The electron micrograph below shows a thin section of the Gram-positive bacterium Micrococcus lysodeikticus.

Compare the cell wall structure of this bacterium with one classified as Gram-negative.

Outline the role of saprotrophic bacteria in the treatment of sewage using reed bed systems.

similarities:
both have walls made of murein net;
both have polysaccharide chains cross-linked by short peptide chains/ peptidoglycan;

differences:
Gram-positive have thicker/more rigid walls while Gram-negative walls are thinner;
Gram-positive walls contain other components/polysaccharides and proteins while Gram negative do not;
Gram-negative walls coated on outside with lipid-rich layer while Gram-positive are not;
To award [2 max] responses need to address a similarity and a difference.

bacteria cause decay by feeding on dead/decaying organic matter (in sewage);
nitrifying bacteria convert ammonia to nitrates /nitrites;
plant/reed roots absorb nitrates;
denitrifying bacteria convert nitrates to nitrogen;

Whilst knowledge of the differences in wall thickness was evident, further comparative detail was not.

Poorly answered, showing little appreciation of the different roles played in sewage treatment by bacteria. Many candidates answering in Spanish seemed to ignore this topic, although the question is nearly an exact statement in the guide.