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Bacteria Morphology and Structures

Bacteria are found in various shapes.

Cocci such as streptococci and staphylococci are spherical and form chains or three-dimensional clusters.

Rods (bacilli) may be very short rods (coccobacilli), curved rods (vibrio), or spiral rods (spirochaetes).

Filamentous bacteria are long and slender and may also contain various branches.

 

Baterial Structures

Flagella are long slender strutures akin to a propeller that enable bacteria to move when rotating in one direction and stop when rotated in the other. Flagella may be monotrichous, in tufts, or around the cell (peritrichous).

Fimbriae are shorter filaments that facilitate adhesion to cells.

Pili are similar to fimbriae and are involved in DNA transfer between bacteria. Plasmids often contain the genes responsible for pili formation.

 

Bacterial Surface and Cell Wall

Capsule

Many bacteria secrete a thick layer of material, often polysaccharide such as hyaluronan, that that protects the cell from the environment and immune response.

Both gram negative and positive bacteria may contain capsules.

Cell Wall

Bacterial cell wallsdepend on whether the bacteria is gram negative or positive. Gram positive bacteria have a cell membrane and a thick peptodiglycan layer, while gram negative bacteria have a thinner peptidoglycan layer between inner and outer cell membranes. The space between membranes is termed the periplasmic space.

Peptidoglycan maintains cell shape and integrity and is an important target for antibiotics. Peptide cross links between polymers contributes to peptodoglycan's strength.

Both outer and inner cell membranes contain proteins involved in wall production, metabolism, and production of secreted molecules. Gram negative bacteria membranes contain endotoxin (lipopolysaccharide).

 

Ribosomes

Bacterial ribosomes contain two subunits and are composed of RNA and proteins different from eukaryotic ribosomes. Ribosomes are important targets of antibiotics.

DNA

Bacteria contain one or two circular chromosomes that are present in the cytoplasm.

Plasmids are separate from chromosomes and are transferred between bacteria through pilli. They often carry genes that contribute to virulence or antibiotic resistance.

Inclusions

Inclusions are storage granules in the cytoplasm.

 

Spores

Endospores form from certian gram positive bacteria, including Clostridium and Bacillus. They develop in the cytoplasm and may remain dormant for long periods. Germination may occur to form a new cell once appropriate conditions exist.