Recent Outbreaks of a Genus of Deadly Bacteria Raise a Major Concern

The bacteria Listeria monocytogenes as seen in a file photo. (Courtesy of U.S. Centers for Disease Control and Prevention)

In brief:

• Listeria has a 20 percent mortality rate, and nearly every outbreak includes a fatality, making any increase in outbreaks a major public health concern.
• Outbreaks have been traced to almost every kind of food.
• On the one hand, this is a reminder to prepare your food in a hygienic way: thoroughly wash produce under running water, and cook and heat meat and ready-to-eat products thoroughly. On the other hand, this is linked to a worryingly high level of antibiotic use in farming across the board.
• China presents a worrying case study, but the U.S. is also using huge amounts of antibiotics in farming.
• The emergence of a “superbug” strain of Listeria is within the realm of possibility, and could be very dangerous given that 20 percent of infections result in deaths. But it’s important to remember that the prime way we fight off Listeria is our innate immune system and a healthy gut—in fact, 5 percent of people have Listeria in their intestines, and no sickness—antibiotics are meant to lend a hand in the fight, not to be the first line of defense.

On July 13, 2022, the Centers for Disease Control (CDC) reported a recent investigation of a Listeria outbreak. The incident began when Big Olaf Creamery in Florida was contaminated with Listeria monocytogenes during the production of its ice cream, and the company recalled all of its ice cream after the incident.

The incident caused cases of Listeria infection in several states. A total of 23 people have been infected, including 22 hospitalizations and one death.

In fact, there have been many outbreaks of Listeria in recent years, but these are often treated as diarrhea and thus the outbreaks haven’t attracted much attention. Listeria is, in fact, not a small matter. It has a high mortality rate, and its high pathogenicity also raises an important question.

Listeria Mortality Rate is 20 Percent, High Risk for 4 Types of People

In healthy people, Listeria infection can cause diarrhea or short-term high fever. During diarrhea, dehydration may occur, but recovery will be rapid with adequate nutrition, vitamins, minerals, and hydration.

There are several groups of people who are at high risk; namely, the elderly over 60 years of age, children, pregnant women, and people who are immunocompromised. For these people, Listeria infections can easily cause serious illnesses and can even be fatal. This is because Listeria can destroy normal cells, and enter the bloodstream through the mucous membrane of the gastrointestinal tract and cross the blood-brain barrier, causing meningitis in severe cases. Infected pregnant women can also infect their fetus through the placenta or birth canal.

The International Journal of Environmental Research and Public Health summarized Listeria outbreak cases in the United States from 2009 to 2018.

In the United States, there are at least a few dozen cases of Listeria infection each year, and almost every outbreak results in deaths.

In 2011, the outbreak of Listeria monocytogene infection numbers were relatively large, the total number of cases reached 209, with 184 hospitalizations and 39 deaths, and a mortality rate of more than 20 percent. Although the incidence numbers may not seem high, the total mortality rate of Listeria is high, between 20 percent and 30 percent, and certainly noteworthy.

Listeria Outbreaks Linked to These Foods

In early October 2021, some packaged salads produced by Dole caused Listeria infections in 18 people, 16 of whom became hospitalized, and three people eventually passed away.

In January 2022, lettuce from Fresh Express was contaminated with Listeria monocytogenes. Patients in all 10 cases were hospitalized, and one person died.

The surface of lettuce is relatively rough, so contaminants cling on easily, and may not be cleaned thoroughly during food preparation. The refrigerated storage temperatures, from 3 to 8 degrees Celsius, are actually suitable for Listeria’s reproduction. Cooking the food can kill Listeria, but people usually do not cook salads. Therefore, it is recommended to wash salads with clean running water to avoid infection.

Furthermore, meat is also a type of food that can be easily infected by Listeria monocytogenes. In April 2021, ready-to-eat chicken products sold by Tyson Foods Inc., a meat producer and nationwide distributor, caused three people to be infected by Listeria. All of them were hospitalized, and one passed away. Tyson Foods Inc. eventually recalled nearly £9 million worth of chicken products.

Cooked meat and sausages can still be contaminated with Listeria via processing or packaging equipment.

A research report analyzed food samples from 28 provinces in mainland China and found that the prevalence of Listeria monocytogenes reached 4.4 percent. The prevalence of Listeria in poultry meat products was as high as 8.9 percent.

Listeria can survive in a hostile environment. The secretions of its bacterial cells can form a mucous layer on the outside of the cells, which eventually accumulates into a biofilm, making it difficult for disinfectants to reach the bacteria. These membranes are sticky in nature, making it easier for bacteria to remain on processing equipment and food packaging.

Other food items, including raw milk, mushrooms, and the skins of melons and fruits stained with soil may also be infected by Listeria monocytogenes.

