The Genus Bacillus (page 4)
(This chapter has 6 pages)
© Kenneth Todar, PhD
Genetics of Bacillus
The discovery of transformation in a strain of Bacillus
1958, focused attention on the genetics of the bacterium. This is
one of relatively few bacteria in which competence for DNA uptake has
found to occur as a natural part of the bacterium's life cycle.
specialized transduction were observed in B. subtilis, and
of the genetics and chromosomal organization of the bacterium quickly
to become second only to that of the enteric bacteria. Furthermore, the
identification of numerous genes affecting sporulation in B.
has provided a means for analyzing the complex developmental program of
Bacteriophages capable of mediating generalized transduction have
been reported in other species of Bacillus, including B.
B. megaterium, B. thuringiensis, B. anthracis, and in Geobacillus
Conjugative plasmids are plasmids capable of bringing
about their own transfer from one bacterium to another. They have been
in several species of Bacillus. The capacity to produce the
delta toxin crystal protein in B. thuringiensis is encoded in
plasmids. These plasmids can be transferred to plasmid-deficient
of B. thuringiensis, as well as to B. cereus, to yield
that produce crystal protein. B. thuringiensis transfers
pXO11 and pXO12 plasmids to B. anthracis and to B. cereus.
The recipients, in turn, become effective donors, and in the case of
inheriting pXO12, also acquire the ability to produce parasporal
Strains of B. anthracis that acquire plasmid pXO12 can
mobilize and transfer nonconjugative plasmids present in the same cell.
The B. anthracis toxin plasmid, pXO1, and the capsule plasmid,
can be transferred to B. anthracis and B. cereus
lacking these plasmids.
The large B. anthracis plasmids are apparently transferred
a process called conduction. This involves formation of
molecules in the donor, and resolution of the cointegrates into pXO12
the respective B. anthracis plasmid in the recipient.
contact is necessary for plasmid transfer and is resistant to DNase,
little is known about the mechanisms or conjugative structures that may
be involved. None of the conjugative plasmids have been found to
and transfer chromosomal markers as is observed with the F plasmid of E.
In addition to the naturally occurring transmissible plasmids of Bacillus,
a conjugative transposon (Tn925) has been identified, which
from Enterococcus faecalis to B. subtilis.
Our understanding of the Bacillus genome, and their means of
DNA transfer, has led to its manipulation. So far, this has resulted in
numerous medical, agricultural and industrial achievements, involving
use of the organism or its products.
This e.m. image of a
(also at the top of page 1) is that of B. megaterium which
been cloned with the Bt gene and is expressing Bt in the form of the
"parasporal" crystal adjacent to the spore.Bt is an insecticidal protein
Due to the resistance of their endospores to environmental stress,
well as their long-term survival under adverse conditions, most aerobic
sporeformers are ubiquitous and can be isolated from a wide variety of
sources. Hence, the occurrence of sporeforming bacteria in a certain
is not necessarily an indication of habitat. However, it is generally
that the primary habitat of the aerobic endospore-forming bacilli is
soil. The great Russian microbiologist, Winogradsky, considered them as
flora" of the soil.
In the soil environment the bacteria become metabolically-active
suitable substrates for their growth are available, and presumably they
form spores when their nutrients become exhausted. This is a strategy
by other microbes in the soil habitat, including the filamentous fungi
and the actinomycetes, which also predominate in the aerobic soil
It is probably not a coincidence, rather an example of convergent
that these three dissimilar groups of microbes live in the soil,
form resting structures (spores), and produce antibiotics in
with their sporulation processes.
Since many endospore forming species can effectively degrade a
of biopolymers (proteins, starch, pectin, etc.), they are assumed to
a significant role in the biological cycles of carbon and nitrogen.
From soil, by direct contact or air-borne dust, endospores
can contaminate just about anything that is not maintained in a sterile
environment. They may play a biodegradative role in whatever they
and thereby they may be agents of unwanted decomposition and decay.
species are especially important as food spoilage organisms.
Generally, standard bacteriological criteria do not
distinguish the aerobic sporeforming bacteria for
or positive identification. An artificial, but convenient, way to
organize aerobic spore-formers for this purpose is to place them into
groups, such as nitrogen-fixers, denitrifiers, insect pathogens,
pathogens, thermophiles, antibiotic producers, and so on. Such an
also allows some speculation concerning the natural history, diversity,
of this important group of bacteria.
Acidophiles: include Acyclobacillus
Bacillus coagulans, and Paenibacillus
and Sporosarcina pasteurii. The optimum pH is 8, and some strains
grow at pH
pantothenticus, Sporosarcina pasteurii. Some strains grow in 10 %
Psychrophiles or psychrotrophs: Sporosarcina
globisporus, Bacillus insolitus, Marinibacillus marinus, Paenibacillus macquariensis,
Bacillus megaterium, Paenibacillus
polymyxa. Two species will grow and form spores at 0oC.
Thermophiles: include Acyclobacillus
Bacillus schlegelii, and Geobacillus stearothermophilus. Acidophiles
are found in this group, too. The upper temperature limit is 65oC.
