Acid Fastness Bacteria include a group of bacteria or eukaryotic cells that react to or resist the decolourisation effects rendered by acids during laboratory staining methods. This characteristic feature is observed in sub – cellular cells too. This mechanism helps in differentiating cells because the “acid – fast” mechanism is different for different species of cells. For instance, the Mycobacterium species that cause tuberculosis and leprosy exhibit its acid - fastness due to the high mycolic acid content in its cell walls which forms a staining pattern due to low absorption and high retention. The AFB Susceptibility Cycloserine test is a way of measuring or detecting the sensitivity in a given sample of sputum towards cycloserine. It helps in detecting the presence of drugs resistant to Tuberculosis or Leprosy.