This is another possibility, depending on the technique used.

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Note added at 18 mins (2018-05-30 11:00:39 GMT)
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https://www.cliffsnotes.com/study-guides/biology/microbiolog...

Because microbial cytoplasm is usually transparent, it is necessary to stain microorganisms before they can be viewed with the light microscope. In some cases, staining is unnecessary, for example when microorganisms are very large or when motility is to be studied, and a drop of the microorganisms can be placed directly on the slide and observed.

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Note added at 21 mins (2018-05-30 11:03:28 GMT)
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https://www.click4tag.com/fr/technologie/marquage-bacteries-...

eg Marquage des bactéries Gram négative
Dumont et col.[1] ont décrit une nouvelle technique pour marquer spécifiquement les membranes de bactéries Gram négative cultivables, via la click chemistry, par l’assimilation d’un analogue azide du Kdo (Kdo-N3), un composé essentiel du lipopolisaccharide.

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Note added at 21 mins (2018-05-30 11:04:05 GMT)
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however staining would involve the use of microscopy

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Note added at 2 hrs (2018-05-30 12:47:58 GMT)
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http://www.cnrs.fr/cw/dossiers/doscel/decouv/xtxt/zhist/Niv2...

e simple marquage
Le simple marquage ou marquage monochromatique permet de mettre en évidence un élément spécifique de la cellule en le rendant fluorescent grâce à l’utilisation d’un marqueur lui-même fluorescent (2).