Salmonella

From Wikipedia, the free encyclopedia

This article is about the bacterium. For the disease, see Salmonellosis.

Salmonella

Scientific classification

Superkingdom: Bacteria

Kingdom: Bacteria

Phylum: Proteobacteria

Class: Gammaproteobacteria

Order: Enterobacteriales

Family: Enterobacteriaceae

Genus: Salmonella Lignieres 1900

Species

S. bongori

S. enterica

Salmonella /ˌsælməˈnɛlə/ is a genus of rod-shaped, Gram-negative, non-spore-forming,

predominantly motile enterobacteria with diameters around 0.8 to 1.5 µm, lengths from 2 to

5 µm, and peritrichous flagella, (flagella that are all around the cell body). They are

chemoorganotrophs, obtaining their energy from oxidation and reduction reactions using organic

sources, and are facultative anaerobes. There are only two species of Salmonella; Salmonella

bongori and Salmonella enterica of which there are around six subspecies and innumerable

serovars. Salmonella belongs to the same family as Escherichia, which has as a species E.coli.

Most subspecies of Salmonella produce hydrogen sulfide,[1]

which can readily be detected by

growing them on media containing ferrous sulfate, such as is used in the triple sugar iron test

(TSI). Most isolates exist in two phases: a motile phase I and a nonmotile phase II. Cultures that

are nonmotile upon primary culture may be switched to the motile phase using a Cragie

tube.[citation needed]

Salmonella is found worldwide in both cold-blooded and warm-blooded animals, and in the

environment. They cause illnesses such as typhoid fever, paratyphoid fever, and food

poisoning.[2]

Contents

1 Salmonella as pathogens

2 Enteritis salmonellosis or food poisoning Salmonella

3 History

4 Salmonella nomenclature

5 Growth kinetics

6 Sources of infection

7 New antibiotic-resistant strains

8 Vaccine status

9 Deaths

10 See also

11 References

12 External links

Salmonella as pathogens

Salmonella bacteria are zoonotic and can be transferred between humans and other animals.

Many infections are due to ingestion of contaminated food. For example, recent FDA studies link

Guatemalan cantaloupes with Salmonella panama.[3]

In speaking of other salmonella serotypes,

enteritis Salmonella and Salmonella typhoid/paratyphoid Salmonella, the latter—because of a

special virulence factor and a capsule protein (virulence antigen)—can cause serious illness, such

as Salmonella enterica subsp. enterica serovar Typhi. Salmonella typhi is adapted to humans and

does not occur in other animals.

Salmonella species are facultative intracellular pathogens[4]

that enter cells via

macropinosomes.[5]

Enteritis salmonellosis or food poisoning Salmonella

This is a group consisting of potentially every other serotype (over a thousand) of the Salmonella

bacteria, most of which have never been found in humans. These are encountered in various

Salmonella species, most having never been linked to a specific host, but can also infect humans.

It is therefore a zoonotic disease.

The organism enters through the digestive tract and must be ingested in large numbers to cause

disease in healthy adults. Gastric acidity is responsible for the destruction of the majority of

ingested bacteria. Bacterial colonies may become trapped in mucus produced in the oesophagus.

Salmonellosis is a disease incurred by eating food (often raw or undercooked, or too frequently

re-heated) which is contaminated by S. enterica. Infection usually occurs when a person ingests

foods that contain a high concentration of the bacteria, similar to a culture medium. In otherwise

healthy adults, the symptoms can be mild. Normally, no sepsis occurs, but it can occur

exceptionally as a complication in the immunocompromised.

However, infants and young children are much more susceptible to infection, easily achieved by

ingesting a small number of bacteria. In infants, contamination through inhalation of bacteria-

laden dust is possible. After a short incubation period of a few hours to one day, the bacteria

multiply in the intestinal lumen, causing an intestinal inflammation with diarrhea that is often

mucopurulent (containing mucus or pus) and bloody. In infants, dehydration can cause a state of

severe toxicity. Extraintestinal localizations are possible, especially Salmonella meningitis in

children, osteitis, etc.

Enteritis Salmonella (e.g., Salmonella enterica subsp. enterica serovar enteritidis) can cause

diarrhea, which usually does not require antibiotic treatment. However, in people at risk such as

infants, small children, the elderly, Salmonella infections can become very serious, leading to

complications. If these are not treated, HIV patients and those with suppressed immunity can

become seriously ill. Children with sickle cell anaemia who are infected with Salmonella may

develop osteomyelitis. Treatment of osteomyelitis, in this case, will be to use fluoroquinolones

(Ciproflaxacin, Levofloxacin, etc. and Nalidixic acid).