Denitrifiers: include Bacillus
azotoformans, Bacillus cereus, Brevibacillus laterosporus, Bacillus
(over half the type species reduce NO3 to NO2).
species are common in agricultural soils, and they are attributed to
in wasteful denitrification (conversion of the farmer's expensive NO3
fertilizers to volatile N2O or N2)
their exact role in the economy of this processes has not been
related process conducted by some Bacillus species, called
nitrate reduction, reduces NO3 to ammonia (NH3),
but this is not considered denitrification.
Nitrogen-fixers: Paenibacillus macerans
macerans is a fairly prominent bacterium in
soil and in decaying vegetable material. The bacteria only fix
under anaerobic conditions because they do not have a mechanism for
of their nitrogenase enzyme from the damaging effects of O2.
In the same way as the role of the bacilli in denitrification and
nitrification, their overall contribution to non symbiotic global
fixation is not known.
antibiotics produced by the aerobic sporeformers are often, but not
always, polypeptides. Known antibiotic producers are Brevibacillus
(e.g. gramicidin, tyrothricin),
Bacillus cereus (e.g. cerexin, zwittermicin),
circulans (e.g. circulin), Brevibacillus laterosporus (e.g.
licheniformis (e.g. bacitracin), Paenibacillus polymyxa (e.g.
colistin), Bacillus pumilus (e.g. pumulin) and
Bacillus subtilis (e.g. polymyxin,
difficidin, subtilin, mycobacillin).
Bacillus antibiotics share a full range of antimicrobial
bacitracin, pumulin, laterosporin, gramicidin and tyrocidin are
against Gram-positive bacteria; colistin and polymyxin are
difficidin is broad spectrum; and mycobacillin and zwittermicin are
As in the case of the actinomycetes, antibiotic production in the
is accompanied by cessation of vegetative growth and spore formation.
has led to the idea that the ecological role of antibiotics may not
with competition between species, but with the regulation of
and/or the maintenance of dormancy.
Pathogens of Insects: Paenibacillus
lentimorbus and Paenibacillus
popilliae are invasive pathogens. Bacillus
thuringiensis forms a parasporal crystal that is toxic to Lepidoptera.
P. larvae, P. lentimorbus and P. popilliae are a
cluster of species, being insect pathogens with
sporangia and typically catalase-negative. They also are unable to
in nutrient broth, probably because it is insufficient in thiamin,
they need as a growth factor. Yeast extract (15g/l) must be added to
media for growth. Also, P.
lentimorbus and P. popilliae are
quite similar in their biochemical properties, virulence and host
They sometimes occur in coinfections.
P. larvae is the causative agent of American foulbrood
of honeybees, which is the most widespread and persistent of the
brood diseases. The organism can be isolated repeatedly from infected
and honeycomb, usually in a pure culture. It has been noted on many
that the natural habitat of the bacterium is remarkably free of
Presumably, the bacterium can be isolated from soil around the hives of
infected bees, but it has not been isolated from other sources. This is
indicative of a very close and specific type of host-parasite
between the bacterium and the honeybee.
P. popilliae is the cause of the most widespread of two milky
diseases of the Japanese beetle, Popillia japonica. Their
in a swollen sporangium, are frequently accompanied by a parasporal
Interestingly, the bacterium sporulates with ease in the hemolymph of
infected insect, but it will not form mature spores in most artificial
Special media have been designed that induce P. popilliae and
lentimorbus to form mature spores. The prospect that P.
together with P. lentimorbus, might be used to control or
the Japanese beetle and the European chafer (Amphimallon majalis)
has drawn attention to these bacteria. P. popilliae is
in naturally-infected grubs far more frequently than P. lentimorbus,
which also causes milky disease.
P. lentimorbus is similar in most ways to P. popilliae.
The most obvious difference is that P. lentimorbus does not
a parasporal body. The bacteria also differ morphologically and
lentimorbus likewise causes one of two milky diseases in the
beetle. The bacterium can only be isolated from the hemolymph of
beetles, although it most certainly exists in soil inhabited with
The principal interest in P. lentimorbus arises from its
to cause disease of Japanese beetle and European chafer larvae, which
cause millions of dollars in damage each year to a variety of plants. P.
lentimorbus is more widespread than P. popilliae, which
causes milky disease in the same hosts. The reason the infections are
"milky disease" is that as the disease develops, the larvae become
in appearance. This is caused by the prolific production of spores in
Spores of the the insect
seen by phase microscopy. U.S. Dept. of Agriculture. A. Paenibacillus
spores from a comb infected with American foulbrood; B. Paenibacillus
from hemolymph of infected Japanese beetle larvae; C. Spores of Paenibacillus
from hemolymph of infected Japanese beetle larvae.
Bacillus thuringiensis is a variety of B. cereus and
therefore considered in the B. cereus-B. anthracis-B. thuringiensis
group. B thuringiensis is distinguished from B. cereus
anthracis by its pathogenicity for lepidopteran insects and by
of an intracellular parasporal crystal
in association with spore
The bacteria and protein crystals are marketed as "Bt" insecticide,
used for the biological control of certain garden and crop pests.
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