Answer to Question #180713 in Microbiology for Emmanuel

Question #180713

1) In stool culture, the following media are used for direct/primary plating:

Salmonella Shigella Agar (SSA) and MacConkey Agar (Mac). With regards to the

two culture media, give the following:

a. media classification (i.e. general, differential, selective) of the 2 culture media

b. sugar/s incorporated in the media

c. colonial appearance of probable enteropathogens

2) With reference to urine culture give the following:

a. method of streaking the primary plates and the reason for such method

b. significance of the different colony count in urine culture

Expert's answer

Q1a. Each bacterial specics can grow in selective medium listed above but not all 8 bacteria are not able to grow in same medium. Because selective medium contains some specific substance/s to cultivate specific type of bacteria.

Q2b. As for example lactose present in Macconky Agar medium permits to grow only lactose fermentor like E. coli not S. aureus. Enriched medium like Blood agar permits to grow only hemolytic bacteria.

Q3c. The colour of macckonky medium is light yellowish before incubation with bacteria. After incubation with lactose fermenting bacteria the colour of the colony becomes red. Because the medium contain lactose, bile salt and neutral red (acid indicator). Lactose fermentor can grow in this medium where lactose is the sole source of carbon. bile salt prevents the growth of gram positive bacteria. During growth in the medium, lactose will convert to lactic acid thereby indicated by neutral red and forming the colony red in colour.

Answer-2a The pour plate method has only one type of bacteria as it was inoculated from the liquid media. The agar plate has the discrete colonies clear and countable with naked eye.the common errors for streaking are the use of the streaking wire multiple time without doing red hot. The streaking performed in dusty area or without laminar air flow. The mixing of the samples or confusion during marking.

Answer-2bthe contaminants are agents other than the desired bacteria that make use of media for growth.

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