Bacteria have contributed significantly to the rise in infectious diase. Bacteria as a class include a huge number of organisms, generally classified by biochemical characteristics (i.e., Gram stain characteristics) or morphological characteristics (e.g., bacilli cocci).

Bacteria are classified taxonomically as to genera and species. Many bacteria are harmless to humans while others cause disease only opportunistically. A subset of bacteria are highly pathogenic.

Differences between Bacterial and Eukaryotics

Bacterial are single cell prokaryotes, which have the following differences with the eukaryotic cell:

(1) lacks a true membrane bound nucleus. Instead, the prokaryotic DNA molecule is found within the cytoplasm of the cell in a discrete area known as the nucleotid. Because there is no physical separation between transcription and translation as with eukaryotes, the two processes are “coupled” (occur simultaneously).

(2) surrounded by a cell wall (everything external to the cytoplasmic membrane) composed of peptidoglycan which interfers with phagocytosis, is mitogenic (stimulates mitosis of lymphocytes) and has pyrogenic activity (induces fever). The B-lactam antibiotics (penicillins and cephalosporins) inhibit peptidoglycan synthesis. Cell envelope architecture differs between gram +/- bacteria.

Plants, fungi and most protists also have cell walls but with a chemical structure different form that of peptidoglycan. 

(3) the cytoplasm is surrounded by a cytoplasmic membrane. (this is true with both gram + and -)

(4) in some cases enclosed in a capsule. The presence of a capsule is associated with virulence because it interferes with phagocytosis. Most capsules consist of repeating sequences of 2-3 sugars. This capsule encases an endospore which is an extremely resistent structure (contains calcium dipicolinate) that encases the genome. When conditions become favorable signals turn on genes that allows the spore to germinate.

The resistance of endospores to antibiotics is of immense concern. The endospore’s most external layer, the exosporium, is composed primarily of proteins, polysacharides and lipids. Inward from the exosporium, the spore coat is composed primarily of proteins, which envelops the peptidoglycan cortex. Interior to the corext, the highly lipid nature of the inner membrane renders the spore core impermeable to several chemical compounds. These protective aleyrs work collectively to passively dehydrate the spore core and maintain a state of dormany. Upon encountering favorable enironmental conditions, suficial sensory mechanisms of variable sportes recognize specific chemical signals, known as germinants and initiate metabilic casades that resultin the germination and outgrowth of the spore to its vegetative form. 

spore-forming bacterial species reported as being pathogen to humans, livestock and insects below to the general Bacillus and Clostridium. 

(5) Various structures like flagella (long helical) and pili (straight and smaller) may protrude from the cell. The presence of pili is a characteristic most common among pathogenic bacteria of the mucosal surfaces like neisseria gonorrhoeae which are able to adhere to genital tract mucosal surfaces.

(6) smaller than eukaryotic cells. Bacteria are about 1 micron whereas eukaryotes are typically greater than 5 microns. The genomes of bacteria are also smaller (2-5 million base pairs which encode about 4k genes)

(7) some bacterial (particularly gram –) contain additional DNA molecules called “plasmids” which replicate independently of the chromosomes and which often code for virulence factors. They are commonly exchanged among related bacteria via conjugation and transduction and occasionally by transformation.

(8) Bacteria reproduce by a process called binary fission rather than mitosis. Since there is only one DNA molecule, a spindle apparatus and other components of the mitotic cycle are unnecessary.

(9) there are no other organelles in their cytosol except ribosomes.

Bacteria are extremely complex at the molecular level, composed of thousands of proteins, large quantities of nucleic acids and a great many types of small organic molecules. However, the host is oblivious to the vast majority of these molecules. Only a handful of them (e.g., LPS, lipopeptides, lipoteichoic acid, flagellin and unmethylated DNA) incite an innate immuen response.