Atypical cell walls:

•  Mycoplasma – is a bacterial genus that naturally lacks cell walls. Their plasma membranes have lipids called sterols, which protect them from osmotic lysis.

•  Archaeabacteria – have pseudomurein (N-acetylalosaminuronic acid) but no peptidoglycan.

•  L forms - are mutant bacteria with defective cell walls .

 

Damage to the cell wall

The pres ence of cell wall is essential to protect bacteria against destruction by osmotic pressure. The bacterial cytoplasm is much more concentrated with solutes than in most microbial habitats which are hypotonic. During osmosis, water moves across selectively permeable membranes such as the plasma membrane from dilute solutions (higher water concentration) to more concentrated solutions (lower water concentration). Usually water generally enters the bacterial cells and the osmotic pressure may reach 20 atmospheres. Plasma membrane cannot resist such high pressures and the cell will swell and be physically disrupted and destroyed, a process called lysis. In hypertonic habitats, the water flows outward, and the cytoplasm shrivels up and pulls away from the cell wall. This phenomenon is called plamolysis and is useful in food preservation because many microorganisms cannot grow in dried foods and jellies as they cannot avoid plasmolysis. The importance of the cell wall in protecting bacteria can be demonstrated by treatment with lysozyme (naturally occurs in eukaryotic cells and is a constituent of tears, mucus and saliva), which attacks the peptidoglycan by hydrolyzing the bond that connects NAM and NAG units. Penicillin inhibits peptidoglycan synthesis. Gram positive cell walls are destroyed and the remaining cellular contents are referred to as protoplast .Gram negative cells are not completely destroyed and the remaining cellular contents are referred to as spheroplast. Protoplast and spheroplast are subject to osmotic lysis. Antibiotics such as penicillin destroy bacteria by interfering with the formation of the peptide cross bridges of peptidoglycan and ultimately cell wall synthesis.

 

REFERENCES:

Text Books:

1. Jeffery C. Pommerville. Alcamo's Fundamentals of Microbiology (Tenth Edition). Jones and Bartlett Student edition.

2. Gerard J. Tortora, Berdell R. Funke, Christine L. Case. Pearson - Microbiology: An Introduction. Benjamin Cummings.

Reference Books:

1. Lansing M. Prescott, John P. Harley and Donald A. Klein. Microbiology. Mc Graw Hill companies.