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Gram Negative Bacilli. Enterobacteriaceae Family : 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,

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Gram Negative Bacilli


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    1. Gram Negative Bacilli Enterobacteriaceae Family: 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 peritichousflagellae) except Klebsiella and Shigella 3- Facultative anaerobes. 4- Oxidase negative and catalase positive. 5- All ferment glucose with acid or acid and gas production and reduce nitrate to nitrite.

    2. Enterobacteriaceae Based on lactose fermentation they are divided into: Lactose fermenters as: Escherichia coli, Klebsiellapneumoniae, Enterobacter and Citrobacter . Non-lactose fermenters as: Salmonella, Shigella, Proteus, Morganella, Yersinia, and Serratia.

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

    4. Enterobacteriaceae Biochemical identification of Enterobacteriaceae:

    5. Enterobacteriaceae The API 20 E strips showing the biochemical reactions of two different Enterobacteriaceaespecies: Anautomated bacterial identification System:

    6. Enterobacteriaceae Escherichia coli:Escherichia coli is a normal flora of human small intestinal tracts. Some species are pathogenic to man. - Facultative anaerobic bacterium. - Metallic green sheen on EMB medium. - Rose pink colonies on MacConkey’s. - Indole production: positive.

    7. Enterobacteriaceae The genus Klebsiella: Klebsiellaspecies are Lactose fermenter that produce mucoid colonies on MacConkey’s agar. Examples: 1- Klebsiellaaerogens. 2- Klebsiellapneumonia. - Mucoidcolonies are capsulated; So it should be stained by Capsule Stain. The capsule could be illustrated microscopically by India Ink stain.

    8. Enterobacteriaceae The Genus Salmonella: Salmonella are found in many animals, including poultry, reptiles, rodents, domestic animals, birds, and human. Serotypes such as Salmonella typhiand Salmonella paratyphiare highly adapted to man. The most common sources of human infections are poultry, eggs, and dairyproducts. Fecal-Oralroute of entry is associated with childreninfection and food poisoning. Salmonella are non-lactosefermenters, and most strains are H2Sproducers. Identification of microbe depends on serotypingreaction with Anti-Salmonella antibodiesin vitro.

    9. Enterobacteriaceae Salmonella species cultivated on XLD agar; the microbe is non-lactose fermenter And H2S producer. The microbe show positive reaction with polyvalent reagent.

    10. Enterobacteriaceae The Genus Proteus: Gram’s negative bacilli belong to Enterobacteriaceae. Non-Lactosefermenters and H2S producers on XLD. On blood agar, the microbe illustrates swarminggrowth. Almost all strains are urease positive.

    11. Pseudomonas The Genus Pseudomonas: - Gram’snegativeshort aerobic bacilli. All species are oxidase positive. Grape-like odor . The exopigmentproduction on nutrient agar discriminate this genus from other. (greenish yellowish pyoverdin) .

    12. Vibrios The Vibrios: Vibrios are gram-negative comma shaped, curved bacilli, motile with a single polar flagellum( 2*0.5 micrometer diameter). Facultative anaerobes. - Vibrio choleraeserogroupsO1 and O139 are responsible of epidemic cholera. SerogroupO1 is non-capsulated while O139 is capsulated. They grow on different media at high pH (8.5-9.5) as alkalinepeptonewater media. TCBS is a selective medium. Oxidase and Indolepositive. - Reduce nitrate to nitrite.

    13. Vibrios Microscopic and cultural characteristics of Vibrio cholera: Curved Gram’s negative bacilli Vibrio’s as seen microscopically. Yellow-Mucoid colonies of Vibrio cholera on TCBS agar.

    14. Coccobacilli Coccobacilli: The Genus Brucella: The organisms are gram-negative non-motile, non-spore forming, non-capsulated short coccobacilli. They are aerobic bacteria; grow best on liver extract agar. They are oxidase and catalasepositive. 5-10% CO2 is required for primary isolation.

    15. Coccobacilli The genus includes: Brucellamelitensis: It is mainly a pathogen of goats and sheep. It the strain found in the Mediterranean area and Arabian Gulf. Brucellaabortus: infects cattle mainly but can infect other animals as sheep, goats camels, dogs and pigs. Brucellasuis: mainly found in pigs. Brucellacanis: mainly found in dogs.

    16. Curved Bacilli The Pleomorphic Curved S or U-Shaped Bacilli: The Genus Helicobacter and Campylobacter: Helicobacter pylori : - Gram’s negative pleomorphic S or U-shaped Bacilli. - Motile, non-spore formers, and capsulated bacteria. - Fastidiousmicrobe grow best on Enriched media supplemented by Bovine serum albumin and antibiotics. - Microaerophilic(5% O2, 10% CO2), acidophilic bacterium, Optimum Temp is 37 ᵒC. - Catalase, oxidase, and urease positive. Electron micrograph for Helicobacter pylori

    17. Curved Bacilli Campylobacter jejuni: - Similar microscopic and cultural characteristics of Helicobacter. - Urease negative. - Oxidase and catalase positive. - Optimumgrowth temp is 42ᵒ C.

    18. Spirochetes Spirochetes: Treponema species, Borrelia species, and Leptospira species Treponemapallidum: Microscopy: Treponemapallidumhas 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 should be examined by Darkfield microscopy.

    19. Spirochetes Treponemapallidumshow characteristic Corkscrew- like motility with 90 ˚ angulation . Electron Microscopy Darkfieldmicroscopy