The genus Shigella contains fewer species than the genus Salmonella and is antigenically less complex.
Clinical dysentery may be caused by Shigella, Salmonella, Entamoebahistolytica, Proteus morganii, and viruses.
Shigelladysenteria was isolated by Shiga in Japan in 1898.SPECIES:
ShigellasonneiMORPHOLOGY AND STAINING:
- Short rods
- Non-spore former
Shigella species are found only in the human intestinal tract.
Carriers of pathogenic strains can excrete the organism up to two weeks after infection and occasionally for longer periods.
Shigella are killed by drying. Shigella are transmitted by the fecal-oral rout.
The highest incidence of Shigellosis occur in areas of poor sanitation and where water supplies are polluted.
All members of Shigella are aerobic and facultative anaerobes.
Grow readily in culture media at pH 6.4 to 7.8 at 10 oC - 40 oC, with optimum of 37 oC.
After 24 hours incubation, Shigella colonies reaches a diameter of about 2 mm.
The colonies are circular, convex, colorless, but moderately translucent with smooth surface, and entire edges.
Small tangled hair-like projections can sometimes be seen at one or more points on the periphery of the colony.
In XLD they appear pinkish to reddish colonies while in Heaktoen Enteric Agar (HEA), they give green to blue green colonies.
If a number of typical colonies present onto the original plate, a tentative diagnosis can be made by direct slide agglutination with polyvalent Shigella antiserum.
In all instances, diagnosis should be confirmed by additional biochemical tests and by specific type agglutination.
=All ferment glucose, some ferments mannitol
=They do not form acetyl-methylcarbinol,
=Does not hydrolyze urea or liquefy gelatin
=TSIA (Alkaline slant over acid butt)
=IMVIC V + - -
O antigen: The ability to survive the passage through the host defenses may be due to O antigen.
Invasiveness: Virulent shigella penetrate the mucosa and epithelial cells of the colon in an uneven manner. Intracellular multiplication leads to invasion of adjacent cells, inflammation and cell death. Cell death is probably due to cytotoxic properties of shiga toxin that interfere with protein synthesis. The cellular death and resulting phagocytosis response by the host accounts for the bloody discharge of mucus and pus and shallow ulcers characteristic of the disease.
Other toxins: It has a protein toxin which may be neurotoxic, cytotoxic, and enterotoxic. The enterotoxic property is responsible for watery diarrhea.
Shigella dysentery’s form a powerful exotoxin, it is associated with epidemics of bacillary dysentery.
In man, shigellosis begins with symptoms of acute gastro-enteritis which is accompanied by abdominal pain and diarrhea.
As it progresses, diarrhea becomes more frequent and is usually accompanied colicky pain.
Later diarrhea losses its fecal characteristic and is followed by mucus with pus and blood.
The disease is usually accompanied by fever and marked prostration. It is also known that children are more frequently attacked than adult persons and the symptoms are more severe.
The only satisfactory method of laboratory diagnosis is to cultivate the bacilli from the patient.
In the early stages of acute shigellosis, isolation of the causative organism from the feces is usually accomplished without difficulties by using the same special media and methods employed for salmonella
1. Cultivation of the bacilli from stool specimen during the first 4-5 days of the
2. Smears: Gram-negative bacilli appearing singly
TSIA = Alkaline/acid (No gas no H2S)
IMVIC reaction : V + - -
3. Serological examination with polyvalent and monovalent anti-sera.
1. Shigella are rarely encountered except in feces, Salmonella are also found in cultures of bile, blood, urine, abscesses and cerebrospinal fluid.
2. Microscopic examination of stained smears is of no use for the identification of the bacilli except the fact that it is a gram negative bacilli.
3. On general and selective media colonies are similar. On Blood agar they are smooth, gray and opaque. Most salmonella species produces H2S and therefore will be black on Bismuth Sulfite Agar.
Points of identifications between Salmonella and Shigella :
1. Water and electrolytes replacement.
2. Antibiotic therapy is required to eliminate the organism. Due to the emergence of resistant strains of shigella, antibiotic sensitivity, must be performed on any shigella
isolate to determine suitable antibiotics:
Sulfonamides, tetracycline, Chloramphenicol, ampicillin and streptomycin are known to be effective against shigella.
