Enterobacteriaceae: They include large heterogeneous group of gram negative rods whose natural habitat is the intestinal tract of man or animals. General characteristics: 1-They are short rods with rounded edge 4-5 microns in length, non-spore formers. 2- All are motile (having peritichous flagellae) except Klebsiella and Shigella 3- Aerobes or facultative anaerobes 4- Oxidase negative and catalase positive. 5- All ferment glucose with acid or acid and gas production and reduce nitrate to nitrite.

a Based on lactose fermentation they are divided into: Lactose fermenters as: Escherichia coli, Klebsiella pneumoniae, Enterobacter and Citrobacter. Non-lactose fermenters as: Salmonella, Shigella, Proteus, Morganella, Yersinia, Serratia, Providencia, Edwardsiella and Hafnia.

a Microscopy: All Enterobacteriaceae species are Gram's negative short bacilli, and non- spore producers. All are motile (having peritichous flagellae) except Klebsiella and Shigella.

Biochemical identification of Enterobacteriaceae: a

The API 20 E strips showing the biochemical reactions of two different Enterobacteriaceae species: An automated bacterial identification System:

Pathogenic Enterobacteriaceae species: Escherichia coli: Escherichia coli is a normal flora of human small intestinal tracts. Some species are pathogenic such as: 1- Enterotoxigenic E.coli: It caused traveler's diarrheal illness. It produces two exotoxins: heat-labile and heat stable toxins. 2- Enteropathogenic E.coli: it causes infantile diarrhea, some causes hemorrhagic syndrome due to ingestion of contaminated beef. 3- Enteroinvasive E.coli: cause inflammatory disease due to Invasion and ulceration of intestinal mucosa.

The genus Klebsiella and Enterobacter : The pathogenic species of this genus mainly summarized by the following: Klebsiella aerogens causes urinary tract infection (UTI). Klebsiella pneumonia causes Friedlander's pneumonia. Klebsiella rhinoscleromatis causes Rhinoscleroma. Enterobacter aerogens and Enterobacter cloacae may be the causative agent for the following: 1-Urinary tract infection. 2-Meningitis. 3-Bacteraemia 4-Wound infection.

Facultative anaerobic Coccobacilli: The Genus Brucella: The organisms are gram-negative non-motile, non-sporing, non-capsulated short coccobacilli. They are aerobic; grow best on liver extract agar. They are oxidase and catalase positive.

a The genus includes: Brucella melitensis: The most frequent and virulent strain. It is mainly a pathogen of goats and sheep but it can also infect cattle. It the strain found in the Mediterranean area and Arabian Gulf. Brucella abortus: infects cattle mainly but can infect other animals as sheep, goats camels, dogs and pigs. Brucella suis: mainly found in pigs. Brucella canis: mainly found in dogs. The Brucella species can be differentiated by the inhibition of their growth in culture by certain pigments, H 2 S production and CO 2 growth requirements.

Treponema pallidum: Microscopy: Treponema pallidum has a characteristic helical shape, tightly coiled bacilli like a telephone cord. All are Gram’s negative spiral bacilli with endoflagella. Because it can not be stained by Gram’s stain; wet-mount Preparation of clinical specimens show a characteristic Corkscrew- like motility with 90 ˚ angulation ( Darkfeild microscopy).