''Salmonella enterica'' (formerly ''Salmonella choleraesuis'') is a
rod-shaped
Bacterial cellular morphologies are the shapes that are characteristic of various types of bacteria and often key to their identification. Their direct examination under a light microscope enables the classification of these bacteria (and archae ...
,
flagellate
A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...
,
facultative anaerobic,
Gram-negative
Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...
bacterium
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the ...
and a
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of the
genus
Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
''
Salmonella
''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...
''.
It is divided into six subspecies, arizonae (IIIa), diarizonae (IIIb), houtenae (IV), salamae (II), indica (VI), and enterica (I). A number of its
serovar
A serotype or serovar is a distinct variation within a species of bacteria or virus or among immune cells of different individuals. These microorganisms, viruses, or cells are classified together based on their shared reactivity between their ...
s are serious human
pathogen
In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
s; many of them are (more specifically) serovars of
''Salmonella enterica'' subsp. ''enterica''.
Epidemiology
Most cases of salmonellosis are caused by food infected with ''S. enterica'', which often infects cattle and poultry, though other animals such as domestic cats and
hamsters have also been shown to be sources of infection in humans. It primarily resides in the intestinal tract of animals and humans and can be found in feedstuff, soil, bedding, litter, and fecal matter.
The primary reservoir for the pathogen is poultry and 70% of human cases are attributed to the consumption of contaminated eggs, chicken, or turkey. Raw
chicken eggs and
goose eggs can harbor ''S. enterica'', initially in the
egg white
Egg white is the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens, it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms a ...
s, although most eggs are not infected. As the egg ages at room temperature, the yolk membrane begins to break down and ''S. enterica'' can spread into the
yolk
Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...
. Refrigeration and freezing do not kill all the bacteria, but substantially slow or halt their growth.
Pasteurizing and
food irradiation are used to kill ''Salmonella'' for commercially produced foodstuffs containing raw eggs such as ice cream. Foods prepared in the home from raw eggs, such as
mayonnaise
Mayonnaise (), colloquially referred to as "mayo" (), is a thick, creamy sauce with a rich and tangy taste that is commonly used on sandwiches, hamburgers, Salad#Bound salads, bound salads, and French fries. It also forms the base for various o ...
, cakes, and cookies, can spread salmonellae if not properly cooked before consumption. Salmonella is the leading foodborne pathogen in the United States, causing the most deaths and having the highest cost burden. It is a resilient microorganism capable of surviving long periods of time in hot and dry environments, increasing its effectiveness as a pathogen and making it able to survive the harsh environments of the gastrointestinal tract and farms.
''S. enterica'' genomes have been reconstructed from up to 6,500 year old human remains across Western Eurasia, which provides evidence for geographic widespread infections with systemic ''S. enterica'' during prehistory, and a possible role of the
Neolithization process in the evolution of host adaptation. Additional reconstructed genomes from colonial Mexico suggest ''S. enterica'' as the cause of ''
cocoliztli'', an epidemic in 16th-century
New Spain
New Spain, officially the Viceroyalty of New Spain ( ; Nahuatl: ''Yankwik Kaxtillan Birreiyotl''), originally the Kingdom of New Spain, was an integral territorial entity of the Spanish Empire, established by Habsburg Spain. It was one of several ...
. In 1545, this outbreak of ''S. enterica'' spread explosively across what is now Mexico. Over the next century, the disease killed up to 90% of the Indigenous population.
Children under the age of five years, the elderly, and
immunosuppressed adults are at an increased risk of systemic dissemination of the disease and need specialized treatment to combat the disease. Drinking extra fluids and
antibiotic
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...
s such as
fluoroquinolones are typical treatments. Complications of the disease often appear as anemia or septicaemia, and the mortality rate is 15% once these symptoms arise.
The serogroup ''S.'' Typhi is the cause of
typhoid fever.
Nomenclature
''S. enterica'' has six subspecies, and each subspecies has associated
serovar
A serotype or serovar is a distinct variation within a species of bacteria or virus or among immune cells of different individuals. These microorganisms, viruses, or cells are classified together based on their shared reactivity between their ...
s that differ by antigenic specificity. ''S. enterica'' has over 2500 serovars. ''
Salmonella bongori'' was previously considered a subspecies of ''S. enterica'', but it is now the other species in the genus ''Salmonella''. Most of the human
pathogenic
In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.