Salmonella bacteria can survive for weeks outside a living body, and they are not destroyed by

freezing.[6][7]

Ultraviolet radiation and heat accelerate their demise; they perish after being heated

to 55 °C (131 °F) for 90 min, or to 60 °C (140 °F) for 12 min.[8]

To protect against Salmonella

infection, heating food for at least ten minutes at 75 °C (167 °F) is recommended, so the centre

of the food reaches this temperature.[9][10]

Most people with salmonellosis develop diarrhea, fever, vomiting, and abdominal cramps 12 to

72 hours after infection. In most cases, the illness lasts four to seven days, and most people

recover without treatment. In some cases, though, the diarrhea may be so severe, the patient

becomes dangerously dehydrated and must be taken to a hospital. At the hospital, the patient

may receive intravenous fluids to treat the dehydration, and may be given medications to provide

symptomatic relief, such as fever reduction. In severe cases, the Salmonella infection may spread

from the intestines to the blood stream, and then to other body sites, and can cause death, unless

the person is treated promptly with antibiotics. The elderly, infants, and those with impaired

immune systems are more likely to develop severe illness.

An infectious process can only begin after living salmonellae (not only their toxins) reach the

gastrointestinal tract. Some of the microorganisms are killed in the stomach, while the surviving

salmonellae enter the small intestine and multiply in tissues (localized form). By the end of the

incubation period, the macro-organisms are poisoned by endotoxins released from the dead

salmonellae. The local response to the endotoxins is enteritis and gastrointestinal disorder. In the

generalized form of the disease, salmonellae pass through the lymphatic system of the intestine

into the blood of the patients (typhoid form) and are carried to various organs (liver, spleen,

kidneys) to form secondary foci (septic form). Endotoxins first act on the vascular and nervous

apparatus, manifested by increased permeability and decreased tone of the vessels, upset thermal

regulation, vomiting and diarrhea. In severe forms of the disease, enough liquid and electrolytes

are lost to upset the water-salt metabolism, to decrease the circulating blood volume and arterial

pressure, and to cause hypovolemic shock. Septic shock may develop. Shock of mixed character

(with signs of both hypovolemic and septic shock) are more common in severe salmonellosis.

Oliguria and azotemia develop in severe cases as a result of renal involvement due to hypoxia

and toxemia.

A small number of people afflicted with salmonellosis experience reactive arthritis, which can

last months or years and can lead to chronic arthritis.

In Germany, food poisoning infections must be reported.[11]

Between 1990 and 2005, the number

of officially recorded cases decreased from approximately 200,000 to approximately 50,000

cases. In the USA, about 40,000 cases of Salmonella infection are reported each year.[12]

According to the World Health Organization, over 16 million people worldwide are infected with

typhoid fever each year, with 500,000 to 600,000 fatal cases.[citation needed]

The AvrA toxin injected by the type three secretion system of Salmonella Typhimurium works to

inhibit the innate immune system by virtue of its serine/threonine acetyltransferase activity, and

requires binding to eukaryotic target cell phytic acid (IP6).[13]

This leaves the host more

susceptible to infection. In a 2011 paper,[14]

Yale University School of Medicine researchers

described in detail how Salmonella is able to make these proteins line up in just the right

sequence to invade host cells. "These mechanisms present us with novel targets that might form

the basis for the development of an entirely new class of antimicrobials," said Professor Dr.

Jorge Galan, senior author of the paper and the Lucille P. Markey Professor of Microbial

Pathogenesis and chair of the Section of Microbial Pathogenesis at Yale. In the new National

Institutes of Health-funded study, Galan and colleagues identify what they call a bacterial sorting

platform, which attracts needed proteins and lines them up in a specific order. If the proteins do

not line up properly, Salmonella, as well as many other bacterial pathogens, cannot "inject" them

into host cells to commandeer host cell functions, the lab has found. Understanding how this

machine works raises the possibility of new therapies that disable this protein delivery machine,

thwarting the ability of the bacterium to become pathogenic. The process would not kill the

bacteria as most antibiotics do, but would cripple its ability to do harm. In theory, this means

bacteria such as Salmonella might not develop resistance to new therapies as quickly as they

usually do to conventional antibiotics.

History

The genus Salmonella was named after Daniel Elmer Salmon, an American veterinary

pathologist. While Theobald Smith was the actual discoverer of the type bacterium (Salmonella

enterica var. Choleraesuis) in 1885, Dr. Salmon was the administrator of the USDA research

program, and thus the organism was named after him by Smith.[15]

Smith and Salmon had been

searching for the cause of common hog cholera and proposed this organism as the causal agent.

Later research, however, would show this organism (now known as Salmonella enterica) rarely

causes enteric symptoms in pigs,[16]

and was thus not the agent they were seeking (which was

eventually shown to be a virus). However, related bacteria in the genus Salmonella were

eventually shown to cause other important infectious diseases. The genus Salmonella was finally

formally adopted in 1900 by J. Lignières for the many species of Salmonella, after Smith's first

type-strain Salmonella cholera.

Salmonella nomenclature

Initially, each Salmonella "species" was named according to clinical considerations,[17]

e.g.,

Salmonella typhi-murium (mouse typhoid fever), S. cholerae-suis. After it was recognized that

host specificity did not exist for many species, new strains (or serovars, short for serological

variants) received species names according to the location at which the new strain was isolated.

