Introduction to Bacteria

 

Microorganisms have been somewhat neglected in the past 10 or 20 years as much research has focused on eukaryotes in the search for cures to cancer and other diseases. However, in what might be described as the revenge of the microbes, microorganisms have come back into sharp focus with the emergence of new diseases and the return of some old enemies.

Some reasons for studying prokaryotes:

 

The story of Lucien Bouchard's fight against Streptococcus is well known to Canadians.

Tuberculosis has become a major health problem in urban centres in the USA where it has spread among immunocompromised individuals.

Year   Microbe  Disease
 1977  Legionella  Legionnaires' disease pneumophilia
 1977  Campylobacter jejuni  Enteric pathogens distributed globally
 1981  Toxic producing strains of Staphylococcus aureus  Toxic shock syndrome (tampon use)
 1982  Escherichia coli O157:H7  Hemorrhagic colitis; hemolytic uremic syndrome
 1982  Borrelia burgdorferi  Lyme disease
 1983  Helicobacter pylori  Peptic ulcer disease
 1989  Ehrlichia chafeensis  Human ehrlichiosis
 1992  Vibrio cholerae O139  New strain associated with epidemic cholera
 1992  Bartonella henselae  Cat-scratch disease; bacillary angiomatosis
 This table is taken from Table 1 in: Lederberg, J. (1997) Infectious Disease as an Evolutionary Paradigm, Emerging Infectious Diseases, 3(4): 417-423.


MRSA -- Methicillin resistant Staphylococcus aureus is a major health concern as are vancomycin resistant enterobacteria (VRE)

Antibiotic resistance has spread into Neisseria gonorrhoeae that now make infections difficult to treat.

Every few months now, there seems to be another outbreak of E. coli O157:H7 in the food supply somewhere. The most notable outbreak to hit the news in Canada occurred in Walkerton, Ontario during the summer of 2000 when their water supply was contaminated with E. coli O157:H7 and a number of people died as a result.

 

Morphology

The prokaryotes are the simplest autonomous organisms known with the smallest cells. However, there is one exception - Epulopiscium fishelsoni. This organism is symbiont which lives in the intestines of the Red Sea surgeonfish (Acanthurus nigrofuscus). It is 250-600 microns in size, which is huge by comparison with the 1-10 micron size of more typical bacterial cells.

Read description of Epulopiscium at Esther R. Angert's (Cornell University) web page

Bacteria are generally unicellular, though multicellular forms exist and multicellular associations occur as part of the life-cycle of some species. For example, many cyanobacteria grow as filaments, actinomycetes form hyphae and stalks, Caulobacter has motile (free-swimming) and non-motile (sessile) forms.

See pictures of organisms being sequenced by the JGI Microbial Genomics program and note the diversity of morphologies

In general, bacteria are not considered to contain distinct internal organelles -- at least by the definition of an organelle as a membrane bound compartment. However, bacteria do contain macromolecular structures which are specialzed for certain metabolic functions -- e.g. carboxysomes (for RuBisCo) and inclusion bodies.

Bacterial cells contain a variety of external structures:

 

Taxonomy

Taxonomy of bacteria has been a difficult and sometimes controversial field. Classical studies relying on physiology and morphology can be misleading. For example, it is very easy to get filamentous mutants of E. coli. Thus a filamentous phenotype may not necessarily be a reliable taxonomic marker.

Modern thinking on and approaches to bacterial taxonomy use nucleotide sequence analysis to establish evolutionary relationships among species. The 16S rRNA has a highly conserved sequence and structure whose use has been pioneered by Carl Woese to derive such relationships.

When Woese first analysed 16S rRNA sequences from bacteria and "higher organisms", he found that all organisms belonged to one of three broad groups:

Scientists have been arguing about this classification ever since!

More recent work suggests that the Archaebacteria are more closely related to eukaryotes than they are to bacteria, i.e. that there are two branches on the basal tree (Ref) not three as was initially suggested by Woese.

[JGI Tree]

A recent analysis (shown in Fig 1.1 of Joset & Guepin-Michel) has the following groups:

A similar evolutionary tree is shown in Fig 1 of the Introduction in your textbook.

Whether any particular one of these trees is more correct than the other does not really matter. In the long run, what matters is that these kind of studies and analyses have spurred scientists to think a lot more intensively about the problem of bacterial taxonomy and the correct way to deal with it.

 

Woese also divided the bacteria into the following categories:

 

Bacterial Chromosomes

In most cases, the genetic makeup of bacteria consists of a single circular DNA chromosome that ranges in size from 750,000 bp to 1.5 x 107 bp. The size of the chromosome generally increases with increasing physiological complexity of the organism.

e.g. The filamentous cyanobacteria have chromosomes that are twice to three times as large as the chromosomes of unicellular cyanobacteria.

The circularity of bacterial most chromosomes was established from the following types of evidence:

 

In addition, nine archaebacterial genomes have been sequenced, including:

Methanococcus jannaschii

  1.66 Mb

Methanobacterium thermoautotrophicum

  1.75 Mb

Archaeoglobus fulgidus

  2.18 Mb

Pyrococcus horikoshii

  2.0 Mb

 

You can find out more about the current state of genome sequencing projects by visiting The Institute for Genome Research Microbial Database.

The US Department of Energy Joint Genome Institute site also contains information on a large number of microbial sequencing sequencing projects.

 

It is now clear that some bacterial genomes have different structures

Bacterial chromosomes can be:

 

Format and Original Material © Martin E. Mulligan, 1997-2002