Shigella dysenteriae is a species of the rod-shaped
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l
genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
ShigellaShigella is a genus of Gram-negative, non-spore forming rod-shaped bacteria closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella cause disease in primates, but not in other mammals. It is only naturally found in humans and apes. During infection,...
. Shigella can cause
shigellosisShigellosis, also known as bacillary dysentery in its most severe manifestation, is a foodborne illness caused by infection by bacteria of the genus Shigella. Shigellosis rarely occurs in animals other than humans and other primates like monkeys and chimpanzees...
(bacillary
dysenteryDysentery is an inflammatory disorder of the intestine, especially of the colon, that results in severe diarrhea containing mucus and/or blood in the feces. If left untreated, dysentery can be fatal....
). Shigellae are
Gram-negativeGram-negative bacteria are those bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
,
non-spore-formingAn endospore is a dormant, tough, and non-reproductive structure produced by Gram-positive bacteria from the Firmicute phylum that forms when a bacterium produces a thick internal wall that encloses its DNA and part of its cytoplasm...
,
facultatively anaerobicA facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation...
,
non-motileMotility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. It can apply to either single-celled or multicellular organisms. Motile marine animals are commonly called free-swimming...
bacteria.
S. dysenteriae, spread by contaminated water and food, causes the most severe dysentery because of its potent and deadly
Shiga toxinShiga toxins are a family of related toxins with two major groups, Stx1 and Stx2, whose genes are considered to be part of the genome of lambdoid prophages. The toxins are named for Kiyoshi Shiga, who first described the bacterial origin of dysentery caused by Shigella dysenteriae. The most common...
, but other species may also be dysentery agents. Contamination is often caused by bacteria on unwashed hands during food preparation.
A stool specimen is Gram-stained to show Gram-negative rods, with no particular arrangement.
Shigella dysenteriae is a species of the rod-shaped
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l
genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
ShigellaShigella is a genus of Gram-negative, non-spore forming rod-shaped bacteria closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella cause disease in primates, but not in other mammals. It is only naturally found in humans and apes. During infection,...
. Shigella can cause
shigellosisShigellosis, also known as bacillary dysentery in its most severe manifestation, is a foodborne illness caused by infection by bacteria of the genus Shigella. Shigellosis rarely occurs in animals other than humans and other primates like monkeys and chimpanzees...
(bacillary
dysenteryDysentery is an inflammatory disorder of the intestine, especially of the colon, that results in severe diarrhea containing mucus and/or blood in the feces. If left untreated, dysentery can be fatal....
). Shigellae are
Gram-negativeGram-negative bacteria are those bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
,
non-spore-formingAn endospore is a dormant, tough, and non-reproductive structure produced by Gram-positive bacteria from the Firmicute phylum that forms when a bacterium produces a thick internal wall that encloses its DNA and part of its cytoplasm...
,
facultatively anaerobicA facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation...
,
non-motileMotility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. It can apply to either single-celled or multicellular organisms. Motile marine animals are commonly called free-swimming...
bacteria.
S. dysenteriae, spread by contaminated water and food, causes the most severe dysentery because of its potent and deadly
Shiga toxinShiga toxins are a family of related toxins with two major groups, Stx1 and Stx2, whose genes are considered to be part of the genome of lambdoid prophages. The toxins are named for Kiyoshi Shiga, who first described the bacterial origin of dysentery caused by Shigella dysenteriae. The most common...
, but other species may also be dysentery agents. Contamination is often caused by bacteria on unwashed hands during food preparation.
Diagnosis
A stool specimen is Gram-stained to show Gram-negative rods, with no particular arrangement. Enrichment is performed by growing the organisms on Selenite-F broth. Then, since the specimen is not sterile, the use of selective plates is mandatory.
XLD agarXylose lysine deoxycholate agar is a selective growth medium used in the isolation of Salmonella and Shigella species from clinical samples and from food. It has a pH of approximately 7.4, leaving it with a bright pink or red appearance due to the indicator phenol red. Sugar fermentation lowers...
,
DCA agarDCA agar - Deoxycholate Citrate Agar is a solid bacteriological growth medium.-Uses:It is particularly useful for the isolation of organisms that cause bacilliary dysentery, salmonella strains that cause food poisoning and Salmonella Paratyphi. It is not so selective for Salmonella Typhi...
, or
HE agarHektoen enteric agar is a selective and differential agar primarily used to recover Salmonella and Shigella from patient specimens. HE contains indicators of lactose fermentation and H2S production; as well as inhibitors to prevent the growth of gram positive bacteria...
are inoculated and colonies are colorless on all of them as the organism is non-
lactoseLactose is a sugar that is found most notably in milk. Lactose makes up around 2–8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey...
a fermentor. Inoculation of a
TSI slantthumb|right|250px|TSI agar slant results: preinoculated ,[[Pseudomonas aeruginosa|P. aeruginosa]], [[Escherichia coli|E...
shows an alkaline slant and acidic butt with no gas or H
2S production. Following incubation on
SIM-Academics:*Singapore Institute of Management*Studio for Interrelated Media, an academic concentration at the Massachusetts College of Art*Student information management, the process of educational institutions recording data about their students...
, the culture appears non-motile with no H
2S production. Addition of
Kovac's reagentThe indole test is a biochemical test performed on bacterial species to determine the ability of the organism to split indole from the amino acid tryptophan...
to the SIM tube following growth typically indicates no
indoleIndole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Indole is a popular component of fragrances and the precursor to many pharmaceuticals. Compounds that contain an...
formation (serotypes 2, 7 and 8 produce indole).
It's noteworthy that
Shigella flexneriShigella flexneri can cause diarrhea in humans. There are several different serogroups of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become resistant. Less severe cases are not usually treated because they...
will produce acid and gas from glucose, and
Shigella sonneiShigella sonnei is a species of Shigella.Together with Shigella flexneri, it is responsible for 90% of shigellosis.Antibiotic resistance has been reported....
is
mannitolMannitol is an organic compound with the formula . This polyol is used as an osmotic diuretic agent and a weak renal vasodilator...
and
ornithineOrnithine is an amino acid which plays a role in the urea cycle.-Role in urea cycle:
L-Ornithine is one of the products of the action of the enzyme arginase on
L-arginine, creating urea. Therefore, ornithine is a central part of the urea cycle, which allows for the...
positive, and is also late lactose fermentor (ONPG positive). Some
ShigellaShigella is a genus of Gram-negative, non-spore forming rod-shaped bacteria closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella cause disease in primates, but not in other mammals. It is only naturally found in humans and apes. During infection,...
species are capable of producing indole.