The term "nocardioforms" is used for describing filamentous microorganisms with rather short
trichomes, truly branched, Gram positive, containing Neisser positive granules which are
frequently found in foaming activated sludges.
The "nocardioforms" are described in Section 17 of Volume 2 of Bergey's Manual of Systematic
Bacteriology as organisms forming an expanded mycelium which breaks up into rod - shaped or
coccoid elements. The microorganisms covered by the term "nocardioforms" are not necessarily
closely related. From nine "nocardioform" genera, two are involved with activated sludge
foaming - Nocardia and Rhodococcus.
The genus Nocardia was defined by two authors as "aerobic, acid - fast to partially
acid - fast actinomycetes that produce primary mycelium that fragments into rod - and coccoid
- like elements". The genus Rhodococcus was established by two researchers to
accommodate a heterogeneous group of aerobic, non - sporing actinomycetes forming a primary
mycelium. These actinomycetes were termed in older literature Mycobacterium rhodochrous or
According to Bergey's Manual, microorganisms of genus Nocardia are characterized by
extensively branched vegetative hyphae with a diameter of 0.5 - 1.2 micro - m. The reaction
to the Gram stain is positive to variable, the microorganisms are aerobic, mesophilic and
chemoorganotrophic. Most Nocardia strains are of soil origin, and some of them are
pathogenic opportunists for man and animals. Nocardia spp. grow readily on a variety
of media containing both simple components like glucose, acetate or propionate, and more
complex substances like proteins, carbohydrates and fats. All Nocardia species grow
in a temperature range of 15 - 37 deg - C; the doubling time is much longer than for other
bacteria (relatively slow - growing actinomycetes). Mycolic acid is a characteristic component
of the chemical composition of Nocardia cells. Some species, including Nocardia
amarae, are able to produce nitrate reductase.
The microorganism found in activated sludge foams can be positioned to species Nocardia
amarae. A novel species, to included in "Bergey's Manual", was isolated from Australian
activated sludges and named Nocardia pinensis. The branching pattern of this species
resembles a pine tree.
Nocardia amarae as a typical representative of the Nocardia genus in activated
sludge was studied by some authors. They concluded that Nocardia amarae is a poor
competitor with both completely mixed activated sludge floc formers and bulking filamentous
microorganisms for acetate and oxygen at steady state. However, Nocardia amarae
exhibited an acetate storage capacity of the same order as Zoogloea ramigera, a typical
floc - former from systems with substrate concentration gradients.
This indicates that Nocardia amarae control based on kinetic selection should not always
be successful. On the other hand, some authors fully confirmed the principles of Nocardia
amarae control based on metabolic selection under anoxic and anaerobic conditions.
The basic kinetic parameters of Nocardia amarae obtained for acetate were:
Nocardia amarae forms PHB and polyphosphate intracellular storage products. The study
by some researchers has also shown that Nocardia amarae can grow in two different
forms: dispersed filaments, and clumps of filaments. As only the dispersed Nocardia
filaments exhibit a high propensity to foam, this finding is of special importance from a
practical point of view. The dispersed growth resulted from a subsurface withdrawal of
Nocardia culture from a cultivation tank, while the surface overflow supported the
Genus Rhodococcus described in "Bergey's Manual" is formed by actinomycetes growing
in different forms, from rods to extensively branched hyphae. The Rhodococcus
actinomycetes are generally described as Gram positive, aerobic and chemoorganotrophic. The
morphology and staining characteristic of Rhodococcus spp. are quite similar to those
of Nocardia spp. These genera cannot be distinguished by means of common microscopic
procedures used for the examination of activated sludges. Special biochemical tests and the
determination of cell composition (different mycolic acids) are necessary to differentiate
Rhodococcus from Nocardia.
A detailed study of Rhodococcus spp. (especially Rhodococcus rubra) and of stable
foams generated by these actinomycetes was performed at the University of Birmingham, UK by
two researchers and published in a series of papers. They confirmed that Rhodococcus rubra
produces biological surface active substances. The surfactant polymers extracted from the cells
of Rhodococcus rubra were capable of inducing foam in nonfoaming activated sludges. The
presence of hydrophobic Rhodococcus rubra cells stabilized the formed foam. As far as
the nutritional requirements are concerned, the authors from Birmingham found that Rhodococcus
rubra is able, besides common substrates, to metabolize higher alkanes but can be inhibited
by C - 12 or C - 14 acids. They stressed the role of fats and oils in promoting Rhodococcus - type
foams. These substrates produce high - surfactancy metabolites, selectively concentrated,
together with biomass in foams. The biomass in the foam is subjected to both nutrient and
oxygen limitation, which restricts the further degradation of the surfactant molecules and
the resultant formation of inhibitory compounds.
According to some authors, the foams formed by Rhodococcus spp. are more stable than
foams generated by Nocardia spp. The reason is the higher hydrophobicity of Rhodococcus
cells. The difference is connected with the different composition of mycolic acids found in the
cells of Rhodococcus and Nocardia.
The relationship between the formation of stable foams by nocardioforms and the presence of
fatty, oily or viscous substances in wastewaters was confirmed by other authors. For instance,
some researchers observed a selective concentration of nocardioform actinomycetes on fat
particles dispersed in the mixed liquor of activated sludge. Two authors experienced
nocardioform foams in an activated sludge system treating wastewaters with ethylene glycol.
After the spillages of viscous ethylene glycol, nocardioforms proliferated and formed scum.
An exhaustive research into the ecology and growth strategies of foam - forming nocardioform
actinomycetes was conducted in Bavaria by some researchers. They consider nocardioforms,
originally soil actinomycetes, to be ideally equipped for life in a scum environment because;
|µmax (1 / day)
|Ks (mg / L)
In studying foaming by nocardioforms in nutrient removal activated sludge plants, two researchers
commented on the synergism of Nocardia amarae with Acinetobacter sp., a bacterium
connected with the mechanisms of enforced biological phosphorus removal.
An author suggested that nocardioform actinomycetes can switch between two growth strategies.
They are ; (1) µmax : high biomass production for readily biodegradable substrates and
Ks : high affinity to substrates under low concentrations for refractory (slowly biodegradable)
If nocardioforms really possess this ability, it would give them a strong selective advantage
in a heterogeneous environment of activated sludge with stable foams. However, a microorganism
capable of both ecological strategies should develop different sets of enzymes to compete in
different environments, which would cost the cells extra energy. Thus, some authors do not
consider this concept very reliable because it is not supported with enough experimental data.
- Nocardioforms are better - adapted to partially dry conditions in scum
- During periods when dissolved nutrients in the scum are scarce, they can survive by
saprophytic growth on activated sludge particles
- Hydrophobic substrates, preferred by nocardioforms, are selectively concentrated in the
- The usually longer retention times in the scum, in comparison to the mixed liquor, suit
the slow - growing nocardioforms