Bacteria

Introduction

Bacteria are small, single cells and are the only ones characterized by prokaryotic organization. Too tiny to see with the naked eye, a bacterial cell is usually one of three basic shapes; bacillus, a rod shaped cell; coccus, a spherical cell; or spirillum, a spiral cell. A few kinds of bacteria aggregate into stalked structures or filaments.

A bacterium's plasma membrane is encased within a cell wall. Members of the kingdom Eubacteria have a cell wall made of peptidoglycan, a network of polysaccharide molecules linked together with chains of amino acids. Some eubacteria have a cell wall covered with an outer membrane layer made of large molecules called lipopolysaccharides. A lipopolysaccharide is a chain of sugar molecules with a lipid attached to one end. Outside of the cell wall and membrane, many bacteria have a gelatinous layer called a capsule.

I. Size-Shape of Bacteria

Eubacteria are classified by differences in their cell walls. A bacterium with a cell wall containing a large amount of peptidoglycan is classified as gram-positive. A bacterium with a cell wall containing a thin layer of peptidoglycan covered by an outer membrane is classified as gram-negative. Priest, Patricia Pp. 56)

These terms refer to a bacterium's reaction to a staining procedure developed by the Danish microbiologist Hans Gram.

Some bacteria form thick-walled endospores around their chromosomes and a small bit of cytoplasm when they are exposed to harsh conditions such as drought or high temperatures. The formation of endospores in the bacterium Clostridium botulinum is responsible for botulism, a disease often considered to be a form of food poisoning. Björkman , Johanna "Virulence of antibiotic-resistant Salmonella typhimurium." PubMed. Gloucester. (BMG. 2001 Pp. 17)

Bacteria, which outnumber all eukaryotes combined, differ from eukaryotes in at least seven important respects.

1.Internal compartmentalizationBacteria are prokaryotes and lack a cell nucleus, unlike eukaryotes. The cytoplasm of bacteria has very little internal organization. It contains no internal compartments or membrane systems.

2.Cell sizeMost bacterial cells are about 1 um (upside down h, m) in diameter; most eukaryotic cells are well over 10 times that size.

3.MulticellularityAll bacteria are single cells. Some bacteria may stick together in a matrix or may form filaments. However, these formations cannot truly be considered multicellular because the cytoplasm in the cells does not directly interconnect as is the case with many multicellular eukaryotes. Also, the activities of the cells are not specialized.

4.ChromosomesBacterial chromosomes consist of a single circular piece of DNA. Eukaryotic chromosomes are linear pieces of DNA that are completely integrated with proteins.

5.Cell divisionCell division in bacteria typically takes place by binary fission, a process in which one cell simply pinches into two cells. In eukaryotes, however, microtubules pull chromosomes to opposite poles of the cell during the nuclear division process called mitoses. Afterward the cytoplasm of the eukaryotic cell divides in half, forming two cells.

6.FlagellaBacterial flagella are simple structures composed of a single fiber of protein that spins like a corkscrew to move the cell. Eukaryotic flagella are more complex structures made of microtubules that whip back and forth rather than ...