Dip in Technology
Intro to Microbiology
SCTL530/0801

Andy Gao

Shigella dysenteriae

Morphology:

Shigella dysenteriae is a species of the bacterial genus Shigella. Shigella can cause shigellosis (bacillary dysentery). It is a human pathogen and it produces the shiga toxin (1). Shigellae are rod shaped, Gram-negative, facultatively anaerobic, non-motile bacteria closely related to Escherichia coli and Salmonella (3). It grows best at 37 degree Celsius, and it does not form spores when conditions for growth become bad (1).The Incubation requires 12 to 15 hours. There is no gas production released from the carbohydrates (2).

Habitat:

Shigella can be contracted by eating fecal contaminated food or water and it might cause bad diarrhea. Most of Shigella infections are the results of the bacterium passing from stools or soiled hands of one person to the mouth of another one. This happens hand-washing habits are ignored and can happen during certain types of sexual activity. Thus the family members and children are the high risk group of the infection.

“Flies can breed in infected human feces and then contaminate food. Contaminated food usually looks and smells normal. Food may become contaminated by infected food handlers who forget to wash their hands with soap after using the bathroom. Water may become contaminated with Shigella bacteria if sewage runs into it, or if someone with shigellosis swims in or plays with it” (5). Vegetables might also be infected if they are irrigated by the water with Shigella in it.

Human Impact:

There are 14,000 cases of shigellosis are reported in the USA every year. Most of them are caused by Shigella dysenteriae. Quite few people with shigellosis require hospitalization. It is more common in summer than winter. “Children, especially toddlers aged 2 to 4, are the most likely to get shigellosis. Many cases are related to the spread of illness in child-care settings, and many are the result of the spread of the illness in families with small children (5).”

Symptoms:

Symptoms may range from mild abdominal discomfort to full-blown dysentery characterized by cramps, diarrhea, fever, vomiting, blood, pus, or mucus in stools or tenesmus. Onset time is 12 to 50 hours.

Isolation and Identification:

Food samples may be tested for Shigella dysenteriae in order to be shown to produce satisfactory results with artificially contaminated produce.

Replicate the prepared food samples onto at least 2 selective differential media, such as MacConkey (low selectivity), XLD (high selectivity) and HE (moderate to high selectivity) agars to obtain well isolated colonies. Then incubate plates at 35°C for 24 hrs. Examine the characteristic Shigella colonies in the plates. The sample colony in MAC is convex, colourless and the diameter is around 2~3 mm. One in XLD is red or colourless and the size of the colony is 1~2 mm. The colonies in HE agar show green colour and 2~3 mm.

Use a sterilized needle, take suspect colonies and inoculate into TSI agar and incubate the media at 35°C for 20-24 h. (A TSI slant agar includes an agar, a pH-sensitive dye, high concentrations of lactose and sucrose, low concentrations of glucose, few sodium thiosulfate and ferric citrate.)(7)

Shigella can be selectively identified by the method TSI slant. It would shows K/A (red slant/yellow butt) and the H2S on TSI is negative.

Reference:

http://home.cvc.org/cvcscience/MS/MS1/MOPM/Shigelladysenteriae.ppt

http://microbes.historique.net/dysenteriae.html

http://en.wikipedia.org/wiki/Shigella_dysenteriae

http://en.wikipedia.org/wiki/Shigella

http://www.cdc.gov/nczved/dfbmd/disease_listing/shigellosis_gi.html

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=124020

http://en.wikipedia.org/wiki/TSI_slant

http://www.hc-sc.gc.ca/fn-an/res-rech/analy-meth/microbio/volume3/mflp25_e.html