Atypical forms of Bacteria: L forms of bacteria

Atypical forms of bacteria include involution forms, pleomorphic forms and L-forms of bacteria often caused by defective cell wall synthesis.

Involution forms: 

• They are swollen and aberrant forms of bacteria (e.g. gonococci and Yersinia pestis) formed in ageing cultures in high salt concentration.

Pleomorphic bacteria: 

• Some bacteria exhibit great variation in the shape and size of individual cells, e.g; Proteus and Haemophilis. This is known as pleomorphism.

Cell wall deficient forms:

• Pleomorphism and involution forms of atypical forms are often caused by defective cell wall synthesis. 

• Involution forms may also be formed due to the activity of autolytic enzymes.

L Form (Cell Wall Deficient Forms):

• L forms are the cell wall deficient bacteria, discovered by E. Klieneberger, while studying Streptobacillus moniliformis.

• She named it as L form after its place of discovery, i.e. Lister Institute, London (1935).

• When bacteria loose cell wall, they become spherical irrespective of original shape.

• This may occur spontaneously or after exposure to penicillin or lysozyme.

• L forms play a role in the persistence of pyelonephritis and other chronic infections.

Types of L forms

• Two types of L forms are distinguished:

 

• Unstable L forms:

• Bacteria lose the cell wall in presence of penicillin, a mechanism of resistance shown by the bacteria against penicillin. Such L forms are maintained only in presence of penicillin.  

• They are capable of dividing but can revert back to the original morphology once penicillin is removed.

• Protoplasts: They are gram positive bacteria whose cell wall is entirely removed.

• Spheroplasts: They are derived from gram—negative bacteria whose cell wall is partially removed.

 

• Stable L forms:

• L forms that are unable to revert to the original bacteria are called stable L forms. 

• Mycoplasma  do not have a true cell wall; the peptidoglycan layer is replaced by sterol. 

• It is postulated that Mycoplasma may represent stable L forms of a yet to be identified parent bacteria.

• But many researchers do not consider Mycoplasma as L forms, since they are not derived from bacteria that normally have cell walls.

• Bacterial Growth Curve 
• Bacterial Flagella
• Bacterial Capsule