Select the incorrect statement given below.

Explanation-

Mycoplasma

• Mycoplasma is an organism that completely lacks a cell wall.
• Thus called as Joker of the plant kingdom.
• They are the smallest living cells known and can survive without oxygen.
• Many mycoplasmas are pathogenic in animals and plants.

Hence 1 statement is correct.

Bacteria

• Bacteria are the sole members of the Kingdom Monera.
• They are the most abundant micro-organisms.
• Bacteria occur almost everywhere. Hundreds of bacteria are present in a handful of soil.
• They also live in extreme habitats such as hot springs, deserts, snow, and deep oceans where very few other life forms can survive.
• Many of them live in or on other organisms as parasites.

Hence 2 statement is incorrect.

Viroids 

• In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease.
• It was found to be a free RNA. 
• It lacked the protein coat that is found in viruses, hence the name viroid.
• The RNA of the viroid was of low molecular weight

Hence 3 statement is correct.

Prions 

• In modern medicine certain infectious neurological diseases were found to be transmitted by an agent consisting of the abnormally folded protein.
• The agent was similar in size to viruses.
• These agents were called prions.
• The most notable diseases caused by prions are bovine spongiform encephalopathy (BSE) commonly called mad cow disease in cattle and its analogous variant Cr–Jacob disease (CJD) in humans.

Hence 4 statement is correct.

Therefore statement 2 is incorrect i.e., Bacteria are small infectious agents that replicate only inside the living cells of the host.