Scientists have found new genetic information that shows how harmful bacteria cause the acute diarrheal disease shigellosis, which kills more than a million people worldwide each year.

The research, which could lead to the development of future treatments, was published today in the journal PLoS ONE. The study was led by Ohio University scientist Erin Murphy and doctoral student William Broach, with contributions from University of Nevada, Las Vegas and University of Texas at Austin researchers.

When the disease-causing bacterium Shigella invades a human host, environmental conditions there, such as changes in temperature or pH, stimulate a genetic expression pathway within the bacterium that allows it to survive and cause disease. Central to this genetic pathway are two proteins, VirF and VirB. VirF functions to increase production of VirB which, in turn, promotes the production of factors that increases the bacterium’s virulence, or ability to cause illness in its host.

“It’s like a domino effect,” says Murphy, assistant professor of bacteriology in the Ohio University Heritage College of Osteopathic Medicine.

Murphy and Broach’s new study, however, suggests that production of VirB can be controlled independently of VirF. It also shows that the VirF-independent regulation is mediated by a specific small RNA, a special type of molecule whose job is to control the production of particular targets. This is the first study to demonstrate that transcription of virB is regulated by any factor other than VirF, Murphy explained.

The research not only reveals the intricate level of gene expression the bacteria employ to survive in the human body, but potentially could lead to new treatments. Currently, antibiotics are prescribed to patients with the disease.

“These findings are feeding into the basic understanding of this gene expression so that future researchers can work to disrupt it,” Broach said. “The more we know about it, the more targets we have to disrupt it and to possibly develop targeted antibiotic treatments.”

For those living in developing countries, where access to clean drinking water can be scarce, an improved medical treatment for shigellosis could mean the difference between life and death.

“In the United States, if we get severe diarrhea we can go to the store and get Gatorade,” Murphy says. “But if you’re already starving to begin with because you don’t have access to good food and clean water, then you get shigellosis on top of that—and you don’t have good water to rehydrate yourself—that’s when the deaths happen.”

The disease, which is transmitted person to person or through contaminated food or water sources, has an infectious dose of just 10 organisms, meaning as few as 10 organisms can cause disease in a healthy person. This infectious dose is exceedingly low compared to other bacteria that require tens of thousands of organisms to cause disease.

While it is often thought to be a Third-World problem, shigellosis causes a reported 14,000 cases in the United States each year. The Centers for Disease Control suggests that the actual number may be 20 times higher, as mild cases often aren’t reported or diagnosed.

“In the United States it’s probably even more underreported than in developing countries because of access to healthy, clean drinking water,” Murphy says. “If you’re a healthy individual and you’ve got access to clean drinking water, chances are you’re going to get severe diarrhea, but you’re not going to die.”

The research was funded by the National Institutes of Health, the Ohio University Research Committee and the Ohio University Heritage College of Osteopathic Medicine.

The collaborators on the PLoS ONE paper are University of Nevada, Las Vegas scientists Nicholas Egan and Helen Wing and University of Texas at Austin researcher Shelley Payne.

Bad Bug Book – Foodborne Pathogenic Microorganisms and Natural Toxins – Second Edition

1. Organism

Shigella are Gram-negative, non-motile, non- sporeforming, rod-shaped bacteria. Shigella species, which include Shigella sonnei, S. boydii, S. flexneri, and

S. dysenteriae, are highly infectious agents. Some strains produce enterotoxins and Shiga toxin. The latter is very similar to the toxins produced by E. coli O157:H7.

Humans are the only host of Shigella, but it has also been isolated from higher primates. The organism is frequently found in water polluted with human feces.

In terms of survival, shigellae are very sensitive to environmental conditions and die rapidly. They are heat sensitive and do not survive pasteurization and cooking temperatures. In terms of growth, shigellae are not particularly fastidious in their requirements and, in most cases, the organisms are routinely cultivated in the laboratory, on artificial media. However, as noted in subsequent sections, the relative difficulty of cultivating this organism is dependent, in part, on the amount of time within which stool or food samples are collected and processed.

Shigella species are tolerant to low pH and are
able to transit the harsh environment of the
stomach. These pathogens are able to survive
and, in some cases, grow in foods with low pH,
such as some fruits and vegetables. They are
able to survive on produce commodities
packaged under vacuum or modified
atmosphere and can also survive in water, with a slight decrease in numbers.

2. Disease

The illness caused by Shigella is shigellosis (also called bacillary dysentery), in which diarrhea may range from watery stool to severe, life-threatening dysentery. All Shigella spp. can cause acute, bloody diarrhea. Shigella spp. can spread rapidly through a population, particularly in crowded and unsanitary conditions.

S. dysenteriae type 1 causes the most severe disease and is the only serotype that produces the Shiga toxin, which may be partially responsible for cases in which hemolytic uremic syndrome (HUS) develops. S. sonnei produces the mildest form of shigellosis; usually watery diarrhea.

S. flexneri and S. boydii infections can be either mild or severe.
In developed countries, S. sonnei is the Shigella species most often isolated, whereas S. flexneri predominates in developing countries.

• Mortality: In otherwise healthy people, the disease usually is self-limiting, although some strains are associated with fatality rates as high as 10-15%. (See Illness / complications section, below.)

