Basic oral microbiology

• Some bacteria have an extracellular called a glycocalyx that is generally made of polysaccharides; if it is well-defined and dense, it is called a capsule and if it is diffuse and amorphous, it is called a slime layer
• The glycocalyx is important for formation of biofilm, protects the cell and helps it evade immune destruction
• The bacterial cell wall is a 3D network/mesh made out of peptidoglycan, and contains polysaccharides and peptide units
• Whether a cell is Gram- or Gram+ reflects fundamental differences in cell wall structure; Gram- have an extra cell membrane and Gram+ have a thicker cell wall
• Gram negative bacteria are also called diderms
• Gram negative bacteria have two membranes, the inner being the plasma membrane and the outer membrane containing proteins and lipopolysaccharides
• The cell wall in Gram negative bacteria is generally thinner and between the two membranes is the periplasm (fluid-filled layer); there are also porins in the outer membrane that can restrict the entry and exit of large molecules
• Examples of Gram negative bacteria include E. coli and P. gingivalis
• Gram positive bacteria are also called monoderms
• Gram positive bacteria have a thicker cell wall that contains lipoteichoic acid and teichoic acid instead of lipoproteins
• Examples of Gram positive bacteria include Streptococci, Lactobacilli, and Actinobacteria
• Obligate aerobes: essentially require oxygen
• Facultative aerobe: oxygen is not essential but is better
• Microaerophiles: essentially require oxygen but at levels lower than atmospheric
• Obligate anaerobes: oxygen is lethal or massively inhibits growth
• Aerotolerant anaerobes: oxygen is not required and does not affect their growth
• Capnophile: likes high carbon dioxide concentration to enhance growth (e.g. in the mouth)
• Cocci: sphere
• Bacilli: rod-shaped
• Fusiform: spindle shaped
• Pleomorphic: bacteria that doesn’t always have the same shape
• Coryneform: club-shaped
• Bacteria growth phases: lag phase, log phase, stationary phase, decline/death phase
• Lag phase: bacteria adjusts to the new environment, synthesises required molecules and proteins
• Log phase: exponential growth in which cells grow and divide regularly, reaching maximum growth rate
• Stationary phase: nutrients begin to run out, toxic molecules inhibit growth, cells become dormant, etc. → equilibrium of cell division and nondivision/cell death
• Decline/death phase: cells have exhausted nutrients, cells still grow but there are many that die or are dormant; may be able to revive when conditions improve
• Bacteria can be classified into fastidious (fussy, requires precise conditions) or non-fastidious
• Syntrophy: bacteria have co-evolved with other bacteria and have a mutual arrangement, so it is difficult to separate them and grow them individually
• Isolate: pure strain of bacteria, all cells should have indistinguishable genetic material
• Selective media: help isolate single species/strains from a complex mixture, e.g. by containing nutrients that promotes the growth of the desired species and agents that inhibit the growth of others
• Mutualism: cooperation, two species work together and both benefit
• Commensalism: one species benefits while the other is relatively unaffected
• Parasitism: one benefits, the other is harmed or sometimes dies
• Mucosal surfaces/soft tissues include the cheek, gingiva, palate, tonsils and tongue; it is a dynamic surface as surface layer cells are constantly sloughed away and replaced
• Mineralised tissues suchas enamel, dentine and cementum have hard and non-shedding surfaces
• Supragingival plaque: located on the tooth crown and gingival margins
• Supragingival plaque is about 75% bacteria and the remaining is organic/inorganic solids, excluding water
• Subgingival plaque: located inferior to the gingival margin, has higher portions of bacterial cells
• The salivary pellicle is a conditioning film of adhered molecules that coats the entire oral cavity, prevents tooth abrasion, attrition and erosion, and helps to maintain tooth mineral homeostasis