To prevent Listeria infection, we can thoroughly cook, or thoroughly heat ready-to-eat foods before we eat them. Wash fruits and vegetables with running water before eating, and have separate cutting boards for raw and cooked foods.

Listeria’s High Risk Raises Major Question

The risk of Listeria infection appears to be present in any food. Access to effective drug therapy is a critical issue for high risk groups.

The treatment for Listeria infection uses antibiotics. Most strains of Listeria monocytogenes are still susceptible to antibiotics. However, worryingly, antibiotic-resistant strains have emerged with increasing frequency in recent years.

Due to its natural resistance, Listeria monocytogenes is resistant to first-generation quinolones, phosphomycins, and third-generation cephalosporins to a certain degree.

In 1988, a tetracycline-resistant Listeria monocytogenes strain emerged. Subsequently, more Listeria strains resistant to multiple antibiotics have emerged.

A study was conducted on aquaculture in mainland China. Among 72 Listeria strains, 9 of them were found to be significantly resistant to drugs; 16 were highly susceptible to antibiotics and could be killed by a small dose of antibiotics; and most of the strains were in the middle of the drug-resistance spectrum.

One eighth of Listeria strains are resistant to drugs, and this is a large proportion.

The mariculture industry in China has been developing rapidly, and the use of antibiotics to increase production is very common, resulting in a seemingly inevitable increase in antibiotic-resistant strains of Listeria.

Another Chinese analysis report published in the journal Foodborne Pathogens and Disease examined 15 commonly used antibiotics. Listeria monocytogenes was found to be most commonly resistant to tetracycline, while resistance to other antibiotics such as chloramphenicol, erythromycin, ciprofloxacin, doxycycline, and streptomycin was also common.

The study also discovered that Listeria strains with drug resistance in food reached 40 percent to 70 percent.

A study from 2010 to 2021 found that many drug-resistant genes emerged in Listerialisteria strains in North America, Asia, the United Kingdom, and Australia.

Today, antibiotics are used in large quantities in farmed animals, aquatic products, and plants. Its first purpose is to prevent diseases, in which case large doses are used for a short period of time. Its second purpose is to promote growth, in which case small doses are used over a long period of time.

A report published in the American Journal of Public Health states that 80 percent of antibiotics are used in agricultural pesticides.

Since tetracyclines are a class of broad-spectrum antibiotics, they are widely used for the prevention and treatment of livestock diseases or as feed additives to promote livestock growth. As a result, the level of resistance to tetracycline in Listeria monocytogenes has increased.

5 Percent of People Have Listeria in Their Intestines, So Why Don’t They Get Sick?

Listeria monocytogenes can actually be detected in the stool of 5 percent of healthy adults. So why do only a small number of people actually get sick from it?

This is because the most important part of the fight against bacteria is the body’s own immune system. For healthy people, their intestinal mucosa is intact and strong enough to resist the invasion of Listeria, and there are various immune mechanisms in the body such as natural killer cells that can effectively kill Listeria monocytogenes. Therefore, for healthy people, there is no need to use antibiotics; they will not get sick; or if they do get infected, they will recover very easily. Antibiotics are only used as a supplementary antibacterial tool when the amount of bacteria in a patient’s body is already large.

However, due to the indiscriminate use of antibiotics in the modern farming industry for poultry, fish, and vegetables, the amount of antibiotics present in the environment is gradually increasing.  When the dose of antibiotics is not large enough to kill the bacteria, then the bacteria will gradually “learn” to resist antibiotics in the process of reproduction. At this point, the bacteria that have survived are likely to be the ones that are resistant to the antibiotics. In addition, when these plants and animals are consumed by people, this will allow people to indirectly ingest antibiotics, which indeed is an indirect abuse of antibiotics.

Once a strain of bacteria becomes antibiotic-resistant to certain antibiotics, higher dosages are often prescribed to patients, it may be necessary to use dozens or even hundreds of times the original dose in order to receive the same treatment effect. However, higher doses of antibiotics likely The use of large doses of antibiotics itself will cause harm to the human body, so the doctors have to balance between drug toxicity and bactericidal effects when treating patients.  Eventually, certain types of antibiotics might become totally ineffective when the bacteria develops complete resistance.; and the treatment may even lose effectiveness completely.  Then people have to develop or choose different types of antibiotics, then another cycle of the same episode starts again.

Major organizations such as the World Health Organization (WHO) have also identified the emergence of antibiotic-resistant strains as one of the most significant public health threats.

Once super bacteria or “superbugs,” emerges with strong resistance to a very broad spectrum of antibiotics, then the current antibiotics will not work against it. This in fact could be very dangerous, and present a large threat to global human health.