Observations on Enterobacteriaceae cultures and stained specimens

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Observations on Enterobacteriaceae cultures and stained specimens

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1, Gram-stained specimens: Escherichia coli and S. typhi Gram-stained red, G- bacilli, bluntly rounded at both ends, medium size.

2、Agar plate culture

(1) Erythromycin blue medium: commonly used medium for the isolation and identification of enteric pathogens. On this medium, E. coli can ferment lactose to produce acid, so that Erythropoietin and American Blue synthesize purple-black compounds, so its colonies are purple-black, and E. typhi does not decompose lactose, so its colonies are colorless or yellowish transparent colonies.

(2) S-S medium: a commonly used medium for the selection of intestinal pathogens, containing sodium citrate, brilliant green has an inhibitory effect on Escherichia coli, bile salts on Salmonella, dysentery bacillus has a growth-promoting effect, but also contains lactose and acid-base indicator neutral red. In this medium, E. coli can decompose lactose to produce acid, so that the colony is red, S. typhi can not decompose lactose, the colony is colorless and transparent.

(3) Double sugar iron slant: it is a kind of high-level slant medium, composed of two layers. The upper layer is made of peptone water with 1% lactose and ferrous sulfate, with phenol red as indicator, it can observe the results of lactose fermentation and hydrogen sulfide test. The lower layer is a semi-solid of peptone water containing glucose, with phenol red as indicator, which can observe the power of bacteria and lactose fermentation ability. When inoculating, first use the inoculation needle to inoculate according to the semi-solid medium inoculation method, puncture inoculation, exit along the original line and then draw a line to inoculate the slant. The bacterial growth was observed after 18-24 hours of incubation in a 37℃ incubator (Table 1).

Table 1 Growth of Escherichia coli and S. typhi on disaccharide iron slants