Mycoplasma is a genus of bacteria that are characterized by their small size, lack of a cell wall, and unique properties. Among the given options, the correct statement about Mycoplasma is that their growth is not inhibited by penicillin (option C).

Explanation:
Mycoplasma bacteria have several distinct characteristics that set them apart from other bacteria. Understanding these characteristics helps us understand why their growth is not inhibited by penicillin.

1. Lack of a cell wall:
Mycoplasma bacteria are unique in that they do not possess a cell wall. The cell wall provides structural support and protects bacteria from osmotic shock. Without a cell wall, Mycoplasma is more flexible and can adopt various shapes. This characteristic also makes them resistant to lysis by osmotic shock, which is the process of bursting due to differences in osmotic pressure inside and outside the cell. Hence, option B is incorrect.

2. Small size:
Mycoplasma bacteria are among the smallest known bacteria, with dimensions ranging from 0.15 to 0.3 micrometers. Due to their small size, they can easily pass through filters that normally retain bacteria. This characteristic also contributes to their ability to evade the host immune system and colonize various tissues.

3. Lack of a rigid cell shape:
As mentioned earlier, Mycoplasma bacteria lack a cell wall and, therefore, do not have a defined shape. They can adopt different shapes, including spherical, filamentous, or pleomorphic forms. This flexibility allows them to colonize various host tissues and evade the immune system. Hence, option A is incorrect.

4. Resistance to penicillin:
Penicillin is an antibiotic that targets the synthesis of the bacterial cell wall. Since Mycoplasma bacteria lack a cell wall, they are inherently resistant to the action of penicillin. This resistance is due to the absence of a target for penicillin to bind and inhibit cell wall synthesis. Therefore, Mycoplasma can continue to grow and multiply even in the presence of penicillin, making option C correct.

5. Pathogenicity to humans:
Mycoplasma species can cause various diseases in humans, animals, and plants. Some of the well-known human pathogens include Mycoplasma pneumoniae, which causes respiratory tract infections, and Mycoplasma genitalium, which is associated with sexually transmitted infections. Hence, option D is incorrect.

In summary, Mycoplasma bacteria are unique due to their lack of a cell wall, small size, and resistance to penicillin. These characteristics allow them to colonize various host tissues and evade the immune system, making them significant pathogens in humans and other organisms.