|
3 号麦康凯琼脂|MacConkey Agar No.3
用途:抑制革兰氏阳性球菌，可以极好地区分乳糖发酵菌和非乳糖发酵菌的选择性培养基
500g 可以配制9.7 升培养基
Directions
Suspend 51.5g in 1 litre of distilled water. Bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.
Description
A more selective modification of MacConkey medium which is suitable for the detection and enumeration of coliform organisms and also for the detection and isolation of Salmonella and Shigella species occurring in pathological and food specimens. Due to the inclusion of a specially prepared fraction of bile salts in addition to crystal violet, the medium gives improved differentiation between coliforms and non-lactose fermenting organisms whilst Gram-positive cocci are completely inhibited.
This formulation corresponds with that recommended by the American Public Health Association1 for the direct plating of water samples for coliform bacilli, for the examination of food samples for food poisoning organisms2 and for the isolation of Salmonella and Shigella species in cheese3.
Amongst other examples of the use of Oxoid MacConkey Agar No.3 are: the count of coli-aerogenes bacteria in poultry faecal specimens4; the count of coli-aerogenes bacteria in cattle and sheep faeces5; the count of coli-aerogenes and non- lactose fermenting organisms in poultry carcases6; bacterial counts on irradiated canned minced chicken7; the recognition of coli-aerogenes bacteria during investigations on the genus Aeromonas8.
Anderson et al 9 added 10mg/ml of kanamycin to MacConkey Agar to isolate epidemic strains of Citrobacter diversus which were causing neonatal meningitis.
The addition of 100mg of 4-methylumbelliferyl-ß-D-glucuronide to one litre of MacConkey Agar detects the enzyme ß-glucuronidase10. The cleaved 4-methylumbelliferyl moiety is fluorescent at 366nm. Thus colonies of Escherichia coli can be detected rapidly in mixed cultures by examining the plate under a UV lamp after overnight incubation at 35°C. However, it should be remembered that other organisms may also be ß-glucuronidase positive.
Technique
After inoculation the plates are usually incubated for 18 to 24 hours at 35°C and for a further 24 hours if non-lactose fermenting organisms are sought and have not appeared. Lower incubation temperatures may sometimes be used for more psychrophilic species. After 18 hours at 35°C, coliforms produce intense violet-red colonies whilst non-lactose fermenters are colourless.
Storage conditions and Shelf life
The dehydrated medium should be stored at 10-30°C and used before the expiry date on the label.
Store the prepared medium at 2-8°C.
Appearance
Dehydrated Medium: Straw coloured, free-flowing powder
Prepared medium: Dark red coloured gel
Precautions
Prolonged incubation may lead to confusing results. Do not incubate beyond 48 hours.
Test the medium with a laboratory stock strain of Shigella species which is in the R-phase. R-phase shigellae should grow satisfactorily on MacConkey Agar. |