Bacitracin blocks the transport of nag and nam across the cytoplasmic membrane to the cell wall. Like other antimicrobials that block cell wall synthesis, this would result in weak cell walls and cell lysis. Antimicrobilas that act by inhibiting cell wall synthesis include beta-lactams, vancomycin and cycloserine and isoniazid and ethambutol among others. They prevent bacteria from increasing amount of peptidoglycan, they have no effect on existing peptodoglycan layer and therefore are effective only for growing cells.
After hundreds or even thousands of generations both alleles for sickle cell anemia are still common in those of African ancestry. This is supported by two mechanisms: heterozygote advantage and gene flow.
Heterozygote advantage
When an allele is said to be heterozygous, it is carrying one copy of a gene. For example if one carries a copy of sickle cell allele, it will stay in the population even after many generations — natural selection will not be able to remove it and it will be passed on through generations. So, having one copy of that allele is advantageous but not deleterious.
Gene flow
With humans migrating throughout the different parts of the world, sickle cell allele is passed from one person to another or in different populations. It is actually advantageous to have one copy of this allele especially if there’s malaria in the area, which the allele is resistant.
