Bacte

• Only about 3% - 5% of known microbes can cause disease
• The vast majority of known microbes are nonpathogens, which do not cause disease
• Pathogenic bacteria are prokaryotic cells that infect eukaryotic hosts
• Antibiotics inhibit bacterial growth without harming eukaryotic host cells
• Antibiotics are less successful in finding drugs effective against parasites, medically important fungi, and viruses, which are eukaryotic and similar to their human hosts
• Cell Wall:

  • A rigid structure that maintains the shape of the cell
  • Prevents bursting of the cell from the high osmotic pressure inside it
  • Serves as a point of anchorage for flagella
  • Determines the staining characteristics of species
• Gram-Positive Cell Wall:

  • Very thick protective peptidoglycan layer
  • Antibiotics effective against gram-positive organisms act by preventing synthesis of peptidoglycan
  • Consists of glycan chains of alternating N-acetyl-d-glucosamine and N-acetyl-d-muramic acid
  • Contains short peptides attached to a carboxyl group on each NAM residue, cross-linked to form a thick network via a peptide bridge
• Gram-Negative Cell Wall:

  • Inner peptidoglycan layer is much thinner than in gram-positive cell walls
  • Contains an additional outer membrane unique to the gram-negative cell wall
  • Contains proteins, phospholipids, and lipopolysaccharide (LPS)
  • LPS contains three regions: antigenic O-specific polysaccharide, core polysaccharide, inner lipid A
  • Acts as a barrier to hydrophobic compounds and harmful substances
  • Acts as a sieve, allowing water-soluble molecules to enter through protein-lined channels called porins
• Endospore:

  • Small, dormant, asexual spores that develop inside the bacterial cell as a means of survival
  • Highly resistant to chemical agents, temperature change, starvation, dehydration, ultraviolet and gamma radiation, and desiccation
  • Active vegetative cell produces one endospore, which germinates under favorable environmental conditions into one vegetative cell
  • Bacillus and Clostridium produce endospores in response to harsh environmental conditions
• Plasma Membrane:

  • Made of phospholipids and proteins
  • Acts as an osmotic barrier
  • Location of the electron transport chain, where energy is generated
• Flagella:

  • Exterior protein filaments that rotate and cause bacteria to be motile
  • Bacterial species vary in their possession of flagella
  • Flagella that extend from one end of the bacterium are polar
• Common Stains Used for Microscopic Visualization:

  • Basic dyes are cationic and adhere to negatively charged molecules
  • Gram stain is the most commonly used stain in the clinical microbiology laboratory
  • Acid-fast stain is used to stain bacteria with high lipid and wax content in their cell walls
• Microscopic Shapes:

  • Cocci: spherical, may occur singly, in pairs, in chains, or in clusters
  • Bacilli: rod-shaped, may vary greatly in size and length, ends may be square, rounded, tapered, or pointed
  • Spirochetes: spiral-shaped
• Acridine Orange:

  • Fluorochrome dye that stains both gram-positive and gram-negative bacteria, living or dead
  • Binds to the nucleic acid of the cell and fluoresces as a bright orange
  • Used to locate bacteria in blood cultures and other specimens
• Calcofluor White:

  • Fluorochrome that binds to chitin in fungal cell walls
  • Fluoresces as a bright apple-green or blue-white, allowing visualization of fungal structures with a fluorescent microscope
• Methylene Blue:

  • Used to stain C. diphtheriae for observation of metachromatic granules
  • Also used as a counterstain in acid-fast staining procedures
• India Ink:

  • A negative stain used to visualize capsules surrounding certain yeasts, such as Cryptococcus spp.
  • Fine ink particles are excluded from the capsule, leaving a dark background and a clear capsule surrounding the yeast
• Endospore Stain:

  • Endospores appear green within pink-appearing or red-appearing bacterial cells
• Major nutritional needs for growth:

  • Carbon (for making cellular constituents)
  • Nitrogen (for making proteins)
  • Energy (ATP, for performing cellular functions)
  • Source of nitrogen (for making proteins)
  • Phosphate for nucleic acids and phospholipids of cell membranes
  • Sulfur for protein synthesis
• Important mineral ions required by bacteria:

  • Na+, K+, Cl−, and Ca
• Autotrophs (lithotrophs):

  • Able to grow simply using carbon dioxide as the sole source of carbon, with only water and inorganic salts required in addition
  • Obtain energy either photosynthetically (phototrophs) or by oxidation of inorganic compounds (chemolithotrophs)
• Heterotrophs:

  • Require more complex substances for growth
  • Require an organic source of carbon (glucose) and obtain energy by oxidizing or fermenting organic substances
• Types of Growth Media:

  • Minimal medium: contents are simple and completely defined
  • Nutrient media: more complex and made of extracts of meat or soybeans
  • Enriched media: contains added growth factors, such as blood, vitamins, and yeast extract
  • Selective media: containing additives that inhibit the growth of some bacteria but allow others to grow
  • Differential media: ingredients in media that allow visualization of metabolic differences between groups or species of bacteria
• Environmental Factors Influencing Growth:

  • pH
  • Temperature
  • Gaseous composition of the atmosphere
• Optimal Temperature For Growth:

  • Psychrophiles (0° to 20° C)
  • Mesophiles (20° to 45° C)
  • Thermophiles (50° to 60 (125)° C)
• Atmospheric Requirements For Growth:

  • Obligate aerobes require oxygen for growth
  • Aerotolerant anaerobes can survive in the presence of oxygen but do not use oxygen in metabolism
  • Obligate anaerobes cannot grow in the presence of oxygen
  • Facultative anaerobes can grow either with or without oxygen
• Bacterial Growth:

  • Bacteria replicate by binary fission
  • Generation time or doubling time is the time required for one cell to divide into two cells
• Growth Curve:

  • Four phases of growth: lag phase, log phase, stationary phase, death phase
• Determination of Cell Numbers:

  • Direct counting under the microscope
  • Direct plate count
  • Density measurement
• Metabolism:

  • Biochemical reactions bacteria use to break down organic compounds and synthesize new bacterial molecules
  • Occurrence of all biochemical reactions in the cell depends on the presence and activity of specific enzymes
• Fermentation and Respiration:

  • Fermentation is an anaerobic process carried out by obligate, facultative, and aerotolerant anaerobes
  • Aerobic respiration is an efficient energy-generating process using molecular oxygen as the final electron acceptor
• Carbohydrate Utilization and Lactose Fermentation:

  • Ability of microorganisms to use various sugars for growth is important for diagnostic identification
  • Fermentation of sugars is usually detected by acid production and a concomitant change of color in the culture medium
• Carbohydrate Utilization and Lactose Fermentation:

  • Ability to ferment lactose is important in classifying members of the family Enterobacteriaceae
  • Utilization of lactose by a bacterium requires two steps: enzyme transport of lactose across the cell wall and enzyme breakdown of the galactoside bond
• Only about 3% - 5% of known microbes can cause disease
• Vast majority of known microbes are nonpathogens - microbes that do not cause disease
• Some nonpathogens are beneficial to us, whereas others have no effect on us at all
• Pathogenic (disease-causing) bacteria are prokaryotic cells that infect eukaryotic hosts
• Antibiotic action inhibits bacterial growth without harming eukaryotic host cells
• Less successful in finding drugs effective against parasites, medically important fungi, and viruses, which are eukaryotic, similar to their human hosts
• Cell Wall is a rigid structure that maintains the shape of the cell