Salmonella mutations – the ultimate opportunistic microbes

With the ongoing SARS COV-2 pandemic, there has been a great shift in public awareness and focus towards personal hygiene and other healthy habits to ensure a long and healthy life. One of these habits – healthy eating – is often characterised by the consumption of leafy greens such as lettuce and spinach.

Great care is taken by the sectors of the food industry that produce leafy greens to ensure the production of a safe food product, by the elimination of various microbiological and other biological hazards that occur in the food chain. However, a recent study by the University of Delaware has indicated that these measures are no longer adequate to prevent the infection of consumers by certain wild strains of Salmonella, which can circumvent plants’ natural defences.

Salmonella is a genus of rod-shaped, gram-negative, opportunistic, pathogenic bacteria. Salmonella bacteria often piggyback on raw vegetables, plants and animal products so as to enter a human host’s digestive tract, resulting in millions of foodborne illnesses every year.

Usually, the bacterial load is decreased (and often eliminated) through various processes in the food chain, such as the washing of leafy greens in a chlorine solution. However, recent research by the University of Delaware has indicated that some wild strains of Salmonella can enter plants by bypassing their natural defences.

This is achieved by the bacteria entering through the stomata of the leaves. Stomata are microscopic pores or openings on plant leaves, surrounded by guard cells. They allow gas exchange during respiration, and regulate water movement in the plant through transpiration. The opening and closing of stomata is an important factor in a plant’s defence mechanisms.

Stomatal pores vary in shape and size, and adapt to environmental conditions and stressors to optimise photosynthesis.

Generally, when the guard cells around the stomata recognise a threat or a stressor (such as the entry of a foreign entity such as a pathogenic microbe), the stomata are closed to prevent the ingress of the threat into the rest of the plant.

Now, Salmonella has mutated in such a manner that it can slip through ‘closed’ pores by reopening them, bypassing the plant’s defence mechanisms and infecting the plant.

What is concerning is that the plants show no signs of invasion, and thus go on to infect humans consuming the plant products. Once Salmonella is inside the plant, it cannot be detected, and it cannot be washed off through conventional disinfection methods.

Pathogenic bacteria such as Salmonella have gained more opportunities for infection through the increase in the number of plants bred to increase yields, which has a detrimental effect on the effectiveness of such a plant’s defence mechanisms. In addition, when crops are grown too close to livestock, pathogenic bacteria have an even greater chance of contaminating those crops.

At the same institution a solution has been developed – in the form of a beneficial microbe, Bacillus subtilis (UD1022), which is used to protect and strengthen root systems. When plants are exposed to UD1022 through irrigation, protection is provided to the plants. This solution is licensed under BASF.

However, it is important to reassess our food safety controls continuously to prevent foodborne illness, by the use of horizon scanning and robust testing schedules for early detection of hazards.

The FACTS multi-disciplinary team includes food scientists with industry experience in validating and verifying control measures throughout the food supply chain. They will give guidance on which controls are most applicable for your product matrix, and assist with risk assessments and with setting up a robust testing schedule to eliminate hazards. FACTS facilitates microbiological testing, with the added value of result interpretation. Contact us for more information.