Environmental Microbiology of Aquatic and Waste Systems

52 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms

(b) Cell aggregates

Cocci can occur in pairs (diplococci), in chains

(streptococci), or in clumps (staphylococci).

Rods may occur in short chains of two or three

or in long chains or filaments.

(c) Flagellation

The flagella may be at one end (polar) and may

occur singly or as a tuft. The flagella may occur

all around the cell when it is peritrichous (see

Fig. 4. 5 ).

(d) Spores and location of spores

Spores are bodies resistant to heat and other

adverse conditions which may be terminal or

placed mid­way in the cell; in either position, it

may be less than the diameter of the cell or may

be wider. The terminal wider spore gives the

shape of a drumstick, and is diagnostic of the

anaerobic rod­like spore­former, Clostridium tet­

anii, the causative agent of tetanus (see Fig. 4. 6 ).

(e) Acid­fast (Ziel–Nieelsen) stain

If the bacterium is suspected to belong to Myco­

bacterium spp., or any of the other acid­fast

bacteria it might be stained with hot basic

fuchsin; acid fast bacteria retain the dye when

decolorized.

4. Utilization of various substrates
   Utilization of various sugars, carbohydrates, and
   nitrogenous sources
   The ability of the organism to produce acid and/or
   gas from a medium containing a particular substrate
   is diagnostic of its ability to utilize it. The utilization
   of a wide range of sugars and other carbohydrates, and
   nitrogen sources including urea is tested by the pres­
   ence of gas in the small (Durham) tube placed in the

Gram–ve cell wall Gram+ve cell wall

Cell membrane

Periplasmic space

Peptidoglycan

Outer membrane

Lipid-polysaccharide

Fig. 4.2 Diagram illustrating the generalized structure of the
bacterial cell wall (Note the comparative thicknesses of the pep­
tidoglycan layers in the Gm+ve and Gm−ve walls)
Note that the peptidoglycan layer is very thick in Gram +ve
walls, but very thin in Gram –ve bacterial cells. This thick
peptidoglycan enables Gram +ve walls retain crystal violet, the
primary stain in the Gram, when decolorized with dilute acid.

Crystal violet is not retained in Gram –ve bacteria because the

peptidoglycan layer is thin. The Gram –ve wall would be color­

less after decolorization with dilute acid. However in the Gram

stain, after decolourization, the cells are counterstained with a

red stain, safranin. On account of this the Gram negative cells

appear red in the Gram stain, while Gram +ve cells are violet

(see text and Fig. 4.3)

Fig. 4.3 Gram staining