1. Causes and significance of salmonella testing:
According to statistics, 70% ~ 80% of bacterial food poisoning in China is caused by salmonella, who once intake contains a lot of salmonella (10 e5 ~ 10 e6 / g) of animal products, can cause bacterial infection, and then under the effect of the toxin causes gastroenteritis outbreak of food poisoning, typhoid and paratyphoid salmonella and numerous medium, meat, eggs, and food processing to sell is very likely to happen in the process of pollution. So testing for salmonella is a big deal.
2. Salmonella detection environment:
Testing for salmonella should be conducted in the secondary biosafety laboratory and the secondary biosafety cabinet (to prevent contamination of laboratory personnel and the environment by aerosols of pathogenic microorganisms from floating out of the laboratory under negative pressure).
3. Interpretation of relevant medium reagent in the detection process:
1. Pre-enrichment (non-selective):
Pepptone buffer (BPW) is a basic culture medium for increasing bacteria, which contains no inhibiting components and is conducive to the recovery of damaged salmonella. Restore the damaged salmonella cells to a stable physiological state.
2. Selective enrichment:
The combination of sodium thiosulfonic acid and sodium tetrasulfonic acid in sodium tetrapulfonate (TTB) can inhibit intestinal symbiosis bacteria. Gallate and chlorophyll inhibit the growth of e. coli and other gram-positive bacteria, while typhoid and paratyphoid salmonella can still grow.
Cysteine selenite (SC) can be used as selective enrichment for typhoid and other salmonella bacteria. The selenite in its components combines with the sulfurous amino acids in peptone to form a complex of selenite and sulfur, which affects the sulfur metabolism of bacteria, thereby inhibiting the proliferation of escherichia coli, enterococcus and proteus.
3. Separation medium (selective):
1) sulfurous acid bismuth AGAR (BS) in the sulfurous acid bismuth indicator inhibit gram-positive bacteria and coliform bacteria, but does not affect the growth of salmonella; Typhoid and other salmonella bacteria can use glucose to reduce bismuth bisulphate to bismuth sulfate, forming a black and brown ring around the black colonies.
BS cannot be high pressure and cannot be heated too much to reduce its selectivity. It should be prepared when needed and stored in the dark. After 48h, it will lose its selectivity. TTB has calcium carbonate used to eliminate and absorb toxic metabolites, which is easy to precipitate. The TTB cannot be reheated after the addition of additives. Or SC(SC contains sodium selenite, a highly toxic substance in SC, which should be safe when used, and be prepared on the day of use.) A higher detection rate can be obtained if used in combination.
Bacterial colony morphology of salmonella on BS: the H2S are black with metallic luster, brown or gray. The surrounding medium of the colony can be black or brown, and some colonies that do not produce H2S form greyish-green colonies, while the surrounding medium remains unchanged.
Potassium, sodium deoxycholic acid, bromothymol and acid redness in potassium HE AGAR inhibit the growth of gram-positive bacteria. Sodium thiosulphate and ammonium ferric citrate are used to detect the production of H2S, making the colony center black.
Bromothymol and acidic rered are pH indicator, and the fungus fermenting the sugar acid-producing acid is orange and yellow, and the colony of non-fermented sugar is cyan.
HE cannot be sterilized under high pressure, boil for more than 1min, cool in the water bath, and keep for only one day.
The pattern of salmonella bacteria on HE AGAR is blue-green or blue. Most strains produce H2S, with black or almost all black at the center. Some strains are yellow, with black centers or almost all black.
Sodium deoxycholic acid in XLD AGAR inhibits the growth of gram-positive bacteria. At this concentration, it also acts as an inhibitor of escherichia coli but does not affect the growth of salmonella and shigella.
XLD flat plate cannot stand high pressure, cannot overheat boil, save for a day. The sensitivity of XLD medium to isolate salmonella and shigella bacteria exceeds that of traditional media, such as EMB, SS and BS. Because there are potential factors to inhibit the growth of shigella, this medium is a reliable medium for isolating and identifying salmonella and shigella. It is widely used abroad.
The colony morphology of salmonella on XLD: pink band or without black center, some strains can present large shiny black center or all black colonies; Some colonies are yellow, with or without black centers.
The colorant culture medium for salmonella bacteria is mainly used for rapid screening and separation. The basic principle is to use the special reaction of salmonella enzyme and chromogenic group to free the chromogen, so that the salmonella bacteria present a specific color on the medium (specific color check the instructions of relevant brands of chromogenic medium) and other intestinal bacteria such as e. coli present other colors.
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