• Infective dose: As few as 10 to 200 cells can cause disease, depending on the age and condition of the host.

• Onset: Eight to 50 hours.

• Illness / complications: In otherwise healthy people, the disease usually consists of self- limiting diarrhea (often bloody), fever, and stomach cramps. Severe cases, which tend to occur primarily in immunocompromised or elderly people and young children, are associated with mucosal ulceration, rectal bleeding, and potentially drastic dehydration. Potential sequelae of shigellosis include reactive arthritis and hemolytic uremic syndrome.

• Symptoms: May include abdominal pain; cramps; diarrhea; fever; vomiting; blood, pus, or mucus in stools; tenesmus (straining during bowel movements).

• Duration: Uncomplicated cases usually resolve in 5 to 7 days. Most of the time, the illness is self-limiting. In some circumstances, antibiotics are given; usually trimethoprim-sulfamethoxazole, ceftriaxone, or ciprofloxacin.

• Route of entry: The fecal-oral route is the primary means of human-to-human transmission of Shigella. With regard to foods, contamination is often due to an infected food handler with poor personal hygiene.

• Pathway: The disease is caused when Shigella cells attach to, and penetrate, colonic epithelial cells of the intestinal mucosa. After invasion, they multiply intracellularly and spread to contiguous epithelial cells, resulting in tissue destruction. As noted, some strains produce enterotoxin and Shiga toxin similar to those produced by E. coli O157:H7.

3. Frequency

A recent Centers for Disease Control and Prevention (CDC) report on foodborne illnesses acquired annually in the United States revealed that about 15,000 laboratory-confirmed isolates are reported each year, with estimates of actual occurrence ranging from 24,511 to 374,789 cases (average of 131,243). About 31% of these are estimated to be foodborne. Estimates of foodborne illness episodes (mean) caused by 31 pathogens placed Shigella as the sixth most frequent cause (after norovirus, Salmonella species, Clostridium perfringens, Campylobacter, and Staphylococcus aureus, in that order).

Episodes of shigellosis appear to follow seasonal variations. In developed countries, the highest incidences generally occur during the warmer months of the year.

4. Sources

Most cases of shigellosis are caused by ingestion of fecally contaminated food or water. In the case of food, the major factor for contamination often is poor personal hygiene among food handlers. From infected carriers, this pathogen can spread by several routes, including food, fingers, feces, flies, and fomites.

Shigella is commonly transmitted by foods consumed raw; for example, lettuce, or as non- processed ingredients, such as those in a five-layer bean dip. Salads (potato, tuna, shrimp, macaroni, and chicken), milk and dairy products, and poultry also are among the foods that have been associated with shigellosis.

5. Diagnosis

Diagnosis is by serological or molecular identification of cultures isolated from stool. Shigella may be more difficult to cultivate if stool samples are not processed within a few hours.

6. Target Populations

All people are susceptible to shigellosis, to some degree, but children 1 to 4 years old, the elderly, and the immunocompromised are most at risk. Shigellosis is very common among people with AIDS and AIDS-related complex.

7. Food Analysis

Shigellae remain a challenge to isolate from foods. A molecular-based method (PCR) that targets a multi-copy virulence gene has been developed and implemented by FDA. Improvements in the bacterial isolation method continue and should be available in the near future.

The window for collecting and processing Shigella from foods, for cultivation, may be days (rather than hours, as is the case with stool), depending on the food matrix and storage conditions; e.g., temperature. Shigella species can be outgrown by the resident bacterial populations found in foods, which may reflect the usual low numbers of the organism present in foods and, in some foods, a very large number of non-Shigella bacteria. Another factor that reduces the chance of isolating Shigella from foods may be the physiological state of the pathogen at the time of analysis. Environmental conditions could affect its ability to either grow or survive in any food matrix.

8. Examples of Outbreaks

The CDC’s Morbidity and Mortality Weekly Reports provide information about Shigella outbreaks.

9. Other Resources

• Loci index for genome Shigella spp.

• GenBank Taxonomy database

• More information about Shigella and shigellosis can be found on the CDC website.

Continue Reading Bad Bug Book – Foodborne Pathogenic Microorganisms and Natural Toxins – Second Edition – Shigella species

According to press reports, the Escambia County Health Department has reminded the public that people can reduce the spread of gastrointestinal illnesses by practicing simple hygiene. “We like to be proactive about this, and we’ve started seeing cases of Shigella, or shigellosis, which is a bacterial illness that can produce nausea, vomiting, diarrhea and sometimes fever in young people,” said Dr. John Lanza, director of the Escambia County Health Department.

Lanza said the county usually sees three to 30 cases of Shigella a month, and this month there have been about 12 to 15 cases.

“We’re seeing this in child-care centers, and that’s where our most-vulnerable population center is — our children — and we just want child-care providers and parents to know what to look for,” Lanza said.

He said simple hygiene practices can keep gastrointestinal illnesses like Shigella at bay.

“The No. 1 way of transmitting these kind of diseases is by not washing your hands with soap and water,” Lanza said. He said it’s important to thoroughly wash hands after using the bathroom, changing diapers and handling food.

Lanza had another health message: If you or your child has a gastrointestinal illness, stay home instead of spreading the germs.

“If you as an adult, or your child, are sick, for at least 48 hours, stay at home. Don’t go to work,” he said.