Biology Dept
Kenyon College
Chapter 11A.
Microbial Development
Fall Section Spring Section 1 Spring Section 2
Introduction to DevelopmentMolecular Microbiology Department, John Innes Centre
Single-cell Differentiation: Stalk and Swarmer
Single-cell Differentiation: Endospore Formation
Differentiated Cells Cooperate: Cyanobacterial Heterocysts
Multicellular Differentiation: Streptomycetes
Multi-species Development: Biofilms (Kolter Laboratory)

Introduction to Development
What is meant by development?
In complex multicellular organisms, development is understood as the process by which a single fertilized egg transforms itself into a multicellular organism with differentiated tissues and organs.  Development is marked by these features:

  • Cells of the developing organism maintain the same DNA content as the original cell.  That is why it's possible to clone a mammal using a somatic cell nucleus.  The major exception to this rule is the immune system, in which DNA is rearranged in separate clones.
  • As cells reproduce, they change into different kinds of cells with different complementary functions.  This process of change is called differentiation.  Most cells (somatic cells) lose the ability to reproduce themselves directly; instead they depend upon germ cells (such as sperm or egg) to reproduce, with identical copies of their DNA.
  • Differentiation occurs by distinctive regulation of gene expression, in which different genes and splicing patterns are turned on in different cell types.
  • The differentiated cells form tissues and organs with different functions that cooperate with each other for the good of the overall organism.  Some lineages of cells are even programmed to die, to enable maturation of organs; this programmed cell death is called apoptosis.
In the evolutionary context, however, multicellular development forms a continuum with developmental processes in microbes--many of which have major significance for the environment and for human health.  Understanding these forms of microbial development can enable a deeper understanding of the more complex systems such as vertebrate animals and vascular plants.

Differentiation of a Single Cell: Stalk and Swarmer.  Top
An example of a single-celled bacterium that undergoes cellular differentiation is Caulobacter crescentus.  Caulobacter is commonly found swimming in swamps and sewage outlets.

  • Caulobacter differentiates into two cell types--but they do not stay together to cooperate.

Yves Brun laboratory

Developmental Biology, Stanford

A motile swarmer cell of Caulobacter finds a nutrient-rich place to attach and grow. It loses its flagellum, replacing it with a stalk that attaches to detritus in the pond. 

The stalk cell then divides, forming two daughter cells: a swarmer (flagellated) and a cell with the original stalk.  The swarmer cell swims off to find a new location, avoiding competition with the stalk cell left behind.

How does the original cell regulate its division to produce two different types of daughter cell?  Regulation involves a complex cascade of cell cycle regulators, including activator proteins and proteases that cleave the activators.

Differentiation of a Single Cell: Endospore Formation.  Top
Some bacteria can form long-lived dormant forms called endospores. Examples include Bacillus anthracis (cause of anthrax), Bacillus thuringensis (inch-worm insecticide) and Clostridium botulinum (cause of botulism; source of Botox protein).  Endospores can remain viable for thousands of years.

  • Bacillus differentiates into two cell types--one of which remains viable to reproduce, while the other supports its development, then dies.  This behavior is analogous to apoptosis, the developmentally programmed death of helper cells--a common occurrence during vertebrate development.
To form an endospore requires a complex process called sporulation.

Bacteriology at U.W. Madison

Step-through sporulation (Doc Kaiser's Page)

The process of sporulation involves:

  • Replication of the bacterial chromosome into two daughter chromosomes.
  • Two sigma factors, sigma A and sigma H, regulate gene transcription to initiate two different developmental programs in the two daughter chromosomes.
  • Compartmentalization of the cell into the forespore (regulated by sigma F) and the motherspore (regulated by sigma E). 
  • The motherspore assists maturation of the endospore (regulated by sigma G and sigma K), but its own DNA ultimately disintegrates.

Multicellular Differentiation: Streptomyces.  Top
Streptomyces species are soil bacteria that produce many kinds of antibiotics, such as streptomycin. Streptomyces form mycelial structures that produce spores, in a life cycle remarkably analogous to that of fungi. 
  • Streptomyces differentiates into several cell types within a colony.  The cells cooperate to form mycelia, which generate spores that can start new colonies.  This life cycle is analogous to that of eukaryotic fungi; that is, it is functionally similar, but evolved independently.

Molecular Microbiology Dept

Camille M. Cao

Genome of Streptomyces coelicolor

More Microbial Development