Question

Which among the following are the smallest living cells, known without a definite cell wall, pathogenic to plants as well as animals and can survive without oxygen?
A. Nostoc
B. Bacillus
C. Pseudomonas
D. Mycoplasma

Answer 1

The surface component, S-layers- contribute to virulence by protecting bacteria against complement and by phagocytes attack. The cell wall of a bacteria is an important structure that protects the delicate protoplast of the cell from osmotic lysis.

Complete answer:
As you know, Cells of Nostoc, are spherical, barrel-shaped, or oval forming unbranched filaments (trichomes) that may contain both heterocyst (thick walled, specialized N-fixing cells) and akinetes (thick walled cell which functions as a resting cell).
On other hand, the cell wall of Bacillus forms the second barrier between the bacterium and the environment, and maintains the rod shape and withstands the generated pressure by the cell's turgor.
In case of Pseudomonas, it has a cytoplasmic membrane which consists of, symmetric phospholipid bilayer and an asymmetric outer membrane.
Now let’s look at Mycoplasma which lack a cell wall around their cell membranes. This property gives them resistance to antibiotics that target cell wall synthesis. Many of its species are pathogenic in humans, including M. pneumoniae also, which is an important reason for "walking" pneumonia and other respiratory disorders, and M. genitalium, which is said to be involved in pelvic inflammatory diseases. Mycoplasma species are believed to be the smallest bacterial cells, and can survive without oxygen, and come in various shapes.

Additional information:
If we want to culture M pneumoniae from sputum or a throat swab is possible but slow that is why its diagnosis is usually based on serologic tests. Now, tests using amplification of specific genomic mycoplasma sequences by the polymerase-chain reaction (PCR) are developed.

Therefore, correct answer is option D.

Note:- Keep in mind, Mycoplasma pneumoniae infection is a disease of the lower and upper respiratory tract. Cough, fever, and headache may occur and persist for weeks. Whereas, urealyticum infection in men causes nongonococcal urethritis, and results in dysuria, urgency, and urethral discharge.