Question

Before starting a purity plate, you can ensure you have a pure culture if (check all that apply)

A)all the colonies are convex but different colors

B) you use cells from only one colony as an inoculum source

C) the inoculum contained only gram-positive cocci

D) all the colonies are the same color but different size

can someone help me with this

Answer 1

A) Frequently one finds that it is necessary to leave cultures for more than two days at room temperature in order to see differences in morphology. Look for different colonies using a colony magnifier lamp. Look for colonies growing within the colony in question, different colors, sizes, colony edges, and textures. Size: pinpoint, small, medium, large
Color: violet, yellow, cream, gray, red, green, etc.
Shape (Elevation): flat, raised, convex, umbonate (knob in center), concave
Margin (edge): smooth, irregular, filamentous
Consistency: wrinkled, dull, powdery, shiny, moist, mucoid, granular, compact

B) Pure culture, in microbiology, a laboratory culture containing a single species of organism. A pure culture is usually derived from a mixed culture (one containing many species) by transferring a small sample into new, sterile growth medium in such a manner as to disperse the individual cells across the medium surface or by thinning the sample manyfold before inoculating the new medium. Both methods separate the individual cells so that, when they multiply, each will form a discrete colony, which may then be used to inoculate more medium, with the assurance that only one type of organism will be present. Isolation of a pure culture may be enhanced by providing a mixed inoculum with a medium favouring the growth of one organism to the exclusion of others.

c)MacConkey Agar (MAC): This is used to isolate Enterobacteriaceae and other related enteric Gram negative rods. Included bile salts and crystal violet inhibit growth of Gram positive bacteria, Gram negative cocci and fastidious Gram negative rods. Lactose is the sole carbohydrate source. Lactose-fermenters produce colonies in varying shades of red due to the conversion of neutral red indicator dye (red below pH 6.8) from the products of mixed acids. Bacteria which do not ferment lactose appear colorless or transparent. Gram negative more resistant to than gram positive bacteria
non enteric are more resistant than enteric.a) Primary Stain- crystal violet, the bacteria retain the dye color
b) Mordant- iodine, increases the interaction between the bacteria cell & dye so the cell is more strongly stained
c) Decolorizer- 95% ethanol Gram-positive keeps the color, gram-negative loses its color
d) Counterstain- sannifranin will stain the gram-negative pink but will not alter the dark purple of the gram-positive. Decolorizer- too little everything is gram-postive, too much everything looks like gram-negative. cell wall- peptioglytin layer- cite of action for crystal violet. Gram-positive has thick layer so crystal violet works well. Gram-negative has thin layer so removed.Gram negative more resistant to than gram positive bacteria
non enteric are more resistant than enteric.

D)

Pure culture will contain bacteria that appear to be all the same color, size, and shape.

Mixed culture bacteria will be different sizes, shapes, and possibly colors.

Pure cultured that has been extensively decolorized - Bacteria will appear to be the same size and shape throughout the field, but the color will vary

Still, recognizing differences in colony morphology is useful when trying to isolate bacteria and also when plates of pure culture have become contaminated. If the majority of colonies in what is supposed to be a pure culture are similar in appearance, but there are a couple of colonies that look different from all of the rest, the experienced microbiology student knows not to sample from the oddball colonies, as these colonies have arisen from contamination of the plate.