Emerging Pathogens – STEC, Salmonella, Vibrio and Protozoa in food of plant and animal origin

STEC: Shigatoxin-forming E. coli

The EHEC outbreak in 2011 showed us again that there is a risk of infection by pathogens through the consumption of raw plant products. In general, STEC/VTEC must be considered when producing or using plant raw materials such as raw vegetables, sprouts, herbs close to the ground, grasses, salads but also cereal meals. This applies in particular if the raw materials could come into contact with the pathogens a) directly via ruminants as the main reservoir or b) indirectly via their pastures.

The detection of STEC pathogens is usually carried out by PCR detection of the gene(s) for the shiga toxin(s) followed by life detection. The assessment is carried out according to the ALTS recommendations for the classification of germs of concern as probably harmful in the sense of Regulation (EC) No 178/2002 (in German). There are no numerical limits, but rather zero tolerance, because the pathogen can multiply in the intestine depending on its adhesion capacity. 

If not already a legal obligation (e.g. sprouts) and in order to ensure the greatest possible consumer safety, inclusion of STEC/VTEC in monitoring and approval programmes should be established for the above products. The EU recommendations for testing plant-based ready-to-eat foods (Regulation (EC) No 2073/2005) include Salmonella, Listeria monocytogenes and STEC/VTEC.

A classification of the terms STEC/VTEC /EHEC as well as information on the occurrence and detection of STEC in cereal flours and raw dough can be found in our article Focus on EHEC - detection of STEC in foods of plant origin and of animal origin.

Salmonella

This bacterial genus, with its more than 2.000 different serotypes, is still present in the food chain as a zoonotic pathogen. The number of reports of human diseases is now quite constant from year to year (Epidemiological bulletin of the RKI, in German) – the annual zoonosis report of the BfR provides information on the spread in food. Recently, there have also been reports and recalls of salmonella in non- or insufficiently heat-treated plant foods such as sesame seeds, nuts, ingredients for chocolates (clarified butter), but of course also regularly in animal foods (meat of various origins; e.g. also reptile meat) (RASSF – please also see our Food Legislation News). Salmonella can cause antibiotic resistance to therapeutically important antibiotics.

Salmonella is usually identified by detection of specific gene sequences in PCR followed by cultural confirmation of the ability to multiply and subsequent serotyping to trace sources of infection. This can be done in much greater detail using whole genome sequencing (WGS) and is also possible for all other pathogens, especially Listeria monocytogenes. This technique is also used among other things for outbreak clarification in official investigations. The storage of isolated strains is prescribed for 3 months according to the Zoonoses Regulation, but is also recommended for infection chain tracking in raw materials and on the farm.

Vibrions

Vibrions, which can cause food-borne infections as well as dangerous soft-tissue infections, have also come under official attention in recent years. The source of infection is ready-to-eat seafood such as shrimps, but also mussels and fish meat from affected waters. 

Vibrions are halophilic (salt-loving or salt-tolerant) and multiply particularly well at high seawater temperatures. The influence of climate change on the increase of microbial hazards can be clearly seen in the example of the North and Baltic Sea.

Vibrions are detected by means of cultural methods. PCR directly for potentially pathogenic species equipped with the corresponding toxin genes is possible. Following a positive culture, the toxin gene profile of the isolate must be determined to evaluate the findings. Toxin-gene-negative Vibrio cholerae are also regularly identified.

Protozoa

Other current food-borne pathogens are the protozoa with annually constant reporting figures (Epidemiological bulletin of the RKI, in German). The most frequent representatives are Giardia (Giardia lamblia) and Cryptosporidium (Cryptosporidium parvum).

Giardia can be ingested orally via ready-to-eat plant foods and surface water if these have been contaminated by animal excretions. Particularly in southern Europe, the parasites are more common due to climatic conditions. The gastroenteritis caused by them tends to become chronic, possibly also due to the fact that it is examined less frequently and thus remain undetected.

The detection of protozoa in food is conceivable for samples that can be swabbed or rinsed, such as lettuce and vegetables, etc. A standardised procedure does not yet exist. One step for sample preparation is DNA concentration from aqueous solutions, as presented at the last DVG meeting in Garmisch (Germany) 10/2022 from matrices with low DNA content (here drinking water) by H. Einicke et al. A downstream specific PCR would detect the parasite. For the detection of food-borne viruses (noro- and hepatitis A virus) from rinsing fluid of plant matrices, elaborate standard methods already exist, but also reliable in-house methods.