4.1 Taxonomy of Microorganisms in Aquatic Environments 63
stressful environmental conditions, the cells
produce endospores resistant to heat, radiation,
chemicals and other unfavorable conditions.Clostridium: These are Grampositive spore
forming obligately anaerobic rods. Individual
cells are rodshaped, and the name comes
from the Greek for spindle. Clostridium
includes common freeliving bacteria as well
as important pathogens, including C. botuli
num, an organism producing a very potent
toxin in food; C. difficile, which can overgrow
other bacteria in the gut during antibiotic ther
apy; C. tetani, the causative organism of teta
nus; C. perfringens, formerly called C. welchii,
which causes a wide range of symptoms, from
food poisoning to gas gangrene. Because
C. perfringens produces much gas, it is also
used as a replacement for yeasts in breadmak
ing. C. sordellii has been linked to the deaths
of more than a dozen women. They are impor
tant in the anaerobic conditions of muds.Heliobacteria: Heliobacteria are strictly
anaerobic, sporeforming photoheterotrophic
members of the Firmicutes. 16s rRNA studies
put them among the Firmicutes (Bacillus and
Clostridium) but they do not stain Gram
positively like the other members. They have
no outer membrane and like certain other fir
micutes (clostridia), they form heat resistant
endospores. They are the only firmicutes
known to conduct photosynthesis. Soluble
periplasmic components appear absent in
heliobacteria and photosynthesis takes place at
the cell membrane, which does not form folds
or compartments as it does in purple pho
totrophic bacteria. A particularity of heliobac
terial photosynthesis is the occurrence of a
unique Bacteriochlorophll (BChl) g. BChl g is
chemically closer to Chl a than to BChl a.
Correspondingly, heliobacteria appear to be
more closely related to oxygenic photosynthe
sis than the green sulfur bacteria (based on
16SrRNA phylogeny as well as on trees built
from sequences of the photosynthetic reaction
center). A small group, it is the only known
phototrophic one among the Gram positives.
Heliobacteria consist of three genera,Heliobacterium (3 spp.), Heliobacillus (1 sp.),
and Heliophilum (1 sp.). They cannot tolerate
sulfide, all known species can fix nitrogen.
They are common in the waterlogged soils of
paddy fields.
(b) High G + C Group: G + C Above 50%
These include Actinomycetes, Mycobacteria,
Micrococcus, and Corynebacterium:
Actinomycetes
Actinomycetes are filamentous and spore
forming (non heat resistant spores), found in
soil. They are very important as antibiotic pro
ducers. Typical example is Streptomyces sp.
They include some of the most common soil
life, playing an important role in decomposi
tion of organic materials, such as cellulose
and chitin and thereby playing a vital part in
organic matter turnover and carbon cycle.
Actinomycetes of the family Actinoplanaceae,
especially Actinoplanes, are readily isolated
from the flowing waters of rivers and streams,
where they are important in the decomposi
tion of wood and other cellulolisic materials.Mycobacterium
This is a slowgrowing acidfast strain (Ziel–
Nieelsen stain) implicated in diseases (M. lep
rae, leprosy; M. tuberculosis, tuberculosis).
Many are however freeliving and inhabit
aquatic environments. These environmental or
waterborne mycobacteria (WBM) inhabit a
diverse range of natural environments and are a
frequent cause of opportunistic infection in
human beings and livestock. Several hospital
and community outbreaks of mycobacterial
infections, including infections as diverse as
lifethreatening pneumonia in patients with arti
ficial ventilation, cystic fibrosis, and chronic
granulomatous disease; outbreaks of skin infec
tion following liposuction; furunculosis after
domestic footbaths; mastitis after body piercing;
and abscess formation in intravenous drug users.Corynebacteria
Corynebacterium is a genus of Grampositive,
facultatively anaerobic, nonmotile, rodshaped
actinobacteria. Most do not cause disease, but
are part of normal human skin flora. Coryne
bacterium diphtheriae is the cause of diphtheria