Bacteria are microscopic unicellular organisms. They possess an outer capsule or alternatively a slime layer for adhesion and that confers virulence. They have a cell wall and a cell membrane around the cytoplasm. The cell wall is made of peptidoglycan.
The cytoplasm has many constituents. It has a circular double stranded DNA coiled into a nucleoid at the centre of the cell. It however is devoid of all the membrane bound organelles such as mitochondria, chloroplast, ER, GA etc. The cell membrane is invaginated at the cytosolic side to form mesosomes. The cytoplasm also has some granules of varied functions. Very important are the 70S ribosomes in the cytoplasm. They also have monocistronic mRNAs.
Respiration occurs by membrane bound components like cytochromes, ubiquinone etc. which establishes a proton gradient. The bacteria utilizes the energy of this gradient to import and export molecules, ions and nutrients.
Bacteria have a full functional enzymatic machinery to carry out DNA replication, transcription and translation to synthesize essential proteins to carry out necessary biochemical reactions and metabolism.
What is unique in bacteria are plasmids. These are small, circular, autonomously self replicating DNA molecules. Plasmids have recently been discovered to spread drug resistance among strains and species as they carry drug resistance genes. So as a result they transmit resistance both horizontally as well as vertically. Plasmids currently find wide application in biotechnology and genetic engineering for cloning genes into prokaryotic systems.
Bacteria multiply by binary fission like mitosis in animal and plant cells. They exchange genetic materials like plasmids and chromosomal fragments by conjugation, transformation or transduction.
Coming to the nature of bacteria, they become malicious to the host because they need to compete with host cells to obtain nutrients in order to survive and thrive. Consequently, they start damaging the host causing various diseases. Some friendly bacteria are Lactobacillus in curd that form curd and act like probiotics (good for human health). Lactobacillus is also found in vagina of females that keeps the environment acidic and prevents proliferation of pathogens. The skin flora competes with the constantly attacking microbes in air and blocks them from infecting us. The gut flora also keeps the pathogens ingested with food from infecting us.
https://www.quora.com/What-are-bacteria/answer/Rituparna-Saha-1