Bacteria

Prokaryotic Cell :
cells are between 1 and 5 um in diameter. Bacteria appear in a variety of different shapes: Spherical (coccus), rodlike (bacillus) , curved rod (vibrios) and spiral (spirilli) . In addition there are flat, triangular, starlike and square shapes, which are however, less frequent. Bacterial cells are surrounded by a semirigid cell wall , which consists of a polysaccharide matrix, crosslinked by polypeptide chains. Based on their cell wall composition bacteria can be divided into two groups: Gram positive and Gram negative.

Gram positive bacteria : react with the so called Gram stain to appear purple, whereas Gram negative bacteria don't react with the stain. The cell wall of gram positive bacteria consists of several layer of peptidoglycan a polymer made up of N-acetylglucosamine chains crosslinked by short peptide chain which contain D-amino acids. In addition, the cell wall of gram positive bacteria contains teichoic acid , which consists of glycerol or ribitol and phosphate. On the external side of the cell wall, prokaryotes have the glycocalyx a gelatinous polymer, consisting of polysaccharide chains. In addition there are extensions protruding from the cell wall such as flagella , fimbriae and pili. Underneath the cell wall gram positive bacteria have a plasma membrane, which controls the flow of metabolites and nutrients in and out of the cell.

Gram negative bacteria : have a much more complex cell wall. On the cytoplasmic side the plasma membrane is connected to a single layer of peptidoglycan , which in turn is connected to a lipoprotein unit. The lipoprotein layer connects to an outer membrane which contains a lipopolysaccharide that is made up of a Lipid A portion (two sugar units connected to a hydroxy fatty acid) and an O-antigen subunit.
The Lipid A portion is also referred as an endotoxin since it is toxic in a host's bloodstream or gastrointestinal tract. The O-antigen portion consists of sugar molecules and it is the primary site of gram negative bacteria, recognized by antibodies . The variability of the O-antigen chain can cause problems with the immune response.

A major classification scheme is based upon staining properties of bacteria

• Chemosynthetic Bacteria
  Autotrophs : Obtain energy (ATP) from the oxidation of inorganic compounds e.g. NH₃ to NO₃^-, NO₂^- to NO₃^-, S to SO₄^{- -}. The electrons from the oxidation are used to make carbon compounds from CO₂ . Thioacillus (NH₃), Nitrobacter( NO₂^-)
  Heterotrophes: Obtain their energy from organic compounds. Energy source and carbon source is the same (e.g, glucose) Organisms are either parasidic or live on dead organic matter. Most bactria are chemoheterotrophes

• Photosynthetic Bacteria
  Autotrophes: use light as source of energy and CO₂ as carbon source.
  Cyanobacteria are the major group of photosynthetic bacteria. importance in the global carbon, oxygen and nitrogen cycles (evolutionary significance)
  green sulfur bacteria : are photosynthetic , use S or H₂S or H₂ instead of water as electron donor to reduce CO₂ and form organic compounds image

  Heterotrophs: have photosystems to make ATP , but have to use organic substances as carbon source. Green nonsulfur bacteria. Some organisms (extreme halophiles) developed a proton pump which uses light enrgy (bacteriorhodopsin)

Pathogenic organisms

• Spirochetes
  Lyme disease is caused by bite of ticks which are infected with Borrelia burgdorferi. The ticks feed on deer , mice, and other mammals. Prevalent in the northeastern and north-central United States. Symptoms : fatigue, chills and fever, headache, muscle and joint pain, swollen lymph nodes.
  Treponema pallidum: Syphilis

• Pseudomonads :
  very diverse group of Gram-negative rods. Free-living organisms in soil and water; they play an important role in decomposition, biodegradation, and the C and N cycles. Degrade many different organic compounds: insecticides, pesticides, herbicides, plastics, petroleum substances, hydrocarbons , cellulose, lignin, pollutants
  Pseudomonas aeruginosa , (colony) is an opportunistic pathogen (P. aeruginosa produces two exotoxins) . The bacterium almost never infects healthy tissue, Causes urinary tract infections, respiratory infections, variety of systemic infections. Often resistance to antibiotics
  Bordetella pertussis: Whooping cough; Neisseria gonorrhoeae ; Neisseria meningitidis (bacterial meningitis) , Legionella pneumophilia ;
  Rhizobium ( pseudomonads) is not a pathogenig organisms, it is one of the most important N₂ fixers

• Enterobacteriaceae:
  E. col: in gastro-intestinal tract of most warm-blooded animals is colonized by E. coli . The bacterium is ingested in foods or water. Grows aerobic and anearobic . Certain strains are virulent (produce toxins) , Yersinia pestis : causes bubonic plague, Salmonella typhimurium: cause of gastroenteritis.

• Vibrios
  common bacteria in aquatic environments. Vibrio cholerae,

• Cocci :
  Streptococcus pneumoniae most common pathogens (most frequent cause of bacterial pneumonia). It is estimated that Streptococcus and Neisseria are the cause of at least a third of all the bacterial infections in humans, including strep throat, pneumonia, food poisoning, various skin diseases .
  Staphylococcus aureus : occurs often as normal flora of humans (skin, nasal membranes, GI tract). Some strains S. aureus are pathogenic : pimples, wound infections, pneumonia, food intoxication and toxic shock syndrome. S. aureus is the leading cause of hospital-acquired infections (notoriously resistant to antibiotics)

• Endospore-forming bacteria:
  Bacillus anthracis Anthrax is primarily a disease of domesticated and wild animals. Cutaneous anthrax ( via injured skin or mucous membranes), inhalation anthrax (inhalation of spore, begins with high fever and chest pain)