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AGf~ICULTURAL RESEARCH, PUSA.

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6 PRACTICAL BACTERIOLOGY<br />

MORPHOLOGICAL STUDY OF THE BACTERIA<br />

UNSTAINED PREPARATIONS. - The morphology of<br />

the bacteria can be studied by examining them first<br />

in the unstained condition, suspended in fluid. In this<br />

way their general shape can bc obscrved and motility<br />

determined. Certain very slender organisms, however,<br />

such as the spirochaetes, arc so feebly refractile<br />

that they cannot be. seen by the ordinary microscopic<br />

method, and da1'k-ground illumi-nai·ion (vide p. 58) is<br />

necessary for their demollstration.<br />

STAINJ.m PREPARATIONS. - The examination of<br />

stained preparations is usually an essential routine<br />

procedure. For this purpose various aniline dycs are<br />

employed, often along with a mordant. For the demonstration<br />

of spirochaetes "negative staining" has been<br />

used-i.e. where the organism is mixcd with some<br />

substance such as Indian ink, which, in fllm preparations,<br />

yields a dark background, while the organism<br />

stands out clear and unstained. Silver impregnation<br />

methods (vide p. 171) are also utilised for the routine<br />

demonstration of spirochaetes.<br />

STAINING REACTIONS. - The staining reactions of<br />

the bacteria are of the greatest importance both in<br />

their morphological study and for their differentiation<br />

and identification. Thus, all the ba.cteria can be eli vided<br />

into two categories by the so-called Gram's staining<br />

reaction (vide p. 152)-i.e. according to whether they<br />

resist decolorisation with aniline oil, alcohol or acetone<br />

after staining with it pararosaniline dye-e.g. gentian<br />

or methyl violet, and subsequent treatment with<br />

iodine. Those retaining the dye arc designatcd<br />

" Gram-positive"; those decolorised by this method<br />

are spoken of as "Gram-negative."<br />

It is supposcrl that the Teuet.ion depends on the pel'lll('nhility<br />

or t.he Imctel'ial C'llvelope. The iodine fOTIlls n compound with<br />

the dye in t.he pl'Otoplasm of the organism. 'l'his compound is<br />

insolllble in water but soluble in the tlecoloriser, and is retainerl<br />

ill the ctOlls (If Gram-pusit.ive lm('tcl'ia hy yirtue of the impermcahility<br />

of the {'llwlopC', whe]'cas it is morc l'C'nrlily rcmoved fmm<br />

t.he Gram-negative t.ypes.

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