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The Genus Bacillus (page 1)
(This chapter has 6 pages)
© Kenneth Todar, PhD
E.M. of Bacillus megaterium.
Gram-positive, Aerobic or Facultative Endospore-forming Bacteria
In 1872, Ferdinand Cohn, a contemporary of Robert Koch, recognized
the bacterium Bacillus subtilis. The organism is
Gram-positive, capable of growth in the presence of oxygen,
and forms a
unique type of resting cell called an endospore.
organism represented what was to become a large and diverse genus of bacteria named Bacillus,
in the Family Bacillaceae.
Koch relied on Cohn's observations in his classic work (1876), The etiology of anthrax based on the life
history of Bacillus anthracis,
which provided the first proof that a specific microorganism could
cause a specific disease.
photomicrographs of Bacillus anthracis. In 1876, Koch
by careful microscopy that the bacterium was always present in the
of animals that died of anthrax. He took a small amount of blood from
an animal and injected it into a healthy mouse, which subsequently
diseased and died. He was able
recover the original anthrax organism from the dead mouse,
for the first time that a specific bacterium is the cause of a specific
The genus Bacillus
remained intact until 2004, when it was split into several families and
endospore-forming bacteria, justifiable on the basis of ssRNA analysis.
In order to
accommodate former members of the genus Bacillus covered in this
title has been changed to "Gram-positive
aerobic or facultative
The unifying characteristic of these bacteria is that they are Gram-positive, form endospores, and grow in the presence of O2. The trivial name assigned to them
is aerobic sporeformers.
The ubiquity and diversity of these bacteria
in nature, the unusual
of their endospores to chemical and physical agents, the developmental
cycle of endospore formation, the production of antibiotics, the
of their spores and protein crystals for many insects, and the pathogen
anthracis, have attracted ongoing interest in these bacteria since
and Cohn and Koch's discoveries in the 1870s.
There is great diversity of physiology among the aerobic
not surprising considering their recently-discovered phylogenetic
collective features include degradation of most all substrates derived
from plant and animal sources, including cellulose, starch, pectin,
agar, hydrocarbons, and others; antibiotic production; nitrification;
nitrogen fixation; facultative lithotrophy; autotrophy; acidophily;
psychrophily; thermophily; and parasitism. Endospore formation,
found in the group, is thought to be a strategy for survival in the
environment, wherein these bacteria predominate. Aerial distribution of
dormant spores probably explains the occurrence of aerobic sporeformers
in most habitats examined.
stain. CDC. Gram-positive or Gram-negative? The cell wall structure of
endospore-forming bacteria is consistent with that of Gram-positive
bacteria, and young
stain as expected. However, many sporeformers rapidly become
when entering the stationary phase of growth.
Classification and Phylogeny
Early attempts at classification of Bacillus species were
on two characteristics: aerobic growth and endospore formation.
resulted in tethering together many bacteria possessing different kinds
of physiology and occupying a variety of habitats. Hence, the
in physiology, ecology, and genetics, made it difficult to categorize
genus Bacillus or to make generalizations about it.
In Bergey's Manual of Systematic Bacteriology (1st ed. 1986),
the G+C content of known species of Bacillus ranges from 32 to
This observation, as well as DNA hybridization tests, revealed the
heterogeneity of the genus. Not only was there variation from species
to species, but there were sometimes profound differences in G+C
within strains of a species. For example, the G+C content of the
megaterium group ranged from 36 to 45%.
In Bergey's Manual of Systematic Bacteriology (2nd ed. 2004),
phylogenetic classification schemes landed the two most prominent types
of endospore-forming bacteria, clostridia and bacilli, in two different
Classes of Firmicutes,
Clostridia and Bacilli.
Clostridia includes the Order Clostridiales and
Family Clostridiaceae with 11 genera including, Clostridium.
Bacilli includes the Order Bacillales and the
In this family there 37 new genera on the level with Bacillus.
This explains the heterogeneity in G+C content observed in the 1986
The phylogenetic approach to Bacillus taxonomy has been
largely by analysis of 16S rRNA molecules by oligonucleotide
This technique, of course, also reveals phylogenetic relationships.
Bacillus species showed a kinship with certain nonsporeforming
including Enterococcus, Lactobacillus, and Streptococcus
at the Order level, and Listeria
and Staphylococcus at the
Family level. Otherwise, some former members of the genus Bacillus were gathered into new
Families, including Acyclobacillaceae,
Paenibacillaceae and Planococcaceae, now on the level
with Bacillaceae. Most of the
bacteria discussed in this article come from one of these four
Families. Their taxonomic
hierarchy (Bergey's 2004) is Kingdom: Bacteria; Phylum: Firmicutes; Class: Bacilli; Order: Bacillales; Family: Acyclobacillaceae (genus: Acyclobacillus); Family: Bacillaceae (genus: Bacillus, Geobacillus); Family: Paenibacillaceae (genus: Paenibacillus, Brevibacillus); Family: Planococcaceae (genus: Sporosarcina).
Notable former members of the genus Bacillus that have been moved to
new families and/or genera are given in the table below.
taxonomic reassignments in the Genus Bacillus (1986-2004).
Manual of Systematic
Bacteriology (1st ed. 1986)
Manual of Systematic
Bacteriology (2nd ed. 2004),
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