Amino Acid Utilization by
Activated Sludge
DALE A, CARLSON, Associate Professor of Civil Engineering
University of Washington
Seattle, Washington
and
L. B. POLKOWSKI, Professor of Sanitary Engineering
State University of Iowa
Iowa City, Iowa
Proteins, lipids and carbohydrates constitute the major organic fractions
of domestic sewage. Each of these classes encompasses a variety of compounds. The diversity in chemical complexity and structure in each class
makes strict classification of these compounds difficult. The proteins, for example, are subdivided by such criteria as solubility, gross structure, state of
degradation or the content of nonprotein constituents. The primary characteristic of the proteins is, however,   that they yield amino acids when hydrolyzed.
PENETRATION OF CELL BOUNDARIES
The utilization of the proteins and their constituent amino acids, as well
as of the lipids and carbohydrates, is accomplished principally by the bacteria as a means of nutrition. These compounds must be able to enter into
the bacterial cell and they must be usable in the cell's metabolic pathways.
The uses of a given compound by a bacterial cell depend on the enzymes the
cell possesses (1). The modes of entrance of compounds into the cytoplasm of
the bacterial cell is illustrated in the following figures.
Cellular boundaries are shown in Figure 1 on a simplified sketch of a
typical bacillus. The interior cytoplasm of this cell is bounded first by a
membrance, or sac. Bacterial form is maintained by the rigid cell wall on
the exterior of this membrane. Surrounding the whole organism is the slime
layer or capsule which varies in thickness from being barely visible with the
electron microscope to being larger than the cell itself.
Nutrient material approaching this bacterial cell would first encounter a
degree of orientation in the sunounding medium which restricts the movement
of ionized particles. Progressing further the moving material encounters
waste products of the bacterial cell and then moves into the denser jelly of
the capsule.
The next obstacle encountered by the nutrient particle is the cell wall
which might be described as a veritable brush pile of rigid girders. Besides
this microfibrillar structure, many cell walls appear to be, in electron micrographs, a lattice of spherical particles in hexagonal or square packing.
Some bacteria, such as the Escherichia coli, apparently have a multilayered
wall. Like a sieve, the cell wall apparently can offer some resistance to the
entrance of particulate matter on the basis of particle size.
The site of the differential permeability exhibited by bacterial cells is
primarily the cell membrane. Several theories have been proposed to explain
the selective entrance of particles through the cell membrane to the interior of
- 77  -