• Short lived; Preparation of oral live attenuated vaccine is on the way to stimulate mucosal IgA.
• Sanitary precautions Good personal hygiene (hand washing).
1. Yersiniapestis Plague
•Gram-negative rods (Coccobacilli)
• Non-motile with the exception of Y.pseudotuberculosis and Y. enterocolitica(motile at 22oC)
• Facultative anaerobe
• Catalase positive
• Oxidase negative
Plague is primarily a disease or rodents that could be transmitted to man.
Plague occurs in three forms: BUBONIC, SEPTICEMIC and PNEUMONIC.
Bubonic plagueis the common form resulting from the cutaneous inoculation of Y. pestis by the bite of an infective flea. The incubation period is 2-7 days after exposure.
The disease is characterized by sudden fever, shaking chills, headache and pain in the area of involved lymph nodes.
Untreated, bubonic plague can result in coma and death.
Classic bubonic plague entails an inflammatory response in the regional lymph nodes which, when enlarged, is referred to as bubo.
Untreated bubonic plague may become septicemic plague. Virtually, 100% of the untreated septicemic plague infections are followed by pulmonary involvement and death.
At this stage human to human transmission may occur.
NB: Plague is an internationally reportable disease, and when suspected, must be reported to the local public health authorities.
The bubonic plague (also known as the Black Death), was killed more than 200 million people in the Middle Ages.
aspirate : an aspirate from buboes is a good specimen if handled properly using a sterile syringe and needle and aseptic technique in collecting.
Special precautions must be taken to avoid the spread of infection. Face masks, gloves and gowns must be routinly worn when in contact with the patient.
Blood: Blooddrawn aseptically is also good specimen for isolation of Y.pestis is septicemicpatients.
sputum: Early morning sputum obtained from deep couph, avoiding contamination with saliva.
The patient must be instructed to rinse the oral cavity three to four times.
Biopsy from infected lymph nodes.
Smears are stained with WAYSON to be examined for bipolar stained rods.
Routine media, selective for gram negative enteric bacteria are good for isolation.
BHIB, Trypticase Soy Broth, Blood Agar and Nutrient Agar are also used.
Identification by Biochemical Tests:
Yersiniapestis is differentiated from other Yersinia species by a negative ureasenegative test and carbohydrate fermentaion tests. Motility at 37oC and 22oC is also used.
Bipolar stained rod
MacConkey agar :
Appears as small non-lactosefermenting colonies
Trypticase soy broth :
..-Incubate : 5% CO2, 35° C , 24 hrs
Have a good growth
1. Identification of suspected Y. pestis isolate: Commercially avialableantiplagueserum is used to agglutinate cells.
2. Patient serum: (Detection of antibodies): Formalinized suspension of known Y. pestis cells is used as antigen. One drop of diluted antigen is added to one drop of patient`s serum. This mixture is shaken gently for 15-30 minutes and examined for agglutination under low power magnification.
Routine sensitivity testing is unreliable because In-Vitro results commonly differ significantly from In-Vivo efficacy.
For example, penicillin may show slight to wide zone of inhibition of Y. pestis, although penicillin has no In-Vivo activity against this organism.
Tetracycline, Streptomycin or Chloramphenicol and sulfonamides are effective in treatment of plague.
Both agents causes yersinosis which appears as enteric disorders, e.g., diarrhea, enteritis, terminal ileitis.
Clinical symptoms can not be distinguished from those caused by other enteric pathogens such as salmonella and shigella.
Faecal-oral rout is the common mode of transmission, although may be transmitted by bites of infective arthropods.
Human to human transmission is well documented.
* Both of them can cause Yersinosis ..
Stool, blood, excised mesenteric nodes, or appendices obtained during surgery.
Yersinia Selective Agar: is an excellent selective medium for the isolation of Y. enterocolitica from stool or sputum.
Cold Enrichment technique: Specimens that are grossly contaminated, are inoculated into buffered saline (pH 7.4-7.6) and stored at 4-7 oC. Both organisms tolerate and even grow at these temperatures, other organisms gradually dies.Biochemical Identification