Archaea and Their Unique Features

Archaea

• Share many features with Eukarya, specifically genes for replication, transcription and translation.
• Share features with Bacteria specifically genes for metabolism.
• Have unique rRNA not found in Bacteria.
• Capable of methanogenesis.
• Predominately known for growth in extreme environments such as anaerobic, hypersaline, high pH and high temperature habitats.
• Also found in more temperate environments such as marine arctic and tropical waters.
• Typically range in sizes of 1-2µm x 1-5 µm for rods and 1-5 µm in diameter for cocci.
• The smallest observed archaea is 0.2 µm in diameter.
• The largest observed archaea is a multicellular form that can reach a length of 30 mm.

Archaeal Lipids and Membranes

• Unlike Bacteria and Eukaryotes, archaeal lipids have branched chain hydrocarbons that are connected to glycerol by ether linkages.
• Some archaeal lipids have a monolayer structure instead of a bilayer structure.

Archaeal vs. Bacterial Cytoplasm

• Both have a very similar cytoplasm, lacking membrane-enclosed organelles.
• Both may contain inclusion bodies such as gas vesicles for controlling buoyancy.
• Ribosomes are 70S in both archaea and bacteria, but the proteins in archaea are more similar to eukaryotes.
• Nucleoid regions in archaea are single circular chromosomes, and some are complexed with histones.
• Polyploidy is observed in some archaea.

Common Features of Eukaryotic Microbes

• Have membrane-delimited nuclei.
• Membrane-bound organelles that perform specific functions.
• Intracytoplasmic membrane complex serves as a transport system.
• Are structurally more complex and generally larger than bacterial or archaeal cells.

Cilia and Flagella for Motility

• Flagella:
  • About 100-200µm long
  • Moves in an undulating fashion.
• Cilia:
  • 5-20µm long
  • Beat with two phases, working like oars.
• Both cilia and flagella contain a set of microtubules in a 9+2 arrangement.

Virus Structure and Life Cycles

• Virion size ranges from 10-400 nm in diameter.
• Most viruses must be viewed with an electron microscope.

General Properties of Viruses

• All virions contain a nucleocapsid, which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid).
• Viruses cannot reproduce independently and cannot carry out cell division.
• Capsids are made up of protein subunits called protomers.
• Capsids can be: helical, icosahedral, or complex.

Icosahedral or Complex Capsids

• An icosahedron is a regular polyhedron with 20 equilateral faces and 12 vertices.
• Some viruses have complex symmetry:
  • Poxviruses are the largest animal viruses.
  • Large bacteriophages have binal symmetry, the head resembles icosahedral, the tail is helical.

Viral Envelopes and Enzymes

• Viral envelopes are usually derived from host cell plasma or nuclear membranes.
• Envelope proteins are viral encoded and are involved in attaching to the host cell.
• Envelope protein may have enzymatic or other activities, for example, neuraminidase of influenza virus.

Viral Life Cycles Have 5 Steps

• Attachment (adsorption)
• Penetration (entry)
• Synthesis (replication and protein production)
• Assembly (packaging)
• Release
• Nonenveloped viruses lyse the host cell to release progeny.
• Enveloped viruses use budding to release progeny.

Bacterial Cell Cycle

• Most bacterial chromosomes are circular.
• Single origin of replication, site at which replication begins.
• Terminus, site at which replication is terminated, located opposite of the origin.
• Replisome, refers to a group of proteins needed for DNA synthesis.

Cytokinesis – Septation

• Septation is the formation of a cross wall between dividing cells.
• Occurs in several steps:
  • Selection of the site of septum formation.
  • Assembly of the Z-ring (composed of protein FtsZ).
  • Assembly of the cell wall synthesizing machinery.
  • Constriction of the cell and septum formation.

Microbial Population Growth

• Generation/doubling time: time it takes for a population to double in size, this can vary depending on the species and environmental conditions.

Theoretical Estimation of Bacterial Growth

• An example is given in the reading showing the theoretical biomass after 48 hours with a generation time of 20 minutes and a weight of 9.5 x 10^-13 g.

Bacterial Growth

• Refers to population growth rather than the individual cells themselves.
• Exponential growth is common but not always the case.

The Growth Curve

• Shows the pattern of microbial growth in a batch culture.
• Usually plotted as the logarithm of cell number versus time.
• Contains five distinct phases:
  • Lag Phase
  • Exponential Phase
  • Stationary Phase
  • Death Phase
  • Prolonged Decline in Growth

Lag Phase and Exponential Phase

• Lag Phase:
  • Cells synthesize new components; replenish spent materials.
  • Adapt to new medium or conditions.
  • Varies in length, can be short or even absent.
• Exponential/Log Phase:
  • Rate of growth and division is constant and maximal.
  • Population is most uniform in terms of chemical and physical properties during this phase.

Stationary Phase

• Total number of viable cells remains constant as active cells stop reproducing or the reproduction rate is balanced by the death rate.
• Reasons for this phase include:
  • Nutrient limitation
  • Limited oxygen availability
  • Toxic waste accumulation
  • Critical population density reached

Death Phase and Prolonged Decline in Growth

• Death Phase
  • Programmed cell death, a fraction of the population may be programmed to die.
  • The population continually evolves, successive waves of genetically distinct variants.
  • Natural selection occurs.

Environmental Factors that Affect Microbial Growth

• Most organisms grow in fairly moderate environmental conditions, but some are extremophiles.
• Extremophiles can grow very well in harsh conditions that would kill most other organisms.
• Organisms exhibit distinct cardinal growth temperatures:
  • Minimal
  • Maximal
  • Optimal

Temperature Ranges for Microbial Growth

• Psychrophiles: 0-20°C
• Psychrotrophs: 0-35°C
• Mesophiles: 20-45°C
• Thermophiles: 45-80°C
• Hyperthermophiles: 80-121°C

Growth Media

• Culture media can be used to grow organisms. There are three types:
  • Defined: Exact chemical composition is known.
  • Complex: Composed of nutrients that are not chemically defined.
  • Selective: Favor the growth of some microorganisms and inhibit growth of others.
  • Differential: Distinguish between different groups of microorganisms based on their biological characteristics.

Selective Media

• Favors the growth of some microorganisms and inhibits growth of others.
• An example is MacConkey agar (bile salts and crystal violet) which selects for gram-negative bacteria.

Differential Media

• Distinguish between different groups of microorganisms based on their biological characteristics.
• Blood agar differentiates between hemolytic and nonhemolytic bacteria.
• MacConkey agar (lactose and neutral red dye) differentiates between lactose fermenters and nonfermenters.

Isolation of Pure Cultures

• Allows for the study of a single type of microorganism in a mixed culture.
• Techniques include:
  • Spread Plate
  • Streak Plate
  • Pour Plate

Microbial Growth on Solid Surfaces

• Colony characteristics that develop when microorganisms are grown on an agar surface help with identification.
• Differences in growth rate from center to edges are due to:
  • Oxygen
  • Nutrients
  • Toxic products
  • Cells may be dead in some areas

Measurement of Microbial Growth

• Direct methods include:
  • Counting chambers
  • Flow cytometry
  • Membrane filters
• Indirect methods include:
  • Turbidity
  • Measures changes in the mass of the population
  • Most probable number (MPN) of viable microbes.