Classification of Microorganisms Lecture 2

External Structures Appendages - Two major groups of appendages: i. Motility - Flagella and Axial filaments ii. Attachment or channels - Fimbriae and Pili Flagella (singular; flagellum) - Long filamentous appendages that propel bacteria. - made up of protein subunit called 'flagellin' - highly antigenic (H antigens) and some of the immune responses to infection are directed against these proteins. - Atrichous - bacteria that lack flagella. - Has three basic parts: i Filament - long, thin, helical structure composed of protein flagellin. ii.Hook - curve sheath iii.Basal body - anchors the flagellum to the cell wall and plasma membrane. bears a set of rings; one pair in gram +ve bacteria and 2 pairs in gram -ve. Arrangement of flagella Monotrichous – single flagellum at one end. Ex: Vibrio cholerae. Lophotrichous – small bunches emerging from the same site.(Bartonella bacilliformis) Amphitrichous – flagella at both ends of the cell. (Spirillum serpens) Peritrichous – distributed over the entire cell.(E.coli) Flagellar Responses • Rotates 360o • Capable of various pattern of motility. • Advantage of motility: enables a bacterium to move toward a favorable environment or away from an adverse one. 'Runs' or 'swim'- bacterium moves in one direction for a length of time. 'Tumbles' - random changes in direction. Flagellar Responses  Taxis - movement of bacterium toward or away from a particular stimulus.  Chemotaxis - chemical stimuli. (positive and negative)  Phototaxis - light stimuli. (positive and negative)  In response to stimuli, information is passed to the flagella. If the chemotactic signal is positive (attractant), - bacteria move towards the stimulus - many runs and few tumbles. If the chemotactic signal is negative (repellent), - frequency of tumbles increased as the bacteria move away from the stimulus. Ex: Helicobacter pylori (agent of gastric ulcer) – bores through the stomach lining. Vibrio cholerae (cause of cholera) - penetrates the small intestine with the help of its flagellum. Periplasmic Flagella / Axial Filaments  Spirochetes - corkscrew-shaped bacteria that have unique structure and motility.  Internal flagellum - enclosed in the space between the outer sheath and cell wall peptidoglycan.  Treponema pallidum - best known spirochetes; causative agent of syphilis. Spirochetes move by means of axial filament, or endoflagella (bundles of fibrils that arise at the end of the cell beneath an outer sheath and spiral around the cell). The rotation of the filaments produces a movement of the outer sheath that propels the spirochetes in a spiral motion - corkscrew move. Nonflagellar Appendages: Fimbriae & Pili Fimbriae • Fine, proteinaceous, hairlike bristles emerging from the cell surface. • shorter and finer than flagella. • Function: - tendency to adhere to each other - involved in forming biofilms and other aggregations on the surface of the liquids, glass and rocks. - help bacteria adhere to epithelial surfaces in the body. (fimbriae on Neisseria gonorrhoeae (gonorrhoeae) - help microbe to colonize mucous membrane and cause disease). - contribute to the pathogenicity of certain bacteria Pili (singular; pilus) • • • • Usually longer than fimbriae. Composed of pilin protein. Involved in motility and DNA transfer. Function to join bacterial cells for partial DNA transfer - conjugation. • Pili of different bacteria are antigenically distinct and elicit the formation of antibodies by the host. • Type IV – found only in gram –ve bacteria. • Neisseria gonorrhoeae – gonorrhea – binding to the epithelial cells of the reproductive tract. Glycocalyx • Polysaccharide coating that is secreted by many bacteria. • composed of polysaccharide, polypeptide or both. • it is viscous(sticky) gelatinous polymer. • It covers surfaces like a film - allows the bacteria to adhere firmly to various structures (skin, heart valves, and catheters). • Two types: Capsule - condensed, highly organized layer closely surrounding the cell, bound more tightly to the cell. Slime layer - unorganized and loosely attached