Bacteria gram stains?

i have a microbiology practical next week and need to be able to identify the bacteria we have stained. the prof gave us the names of the bacteria, the arrangement, and if it was gram negatuive or positive. however, when i find images, a lot of the ones she said are negative are stained purple, and some of the ones she said were gram positive are stained red. is it possible for the same bacteria to be either positive or negative? and how do you differentiate between two organisms that are both, let's say, steptococcus and gram positive?


thanks in advance!

Bacteria gram stains?
In proper Gram staining, gram + should be purple and gram -


should be pink. However, the decolorization step has to be optimal to attain this. Too much decolorization makes gram + appear pink (like gram -) while too little leaves crystal violet on, making gram negatives look purple.


If you were looking at images on a book it would be easier (because they would be right) but practicals are always a little harder. The best way to pass is by making sure you nail at least the common bacteria. You have to know that Staphylococci will be purple and will be in clusters (like grapes). Streptococci which are also gram + will be in chains. Now, if you see Streptococci in twos (diplococci) those are S.pneumoniae (but you have to be sure that it is most of the slide and not just two broken from a chain). If you see a rod-shaped gram + bacteria, it will most likely be a Bacillus sp. (species). If you see longer purple rods that look like clubs at ends these will be Corynebacteria sp. For gram negatives: remember pink color and most will be rods. E. coli will be a rod; Vibrio will be like a comma; cocci, especially diplococci, will definitely be Niesseria sp.


It is hard to distinguish most species by gram stain only and that is why gram stains maybe followed by biochemical testing etc.
Reply:ACTULLY SOME TIMES CHANGE IN pH MAY CAUSE SOME BACTERIA TO SHOW GRAM +ve OR -ve NATURE

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