Unusual Types of Bacteria (Part 3)

. Actinomycetes

Actinomycetes are gram-positive filamentous bacteria characterized by mycelial growth (i.e., branching filaments), which is analogous to fungal growth. However, the diameter of the filaments is similar in size to bacteria (approximately 1 mm). Most actinomycetes are strict aerobes, but a few of them require anaerobic conditions. Most of these microorganisms produce spores, and their taxonomy is based on these reproductive structures (e.g., single spores in Micromonospora or chains of spores in Streptomyces). They are commonly found in water, wastewater treatment plants, and soils (with preference for neutral and alkaline soils). Some of them (e.g., Streptomyces) produce a

characteristic “earthy” odor that is due to the production of volatile compounds called geosmins. They degrade polysaccharides (e.g., starch, cellulose), hydrocarbons, and lignin. Some of them produce antibiotics (e.g., streptomycin, tetracycline, chloramphenicol). Two well-known genera of actinomycetes are Streptomyces and Nocardia (now called Gordonia). Streptomyces forms a mycelium with conidial spores at the tip of the hyphae. These actinomycetes are important industrial microorganisms that produce hundreds of antibiotic substances. Gordonia is commonly found in water and wastewater and degrades hydrocarbons and other recalcitrant (i.e., hard to degrade) compounds. Gordonia is a significant constituent of foams in activated sludge units.

. Cyanobacteria

Often referred to as blue-green algae, cyanobacteria are prokaryotic organisms that differ from photosynthetic bacteria in the fact that they carry out oxygenic photosynthesis. They contain chlorophyll a and accessory pigments such as phycocyanin (blue pigment) and phycoerythrin (red pigment). The characteristic bluegreen color exhibited by these organisms is due to the combination of chlorophyll a and phycocyanin. Cyanobacteria occur as unicellular, colonial, or filamentous organisms. They propagate by binary fission or fragmentation, and some may form resting structures, called akinetes, which, under favorable conditions, germinate into a vegetative form. Many contain gas vacuoles that increase cell buoyancy and help the cells float to the top of the water column where light is most available for photosynthesis. Some cyanobacteria (e.g., Anabaena) are able to fix nitrogen; the site of nitrogen fixation is a structure called a heterocyst. Cyanobacteria are ubiquitous and, owing to their resistance to extreme environmental conditions (e.g., high temperatures, desiccation), they are found in desert soils and hot springs. They are responsible for algal blooms in lakes and other aquatic environments, and some are quite toxic.