Microbiology - Bacteria and Archaea

Chapter Four- Bacteria and Archaea Form and function: ○ Sometimes bacteria can act as a group. When bacteria are close to one another in colonies or in biofilms, they communicate with each other through chemicals that cause them to behave differently than if they were living singly. ○ Bacteria exhibit considerable variety in shape, size, and arrangement. In terms of size, bacterial cells have an average size of about 1 micron (μm) ○ First, if the cell is spherical or ball-shaped, the bacterium is described as a coccus ○ Second, a cell that is cylindrical (longer than wide) is termed a rod, or bacillus ⇉ When a rod is short and plump, it is called a coccobacillus ○ Third general shape for bacterial cells is curved. If it is gently curved, it is a vibrio ○ A bacterium having a slightly curled or spiral-shaped cylinder is called a spirillum-spirochete, a more flexible form that resembles a spring. ○ Because bacterial cells look two-dimensional and flat with traditional staining and microscope techniques, they are best seen with a scanning electron microscope ○ Bacterial cells can also be categorized according to arrangement, or style of grouping. The greatest variety in arrangement occurs in ○ Cocci- singles ○ Diplococci- in pairs ○ Tetrads- groups of four ○ Irregular clusters- as in staphylococci and micrococci, or in chains of a few to hundreds of cells ○ An even more complex grouping is a cubical packet of 8, 16, or more cells called a sarcina Appendages: ○ Appendages can be divided into two major groups: 1. those that provide motility (flagella and axial filaments) & 2. those that provide attachment points or channels (fimbriae, pili, and nanowires) ○ The primary function of flagella is to confer motility, or self-propulsion4that is, the capacity of a cell to swim freely through an aqueous habitat ○ Flagellated bacteria can perform some rather sophisticated feats. They can detect and move in response to chemical signals4a type of behavior called chemotaxis. ○ Positive chemotaxis is movement of a cell in the direction of a favorable chemical stimulus (usually a nutrient) ○ Negative chemotaxis is movement away from a repellent (potentially harmful) compound. ○ Some photosynthetic bacteria exhibit phototaxis, movement in response to light rather than chemicals ○ Corkscrew-shaped bacteria called spirochetes show an unusual, wriggly mode of locomotion caused by two or more long, coiled threads, the periplasmic flagella or axial filaments. ○ A periplasmic flagellum is a type of internal flagellum that is enclosed in the space between the cell wall and the cytoplasmic membrane. The filaments curl closely around the spirochete coils yet are free to contract and impart a twisting or flexing motion to the cell. ○ Although their main function is motility, bacterial flagella can be used for attachment to surfaces in some species. ○ Two other structures, the pilus and the fimbria, are bacterial surface appendages that provide some type of adhesion, but not locomotion ○ Often, the biofilm is stratified, with the aerobic microbes near the surface where the oxygen levels are high and the anaerobic microbes near the bottom where oxygen levels are low. Each member of the biofilm community finds its niche ○ A pilus is a long, rigid, tubular structure made of a special protein, pilin. True pili have been found only on gram-negative bacteria, where they are utilized in a