Salmonella is a bacterium that causes one of the most common enteric (intestinal) infections in the United States – salmonellosis. The term Salmonella refers to a group or family of bacteria that variously cause illness in humans. Salmonella serotype typhimurium and Salmonella serotype enteritidis are the most common in the United States.
Salmonella are found in the intestinal tract of wild and domesticated animals and humans. Some serotypes of Salmonella, such as S. Typhi and S. Paratyphi are only found in humans. For ease of discussion, it is generally useful to group Salmonellae into two broad categories: typhoidal, which includes S. Typhi and S. Paratyphi, and non-typhoidal, which includes all other serotypes.
Salmonella can be grouped into more than 2,400 serotypes. The two most common serotypes in the U.S. are S.Typhimurium and S. Enteritidis. S. Typhi, the serotype that causes typhoid fever, is uncommon in the U.S. But globally, typhoid fever continues to be a significant problem, with an estimated 12-33 million cases occurring annually. Moreover, outbreaks in developing countries have a high deathrate, especially when caused by strains of the bacterium that are resistant to antibiotic treatment.
In 2009, over 40,000 cases of Salmonella (13.6 cases per 100,000 persons) were reported to the Centers for Disease Control and Prevention (CDC) by public health laboratories across the nation, representing a decrease of approximately 15% from the previous year, but a 4.2% increase since 1996. Overall, the incidence of Salmonella in the United States has not significantly changed since 1996. Only a small proportion of all Salmonella infections are diagnosed and reported to health departments. It is estimated that for every reported case, there are approximately 38.6 undiagnosed infections. The CDC estimates that 1.4 million cases, 15,000 hospitalizations, and 400 deaths are caused by Salmonella infections in the U.S. every year.
Listeria is a gram-positive rod-shaped bacterium that can grow under either anaerobic (without oxygen) or aerobic (with oxygen) conditions. Of the six species of Listeria, only L. monocytogenes causes disease in humans. These bacteria multiply best at 86-98.6 degrees F (30-37 degrees C), but also multiply better than all other bacteria at refrigerator temperatures, something that allows temperature to be used as a means of differentiating Listeria from other contaminating bacteria.
Called an “opportunistic pathogen,” Listeria is noted to cause an estimated 2,600 cases per year of severe invasive illness. Perhaps not surprisingly then, “foodborne illness caused by Listeria monocytogenes has raised significant public health concern in the United States, Europe, and other areas of the world.” Given its widespread presence in the environment and food supply, the ingestion of Listeria has been described as an “exceedingly common occurrence.”
Listeria bacteria are found widely in the environment in soil, including in decaying vegetation and water, and may be part of the fecal flora of many mammals, including healthy human adults. According to the FDA, “studies suggest that 1-10% of humans may be intestinal carriers of Listeria.” Another authority notes that the “organism has been isolated from the stool of approximately 5% of healthy adults.” Overall, seasonal trends show a notable peak in total Listeria cases and related deaths from July through October.
Ingested by mouth, Listeria is among the most virulent foodborne pathogens, with up to 20% of clinical infections resulting in death. These bacteria primarily cause severe illness and death in persons with immature or compromised immune systems. Consequently, most healthy adults can be exposed to Listeria with little to any risk of infection and illness.
Among adults 50 years of age and older, infection rates were estimated to have declined from 16.2 per 1 million in 1989 to 10.2 per 1 million in 1993. Perinatal disease decreased from 17.4 cases per 100,000 births in 1989 to 8.6 cases per 100,000 births in 1993. Neonatal infections are often severe, with a mortality rate of 25-50%.