University of Tabuk Faculty of Applied Medical Science Department of Medical Laboratory Technology Medical Microbiology II (p)Identification of Gram Negative Bacilli Culture Characteristics Mr.AYMAN.S.YOUSIF M.SC IN Microbiology &IMMUNOLOGY Academic Year: (1434-1435-2013-2014)
Characters of Enterobacteriaceae • All Enterobacteriaceae. • Gram-negative rods. • Ferment glucose with acid production. • Reduce nitrates into nitrites. • Oxidase negative. • Facultative anaerobic. • Motile exceptshigella and klebsiella . • Non-capsulated except Klebsiella. • Non-fastidious. • Grow on bile containing media (MacConkey agar).
Enterobacteriaceae • Some Enterobacteriaceae are true pathogens • Salmonella spp. • Shigella spp. • Yersinia spp. • Certain strains of E. coli (ETEC, EPEC, EIEC, EHEC) • Most members of the Enterobacteriaceae are opportunistic or cause secondary infections of wounds, the urinary and respiratory tracts, and the circulatory system e.g. E. coli.
specimens Morphologic Identification Microscopy & Staining Biochemical tests ( Identification and Isolation ) Sub culture in the special types of media for confirmation Serological Test Susceptibility Testing ( to select the suitable antibiotics for treatment the pathogenic isolated bacteria from the specimen ) General Procedureof Bacteriological Diagnosis Cultivation in suitable types of media
Identification of Enterobacteriaceae • Gram stain • All Enterobacteriaceae are Gram-negative rods • Arranged in single
Classification of Enterobacteriaceae • There are several selective and differential media used to isolate distinguishes between LF & LNF • The most important media are: • MacConkey agar • Eosin Methylene Blue (EMB) agar • Salmonella Shigella (SS) agar • In addition to Triple Sugar Iron (TSI) agar
Differentiation Between LF & NLF ByGrowth on Macconkey Agar • MacConkey agar is selective & differential medium for Enterobacteriaceae MacConkeyAgar Contains Bile salts Crystal violet Lactose Neutral red pH indicator Acidic: Pink Inhibit growth of G+ve bacteria Cause of differential Cause of selectivity Lactose non fermenters colorless colonies Lactose feremnters Pink colonies
1/10 II I I 1/5 Rotate 90 III 1/4 IV Streak-plate technique four-area streak plate technique Rotate plate 90 Flame loop Flame loop Rotate 90
Identification of EnterobacteriaceaeDifferentiation between LF & NLF byGrowth on MacConkey agar • Method: • MacConkey agar is inoculated with tested organismusing streak plate technique. • Incubate the plate in incubator at 37 C/24 hrs • Results : • LF organism appears as pink colonies (e.g. E. coli) • NLF organism appears as colorless colonies (e.g. Shigella)
Growth of Enterobacteriaceae on MacConkey agar Colorless colonies Pink colonies Lactose non feremters Salmonella, Shigella, Proteus Lactose feremters E. coli, Citrobacter Klebsiella, Enterobacter Uninoculated plate
Reaction on Salmonella Shigella (SS) agar • SS agar is a selective & differential medium used for isolation of Salmonella and Shigella • The selective agents are bile salts, and brilliant green dye, which inhibit gram-positive organisms • The medium contains only lactose as a differential agent and thus differentiates on the basis of lactose fermentation • The formation of acid on fermentation of lactose causes the neutral red indicator to make pink colonies. • Non lactose fermenting organisms are colorless on the medium. • SS agar contains sodium thiosulfate and ferric ammonium citrate allows the differentiation of organisms that produce H2S • Lactose fermenters, such as E. coli, have colonies which are pink • Shigella appears transparent or amber • Salmonella appears transparent or amber with black centers due to H2S production Lactose fermenter Neutral red Lactose Acid Pink colonies Ferrous sulfide Black precipitate H2S + Ferric ammonium citrate
Growth of Enterobacteriaceae on SS agar A.Klebsiella pneumoniaeB.Escherichia coliC: Salmonella sp.D: Proteus mirabilisE: Ps. aeruginosa. Bothare lactose fermenters BothSalmonella sp. & Proteus product H2S Pseudomonascolonies are nearly colorless
Growth of Enterobacteriaceae on EMB agar • Constituents: Nutrients, lactose, neutral red, eosin and methylene blue. • Appearance: A clear red medium • A selective stain for Gram-negative bacteria. It is a mix of two stains, eosin and methylene blue in the ratio of 6:1. • Acommon application of this stain is in the preparation of EMB agar, a differential microbiological medium, which inhibits the growth of Gram-positive bacteria and provides a colour indicator distinguishing between organisms that ferment lactose (e.g., E. coli) and those that do not (e.g., Salmonella, Shigella).
Eosin Methylene Blue (EMB) • Lactose fermentation produces acids, which lower the pH. This encourages dye absorption by the colonies, which are now coloured purple-black. • Lactose non-fermenters may increase the pH by deamination of proteins. This ensures that the dye is not absorbed. • On EMB if E. coli is grown it will give a distinctive metallic green sheen
Growth of Enterobacteriaceae on EMB agar colonies are very dark, almost black e.g. E. coli