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 Nocardia rhodochrous.

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:

max (1 / day) 2.5
Ks (mg / L) 0.5

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 clumped growth.

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;
  • 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 scums
  • The usually longer retention times in the scum, in comparison to the mixed liquor, suit the slow - growing nocardioforms

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) max : high biomass production for readily biodegradable substrates and Ks : high affinity to substrates under low concentrations for refractory (slowly biodegradable) substrates.

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.