Environmental Microbiology of Aquatic and Waste Systems

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.

16. 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.


17. 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