Question

A mixture smear of staph epidermis is and ecoli is gram stained.describe how the smear will stain under the following conditions. 1)smears stained for 24 hour old culture . 2)smears stained from 7day old culture

Answer 1

Dear student,

Please note that:

?s.epidermis is a gram+ve bacteria.

? e.coli is a gram -ve bacteria.

Now, when gram staining is done s.epidermis retains the purple colour upon decolorising because its thick peptidoglycan layer retains the purple stain, and it is thus gram positive.

On the other hand, e.coli loses purple color upon decolorising because it's thin peptidoglycan layer cannot retain the purple stain, thus it gets stained pink. And called gram negative.

These observations are valid for about 16-28 hours, after which even the gram positive bacteria begin to loose the purple stain and get stained pink.

Hence, after 7 days:

?s.epidermis would have lost all purple stain and taken up pink stain . Thus it would now appear as gram negative.

?And e.coli will of course show pinkish stain, gram negative.

Summary of the answer:

	S.epidermis	e.coli
1. 24 hours	gram positive, purple/violet	Gram negative, Pink/red
2. 7days old	gram negative, Pink/red	Gram negative, Pink/red