Later, molecular findings led to the hypothesis that Salmonella consisted of only one species,[18]

S. enterica, and the serovars were classified into six groups,[19]

two of which are medically

relevant. But as this now formalized nomenclature[20][21]

is not in harmony with the traditional

usage familiar to specialists in microbiology and infectologists, the traditional nomenclature is

common. Currently, there are two recognized species: S. enterica, and S. bongori. In 2005 a third

species was thought to be added Salmonella subterranean, but this has since been ruled out and

is seen as another serovar.[22]

There are six main subspecies recognised: enterica (I), salamae

(II), arizonae (IIIa), diarizonae (IIIb), houtenae (IV), and indica (VI).[23]

Historically, serotype

(V) was bongori, which is now considered its own species.

The serovar (i.e. serotype) is a classification of Salmonella into subspecies based on antigens that

the organism presents. It is based on the Kauffman-White classification scheme that

differentiates serological varieties from each other. Serotypes are usually put into subspecies

groups after the genus and species, with the serovars/serotypes capitalized but not italicized: an

example is Salmonella enterica serovar Typhimurium. Newer methods for Salmonella typing

and subtyping include genome-based methods such as pulsed field gel electrophoresis (PFGE),

Multiple Loci VNTR Analysis (MLVA), Multilocus sequence typing (MLST) and (multiplex-)

PCR-based methods.[24][25]

Growth kinetics

Mathematical models of salmonella growth kinetics have been developed for chicken, pork,

tomatoes, and melons.[26][27][28][29][30]

Salmonella reproduce asexually with a cell division rate of

20 to 40 minutes under optimal conditions.[citation needed]

Sources of infection

An infographic illustrating how Salmonella spreads from the farm

Infected food, often gaining an unusual look or smell, then is introduced into the stream

of commerce;

Poor kitchen hygiene, especially problematic in institutional kitchens and restaurants

because this can lead to a significant outbreak;

Excretions from either sick or infected but apparently clinically healthy people and

animals (especially dangerous are caregivers and animals);

Polluted surface water and standing water (such as in shower hoses or unused water

dispensers);

Unhygienically thawed fowl (the meltwater contains many bacteria);

An association with reptiles (pet tortoises, snakes, iguanas[31][32]

aquatic turtles, and also

amphibians such as frogs) is well described.[33]

Salmonella bacteria can survive for some time without a host; thus, they are frequently found in

polluted water, contamination from the excrement of carrier animals being particularly

important.

The most recent case of salmonella infection had been detected mid-2012 in seven EU countries.

Over 400 people had been infected with Salmonella enterica serovar Stanley (S. Stanley) that

usually appears in the regions of Southeast Asia. After several DNA analyses seemed to point to

a specific Belgian strain, the "Joint ECDC/E FSA Rapid Risk Assessment" report detected turkey

production as the source of infection.[34]

Finally, the European Food Safety Authority (EFSA) highly recommends that when handling

raw turkey meat consumers and people involved in the food supply chain shall pay attention to

personal and food hygiene.[35]

New antibiotic-resistant strains

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Non-typhoidal salmonella (iNTS) Africa, a new form of the germ, emerged in the southeast of

the continent 75 years ago, followed by a second wave, which came out of central Africa 18

years later. The second wave of iNTS began 35 years ago, possibly in the Congo Basin, and early

in the event picked up a gene making it resistant to the antibiotic chloramphenicol. There is an

urgency to develop an effective salmonella vaccine because of the recent outbreaks in Africa of

antibiotic-resistant strains of the food-borne bacteria that is killing hundreds of thousands of

people there, as well as the heavy annual worldwide death toll each year. People with HIV are

greatly affected. A recently identified set of antigens (molecules in the invading bacteria that

trigger an immune response) that is common to both mice and humans, provide a foundation for

developing a protective salmonella vaccine that could be on the market as early as 2016. This is

good news because no new, effective antibiotics are on the horizon. In sub-Saharan the variant is

the cause of an enigmatic disease called invasive non-typhoidal salmonella (iNTS), which affects

Africa far more than other continents. Its genetic makeup is evolving into a more typhoid-like

bacteria, able to efficiently spread around the human body.

Vaccine status

Researchers say they have paved the way toward an effective Salmonella vaccine by identifying

eight antigenic molecules from human and mouse infections. These antigens provide the research

community with a foundation for developing a protective salmonella vaccine. [36]

Deaths

An estimated 142,000 Americans are infected each year with Salmonella Enteritidis from

chicken eggs,[37]

and about 30 die.[citation needed]

The shell of the egg may be contaminated with

Salmonella by feces or environment, or its interior (yolk) may be contaminated by penetration of

the bacteria through the porous shell or from a hen whose infected ovaries contaminate the egg

during egg formation.[38][39]

Nevertheless, such interior egg yolk contamination is theoretically unlikely.[40][41][42][43]

Even

under natural conditions, the rate of infection was very small (0.6% in a study of naturally

contaminated eggs[44]

and 3.0% among artificially and heavily infected hens[45]

). However, the

natural infection rate would result in roughly one in fourteen cartons (one dozen eggs) to contain

at least one egg with interior egg yolk contamination.

In 2010, an analysis of death certificates in the United States identified a total of 1,316

Salmonella-related deaths from 1990 to 2006. These were predominantly among older adults and

those who were immunocompromised.[46]