64 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms
in humans. The genes encoding exotoxins
that are the cause of diphtheria (caused by
Corynebacterium) as well as cholera and some
other bacterial diseases are mobile in aquatic and
terrestrial environments and have been found
in sediments and in river water using PCR.
Micrococcus
These are cocci in bunches and very similar
to staphylococci. They are distinguished from
each other according to the properties shown
in Table 4. 7.
- Nitrospira
Nitrospira are nitriteoxidizing bacteria that are
important in marine habitats. In aquaria, for exam
ple, if the ammonia/nitrite/nitrate cycle is exhausted,
the ecosystem suffers and fish can get sick or die.
Therefore, nitriteoxidizing bacteria as well as the
other bacteria in this system are important for
healthy marine ecosystems. In addition, Nitrospira
like bacteria are the main nitrite oxidizers in waste
water treatment plants. - Proteobacteria (including purple bacteria)
The Proteobacteria are a major group of bacteria.
They include a wide variety of pathogens, such as
Escherichia, Salmonella, Vibrio, and Helicobacter.
Others are important agriculturally or industrally;
still others are freeliving, and include many of the
bacteria responsible for nitrogen fixation. The
group is defined primarily in terms of ribosomal
RNA (rRNA) sequences, and is named after the
Greek god Proteus, who could change his shape,
because of the great diversity of forms found
within the group. Proteus is also the name of a
bacterial genus within the Proteobacteria.
All Proteobacteria are Gramnegative, with an
outer membrane mainly composed of lipopolysac
charides. Many move with flagella, but some are
nonmotile or move by bacterial gliding. The latter
include the myxobacteria, a unique group of bac
teria that can aggregate to form multicellular fruit
ing bodies.
There is also a wide variety in the types of
meta bolism. Most members are facultatively or
obligately anaerobic and heterotrophic, but there
are numerous exceptions. A variety of genera,
which are not closely related to each other, convert
energy from light through photosynthesis. These
are called purple bacteria, referring to their mostly
reddish pigmentation.
The Proteobacteria are divided into five sec
tions, referred to by the Greek letters alpha through
epsilon, again based on rRNA sequences.
Alpha (a) Proteobacteria
The Alphaproteobacteria comprise the most pho
totrophic genera, but also several genera meta bo
lizing C1compounds (compounds with a single
carbon atom e.g., Methylobacterium, symbionts of
plants (e.g., Rhizobia) and animals, and a group
of intracellular pathogens, the Rickettsiaceae.
Moreover, the precursors of the mitochondria of
eukaryotic cells are thought to have originated in
this bacterial group.
Beta (b) Proteobacteria
The Betaproteobacteria consist of several groups of
aerobic or facultative bacteria which are often
highly versatile in their degradation capacities, but
also contain chemolithotrophic genera (e.g., the
ammoniaoxidizing genus Nitrosomonas) and some
phototrophs (genera Rhodocyclus and Rubrivivax).
Beta Proteobacteria play an important role in nitro
gen fixation in various types of plants, oxidizing
ammonium to produce nitrite an important chemi
cal for plant function. Many of them are found in
environmental samples, such as waste water or soil.
Pathogenic species within this class are the
Neisseriaceae (gonorrhea and meningoencephali
tis) and species of the genus Burkholderia.
Gamma (g) Proteobacteria
The Gammaproteobacteria comprise several
medically and scientifically important groups of
bacteria, such as the Enterobacteriaceae, Vibrion
aceae, and Pseudomonadaceae. Many important
pathogens belong to this class, e.g., Salmonella
(enteritis and typhoid fever), Yersinia (plague),
Vibrio (cholera), Pseudomonas aeruginosa
(lung infections in hospitalized or cystic fibrosis
patients), and E coli.
Species Cells arrangement G + C ratio Oxygen requirement
Micrococcus Clusters, tetrads (fours) 66–73 Strictly aerobic
Staphylococcus Clusters, pairs 30–39 Microaerophilic
Table 4.7 Properties of
Micrococci and Staphylococci