packages ranged from 0-13%. At 8°C (temperature abuse conditions), L. monocytogenes increases were no greater than 100-fold after 7 days and approached 1000-fold increases after 14 days. The oxygen content of temperature abused packaged lettuce ranged from 0-11%.

Implications for industry

The possibility of contamination and growth of psychrotrophic foodborne pathogens is of concern for the safety of minimally processed, packaged and cut lettuce. The key to minimising health risks associated with the presence of the psychrotrophic pathogens considered in our study are (i) strict adherence to good manufacturing practices and sanitation; (ii) effective temperature control during storage and distribution and (iii) limiting the shelf-life expectancy with a 'use-by-date' that does not allow sufficient time for pathogens to grow to elevated levels.

For further information contact:
Dr Lisa Szabo
Food Science Australia, Sydney
Telephone 02 9490 8356

Food Safety and Hygiene
Prepared by Keith Richardson and Beverley George
Food Science Australia
PO Box 52, North Ryde 1670. Tel +61 2 9490 8397 Fax +61 2 9490 8499
Email enquiries@csiro.au