The term ...
''Salmonella'' serovars belong to the ''
enterica'' subspecies. These serogroups include ''S.'' Typhi, ''S.'' Enteritidis, ''S.'' Paratyphi, ''S.'' Typhimurium, and ''S.'' Choleraesuis. The serovars can be designated as written in the previous sentence (capitalized and nonitalicized following the genus), or as follows: "''S. enterica'' subsp. ''enterica'', serovar Typhi".
Subspecies ''S. e.'' ''arizonae'', named after the state of
Arizona
Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
, is most commonly found in cold-blooded animals (especially snakes), but can also infect turkey, sheep, and humans. It is endemic in southwestern United States. The similar ''S. e.'' subsp. ''diarizonae'' also infects snakes and occasionally humans.
Pathogenesis
Secreted proteins are of major importance for the
pathogenesis of infectious diseases caused by ''S. enterica''. A remarkably large number of
fimbrial and nonfimbrial
adhesins are present in ''Salmonella'', and mediate
biofilm
A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
formation and contact to host cells. Secreted proteins are also involved in host-cell invasion and intracellular proliferation, two hallmarks of ''Salmonella'' pathogenesis.
Regulatory proteins such as
IgaA are also involved in maintaining envelope integrity and modulating stress responses during pathogenesis.
DNA repair capability
Exposure of ''S. enterica'' to
bile salts
Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver in peroxisomes. Bile acids are conjugated with taurine or glycine residues to give anions called bile ...
, such as sodium
deoxycholate, induces the
SOS DNA damage response indicating that in this organism bile salts cause
DNA damage.
Bile salt exposure is found to increase
GC to AT transition mutations and also to induce genes of the
OxyR and SoxRS regulons suggesting further that bile salts specifically cause oxidative DNA damage.
[ Mutants of ''S. enterica'' that are defective in enzymes required for the process of base excision repair are sensitive to bile salts. This indicates that wild-type ''S. enterica'' uses base excision repair to remove DNA damages caused by the bile salts.][ The ]RecBCD
Exodeoxyribonuclease V (EC 3.1.11.5, RecBCD, Exonuclease V, ''Escherichia coli'' exonuclease V, ''E. coli'' exonuclease V, gene recBC endoenzyme, RecBC deoxyribonuclease, gene recBC DNase, gene recBCD enzymes) is an enzyme of ''E. coli'' that ini ...
enzyme which functions in recombinational repair of DNA is also required for bile salt resistance.
Small noncoding RNA
Small nonprotein-coding RNAs ( sRNA) are able to perform specific functions without being translated into proteins; 97 bacterial sRNAs from ''Salmonella'' Typhi were discovered.
AsdA (antisense RNA of dnaA) is a ''cis-''encoded antisense RNA
Antisense RNA (asRNA), also referred to as antisense transcript, natural antisense transcript (NAT) or antisense oligonucleotide, is a single stranded RNA that is complementary to a protein coding messenger RNA (mRNA) with which it hybridizes, and ...
of ''dnaA'' described in ''S. enterica'' serovar Typhi. It was discovered by deep sequencing and its transcription was confirmed by Northern blot and RACE analysis. AsdA is estimated to be about 540 nucleotide
Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...
s long, and represents the complementary strand to that encoding DnaA, a protein that plays a central role in the initiation of DNA replication and hence cellular division. In rich media, it is highly expressed only after reaching the stationary growth phase, but under limiting iron or osmotic stress, it is already expressed during exponential growth. Overexpression of AsdA stabilizes dnaA mRNA, increasing its levels and thereby enhancing its rate of translation. This suggests that AsdA is a regulator of DNA replication.
See also
* 1984 Rajneeshee bioterror attack
* AsrC small RNA
* Bacterial small RNA
* HilD 3'UTR
* IsrM small RNA
* PinT small RNA
* Typhoid Mary
References
External links
Notes on ''Salmonella'' nomenclature
*
Current research on ''Salmonella typhimurium '' at the Norwich Research Park
*
Type strain of ''Salmonella enterica'' at Bac''Dive'' – the Bacterial Diversity Metadatabase
{{Authority control
Salmonella
Gram-negative bacteria
Typhoid fever