Gram‐Negative Rods and Cocci

Bdellovibrios. Bdellovibrios are aerobic Gram‐negative, curved rods that prey on other bacteria. The organism attaches to the surface of a bacterium, rotates, and bores a hole through the host cell wall. It then takes biochemical control of the host cell and grows in the space between the cell wall and plasma membrane. The host bacterium is killed in the process. The comma‐shaped Bdellovibrio bacteriovorus is the most thoroughly studied species of the group.

Pseudomonads. Pseudomonads are aerobic, Gram‐negative rods that are motile with polar flagella. Over 30 species are found in the group, and Pseudomonas fluorescens is a well‐known producer of a yellow‐green pigment. Another species, P. aeruginosa, causes urinary tract infections and infections of burned tissue.

Azotobacter and Rhizobium. Species of Azotobacter and Rhizobium are extremely important for their ability to fix nitrogen in the environment. These Gram‐negative rods live free in the soil (Azotobacter) or on the roots of legume plants (Rhizobium) and use their enzymes to convert atmospheric nitrogen into organic molecules useful to the plant. The plants then use the nitrogen compounds for the synthesis of amino acids and proteins, which serve as an extremely valuable food source for animals and humans. Members of the genus Azotobacter form a resting cell called a cyst, which withstands drying and environmental stresses.

Enterobacteria. Enterobacteria are facultatively anaerobic, Gram‐negative rods that inhabit the human intestine. Members of the enterobacteria group are members of the family Enterobacteriacae classified in section 5 of Bergey's Manual.

Over 25 genera of enterobacteria are recognized, many with pathogenic importance. Among the medically important enterobacteria are Salmonella species that cause intestinal disease known as salmonellosis; Yersinia pestis, the cause of plague; Klebsiella species, the causes of pneumonia, intestinal disease, and other infections; and species of Serratia and Proteus. The well‐known organism Escherichia coli is also a member of this group. All enterobacteria have peritrichous flagella.

Vibrios. Vibrios are curved, Gram‐negative, facultatively anaerobic rods. They belong to the family Vibrionaceae. One species, Vibrio cholerae, is the cause of cholera in humans. Members of the genus Aeromonas and Plesiomonas are involved in human intestinal disease. Species of Photobacterium are marine organisms known for their ability to produce light as a result of chemical actions stimulated by the enzyme luciferase. This production of light is known as bioluminescence.

Pasteurellas. The pasteurellas belong to the family Pasteurellaceae. They are distinguished from vibrios and enterobacteria by their small size and inability to move. The genera Pasteurella, Haemophilus, and Actinobacillus are among the important members of the group. The species H. influenzae is a cause of meningitis in children, while P. multocida causes cholera in fowl.

Sulfur bacteria. The sulfur bacteria use sulfur or sulfur compounds as electron acceptors in their metabolism. These bacteria produce large amounts of hydrogen sulfide during their growth, and therefore, they produce foul odors in water and mud. Members of the genus Desulfovibrio are particularly important in the sulfur cycle for their ability to use sulfur and convert it to other compounds that can be used by plants to synthesize sulfur‐containing amino acids.

Bacteroides. The bacteroides are genera of anaerobic bacteria having unique motility and flagellation patterns. Several species digest cellulose in the rumen of the cow and thereby break down plants. Human feces contains large numbers of bacteria belonging to the genus Bacteroides, which may be helpful in digestive processes. One species, B. fragilis, is a possible cause of human blood infections.

Veillonella. Among the Gram‐negative cocci are a group of anaerobic diplococci belonging to the genus Veillonella. Veillonella species are part of the normal flora of the mouth and gastrointestinal tract, and they are found in dental plaque. They are anaerobic organisms that may also cause infections of the female genital tract.

Gliding bacteria. Certain bacterial species are able to move by gliding in a layer of slime, which they produce. Wavelike contractions of the outer membranes help the bacteria propel themselves. Members of the group include species of Cytophaga and Simonsiella.

Two important genera of gliding bacteria are Beggiatoa and Thiothrix. Species of these organisms live in sulfur environments and break down hydrogen sulfide to release sulfur in the form of sulfur granules. For this reason, the bacteria are very important in the recycling of sulfur in water and soil. The bacteria are Gram‐negative.

Myxobacteria are gliding bacteria that are Gram‐negative, aerobic rods. They are nonphotosynthetic species and have a unique developmental cycle that involves the formation of fruiting bodies. When nutrients are exhausted, the bacteria congregate and produce a stalk, at the top of which is a mass of cells. These cells differentiate into spherical cells, similar to cysts, which are resistant to environmental extremes.

Sheathed bacteria. Sheathed bacteria are filamentous bacteria with cell walls enclosed in a sheath of polysaccharides and lipoproteins. The sheath assists attachment mechanisms and imparts protection to the bacteria. The genus Sphaerotilus is in this group.

Photoautotrophic bacteria. Photoautotrophic bacteria are Gram‐negative rods which obtain their energy from sunlight through the processes of photosynthesis. In this process, sunlight energy is used in the synthesis of carbohydrates. Certain photoautotrophs called anoxygenic photoautotrophs grow only under anaerobic conditions and neither use water as a source of hydrogen nor produce oxygen from photosynthesis. Other photoautotrophic bacteria are oxygenic photoautotrophs. These bacteria are cyanobacteria. They use chlorophyll pigments and photosynthesis in photosynthetic processes resembling those in algae and complex plants. During the process, they use water as a source of hydrogen and produce oxygen as a product of photosynthesis.

Cyanobacteria include various types of bacterial rods and cocci, as well as certain filamentous forms. The cells contain thylakoids, which are cytoplasmic, platelike membranes containing chlorophyll. The organisms produce heterocysts, which are specialized cells believed to function in the fixation of nitrogen compounds.

Chemoautotrophic bacteria. Chemoautotrophic (or chemolithotrophic) bacteria are a group of Gram‐negative bacteria deriving their energy from chemical reactions involving inorganic material. Certain chemoautotrophic bacteria use carbon dioxide as a carbon source and grow in a medium containing inorganic substances. By comparison, members of the genus Thiobacillus metabolize sulfur compounds, and members of the genera Nitrosomonas and Nitrobacter metabolize nitrogen compounds. Certain chemoautotrophic bacteria use hydrogen gas in their chemical reactions, and others use metals such as iron and manganese in their energy metabolism. These unusual types of biochemistry are characteristic of organisms found outside the body in the soil and water environment.