I need an explanation for this Biology question to help me study.

Needing the answers to the following 3 questions. The first two questions are worth 3 marks so need 3 clear points and same for the third question which is 2 points

Q3 Lactobacillus, one of the two cultures employed to produce the natural yoghurt used in this practical, is not a very robust strain and so may appear as Gram-negative long, slender bacilli in the Gram stain. Explain why this may be the case.(3 marks)

Q4a) The Gram stain is a differential stain. Explain what is meant by a differential stain and why the Gram stain can be classified as being such.(3 marks)

Q4b) Which step in the Gram staining protocol is regarded as the differentiating step, and why must great care be exercised when performing this step?(2 marks)