Certain strains of Mycobacterium have been identified as: • multiple drug-resistant (MDR) - resistant to the drugs most frequently used to control tuberculosis (TB) • extensively drug-resistant (XDR) - resistant to the drugs most frequently used to control TB and to some of the drugs less frequently used to control TB • totally drug-resistant (TDR) - resistant to all known drugs used to control TB. When some of these Mycobacterium strains were compared, differences were seen in the thickness of their cell walls, as shown in the table. Mycobacterium: non-resistant $15\,\text{nm}$; MDR $17\,\text{nm}$; TDR $20\,\text{nm}$. What conclusions can be drawn from this information?
- ABacteria secrete thicker cell walls when in contact with a mixture of drugs.
- BThe cell walls of TDR bacteria are impermeable to drugs.
- CThicker cell walls may form a physical barrier to drugs.
- DXDR bacteria have cell walls between $17$ and $20\,\text{nm}$ thick.