Enteric bacteria may be leading a secret life in plants

November 2005

Are some enteric pathogens leading a double life? That is a possibility, according to Robert V. Tauxe, MD, MPH, of the CDC.

“Recent conclusions suggest that some of our favorite enteric pathogens are surprisingly at home in plants,” said Tauxe at the 43rd Annual Meeting of the Infectious Diseases Society of America, held in San Francisco.

Produce represents a small but important part of the foodborne disease chain in this country and has implications regarding pathogens previously thought to be understood, according to Tauxe.

Seeds, roots and leaves

The CDC discovered through retrospective analysis an increase in produce-related foodborne disease between 1973 and 1997. Because of the increase in outbreaks, Tauxe said infectious disease physicians began consulting with food microbiologists and plant pathologists. Together, they noted some preliminary observations about the behavior of bacterial pathogens around produce.

To make his point, Tauxe mentioned a simple experiment of putting cilantro plant leaves into Salmonella for two seconds, and then letting them sit for a few days. Within two days the bacteria count had increased nearly 100 logs, or 100-fold. He reported a similar experiment with tomatoes and even surface washed them before finding the surface loaded with Salmonella. The bacteria persisted at temperatures as low as 20°F, according to Tauxe.

Campylobacter jejuni is another pathogen found on produce. Models using spinach and radishes have produced results similar to the findings with Salmonella. However, Campylobacter thrives on the roots of the plant rather than the leaves. It will slowly decline but persists for at least four weeks.

Escherichia coli and Salmonella persist on seeds. Research has documented that E. coli survives on dry seeds for at least 60 days, according to Tauxe.

“In fact, we’ve had outbreaks traced to seeds that probably were contaminated 10 years before,” Tauxe said. “We suspect that it will survive indefinitely on the surface or in the cracks of the seed.”

Aside from the pathogens living on the outside of the plant, research has shown that pathogens can enter the plant and persist inside where it is impossible to wash off or disinfect.

This has occurred in many plants and with several bacterial species. There are unsurprising ports of entry like cuts or bruises, but an apple can also be contaminated by a pathogen entering through capillary action.

There are more complex ways for a pathogen to enter a fruit and one is by temperature differential. For example, Tauxe explained, “if you put a warm piece of fruit in a cold bath, the gas spaces in the fruit contract in the cold water and create suction. This will in turn pull up the fruit so that the fluid enters through any available port and any bacteria in the water will now be inside the fruit.”

This has been demonstrated in mangoes, apples, oranges and melons.

Contamination can also occur through irrigation. According to Tauxe, studies have shown that mature lettuce plants irrigated with E. coli O157:H7 can within a day have E. coli throughout the leaves, stems and roots of the lettuce plants at about 10² or 10³ per gram.

“Plants actively take up small particles in the irrigating fluid and distribute it throughout the plant. We should not be surprised that bacteria are among those particles,” Tauxe said.

Why does this occur?

A study from Norway suggests that soil may play a part in how well a pathogen grows on produce. The life cycle of the plant may also have an effect. This was studied closely after the first alfalfa sprout outbreak, according to Tauxe. Seeds that are kept moist and warm will germinate as sprouts and any bacteria present in the seed may find that a good point to grow. E. coli, Salmonella typhii or non-typhoidal Salmonella will flourish in that environment, according to Tauxe.

“If you had seeds with [about] 10³ and you germinated them, over the next few hours they grow up to 10⁷ log in your alfalfa sprouts and then stay there indefinitely as long as you care to keep the sprouts around,” Tauxe said.

Further studies of this with E. coli O157:H7 have shown the pathogen present on the seed coat. Then, it localizes to the root and root hairs as soon as they appear. It penetrates the plant and appears throughout the tissues and the fluid sap within the sprout and colonizes all tissues in the sprout, including the new leaves. This all occurs without causing the plant any particular harm.

Starting at a different part in the life cycle, Tauxe outlined a study of eight tomato plants with a total of 100 flowers that were brushed with a mixture of five serotypes of Salmonella. Eighty percent of the flowers dropped off, but of the 20 flowers that remained, eight developed mature tomatoes. Upon ripening, two of those eight were picked and washed and yielded Salmonella of three different serotypes.

“This is Salmonella going in with the pollen tube into the ovule and colonizing the forming fruit and a month later being found inside the tomato,” Tauxe said.

Early studies

In 1942, a study was published describing Pseudomonas aeruginosa as the cause of soft rot of tobacco seedlings. An irrigation challenge study of sweet basil conducted in 2004 found that clinical P. aeruginosa strains destroyed those plants as well.

“It’s clearly a broad-range host pathogen,” Tauxe said.

Pseudomonas cepacia (now known as Burkholderia cepacia) is similar to P. aeruginosa in produce. Researchers have now clarified that the onion pathogen is the exact same biogenovar that causes chronic infections in cystic fibrosis.

“It’s an opportunistic infection of young onions and an opportunistic infection of young kids,” Tauxe said.

A brand new emerging disease of pumpkins has been identified as Serratia marcescens and is “not too far from the one causing opportunistic infections in humans, although it is from a recently evolved cluster,” said Tauxe.

“There is something going on here that is really kind of fascinating,” he said. “There is tremendous overlap in the capacity and ability to infect both plants and animals.”

Reflections on plausibility

Some enteric bacterial and other pathogens are well adapted to plants. They persist on the leaves and easily penetrate and persist in the stems and edible leaves. Tauxe suggested that they may even participate in the seed-flower-fruit cycle. The gram-negatives are not harmful to the plant, although there are cross-over pathogens that are pathogenic in both plants and animals.

“Why would enteric bacteria, whose home is the gut of vertebrates, have a secret life in plants? Why would they want to do this in the first place?” Tauxe asked. “We tend to think of them as meat associated; however, we should remember that the meat we are eating basically comes from herbivores. Carnivores make up a very small part of the human diet. So the capacity to sometimes colonize the plants that the herbivores eat makes sense from an evolutionary view. The point of a fruit is to be eaten by a mobile herbivore that will wonder off and excrete the seeds somewhere else. Riding along with those seeds and colonizing next year’s growth as it sprouts may get the bacteria to the next herbivore.”

There may also be some practical implications. If the enteric bacteria are well adapted to plants, Tauxe said he wonders if that affects their ability to cause illness in humans.

“I don’t think we know, but I do think there are a lot of unexplained genes in Salmonella and E. coli O157 that we are unsure of and maybe we’re using the wrong model,” he said.

The outbreaks are related to plant-derived foods. The problem is that researchers are still investigating how to prevent them and consumers cannot prevent them by simply washing the produce when the pathogens are on the inside.

With that point in mind, Tauxe outlined some important questions to be asked: Can we reduce the risk of plant-derived food contamination by learning more about how the contamination occurs? Can we block the uptake of these pathogens into plants? Can we promote the uptake of beneficial bacteria? Can we select varieties of new produce that will not become contaminated so easily?

Tauxe continued with what he called “larger questions to be asked”: What are the plant-host ranges of these enteric pathogens? The answer is that no one has looked. What is the survival of these pathogens at various life stages of the plants and do they participate in the life cycle?

“No one has yet taken an infected alfalfa seed, grown an alfalfa sprout and then let that sprout go up to full height and let it seed to see if the Salmonella is in it,” Tauxe said. “That would be interesting to know.”

For more information:

• Tauxe RV. Emerging infections. Symposium #59. Presented at the 43rd Annual Meeting of the Infectious Diseases Society of America. Oct. 6-9, 2005. San Francisco.