Revised 1997 FEB 18 (incomplete)

Place a strip graphic here illustrating bacterial shapes. rod, coccus, spiral, packets, filaments, etc.

A Short Guide to Some Bacteria Genera
By Harold Eddleman, Ph. D.

Man has been discovering bacteria and giving them scientific names for 130 years. No two bacterial strains isolated from nature have the exact same characteristics, just as no two humans are exactly the same. Humans differ greatly in age, size, color, metabolism, and shape; yet, we easily recognize all of them as being humans. Likewise, a group of isolates of the same bacterial species will vary within an accepted range of traits. This page lists most of the better known genera of bacteria but these are a small fraction of the known genera. Most beginning bacteriology courses cover about half of these genera.

This page will give beginners an introductory idea of how bacteria are divided into genera. That will give an idea of the range of bacteria types found in nature and how they benefit the environment, keep humans healthy, and enable industrial and food processes.

This page is intended to provide one step in the quest of beginners who want to find some "safe" bacteria to work with. Next you should study lists of pathogenic species (a link here later). Sadly, you can never be totally sure a culture is safe because it could have become contaminated. Bakers yeast, buttermilk, and other fermented foods are usually free of pathogens, but you have to know your organisms to judge whether you have a safe culture. See our guide to isolating organisms from foods (to be written in 1997).

Bacteria often carry viruses and such bacteria may differ somewhat from those not infected. Other pages in this site will cover bacterial viruses in detail. Here are two examples. Diptheria bacteria cause the disease only if carrying a certain virus. Clostridium botulinum produces toxins in food only when carrying a certain virus. Bacterial viruses do not attack humans or other animals, but they can cause a harmless bacterium to become a lethal pathogen.

The word in Bold italics is the GENUS name:

Acetobacter species oxidize ethanol to acetic acid which clears calcium carbonate agar. They further oxidize acetic acid to CO2 and water. Aerobic. Motile (usually) rod.

Agrobacterium species are noted for producing galls or hairy roots on plants. Most species produce much slime on carbohydrate media. Motile rods. No spores. Gelatin slowly liquified. The type of gall or hairy root is determined by the plasmids carried by a strain. Plant pathogens and can destroy sensitive cultivars of grape and raspberry.

Arthobacter species produce branching filaments which, with age, break up to form small cells. In a colony, filaments are found at the edges and cells at center. This "life cycle" is the chief chacteristic of the genus. Aerobic. Common in soil. Some genera having similar life cycle are Brevibacterium. etc.

Azomonas species are able to fix nitrogen (nitrogen not required in medium). Large ovoid cells 2um or larger are single, in pairs, or in irregular clumps. Marked variation in cell shape. Motile. May produce slime. Catalase positive. Strict aerobe. Not proteolytic. Some strains produce soluble pigment which appears white under ultraviolet light.

Bacillus species are large (often 4 or 5 um long) rods able to produce endospores in air. Some species can grow anerobically by getting oxygen from nitrates (NO3-->NO2). A very large genus (numerous species) readily isolated from pasteurized soil. Bacillus anthracis produces anthrax disease in sheep and man. B. cereus produces soluble toxins which may cause food poisoning. B. thuringiensis, larvae, popilliae, and some others produce disease in certain insects. Indiana Biolab has a collection of about 40 strains noted for insect bio-control. We plan several pages on Bacillus.

Caulobacter has curved cells attached to substrate by a stalk and produces motile swarmers after cell division. Yellow to pink, strict aerobes able to grow on dilute peptone and yeast extract media. On rich media, other genera overgrow caulobacter during isolation. Occurs in water.

Clostridum is a large genus of anaerobes. Some clostridia can grow slightly near surface of culture but no Clostridium species can produce endospores in presence of oxygen. Large rods or clubs. Most have special nutritional needs and grow best on liver medium, but some grow better on boiled cornmeal media. Common in soil, stable manure, and other rich enviroments. Many species are pathogenic: Clostridium botulinum (lethal food toxin), C. tetani (lockjaw = tetanus), C. perfrigens (gas gangrene), and others.

Flexibacter species have very long flexible cells (0.5 by 5 to 100 um) without flagella, but exhibiting gliding motility. Strict aerobes. Cell masses are yellow to orange.

Gluconobacter species form acetic acid from glucose or ethanol. Acetic acid not oxidized to carbon dioxide. 5-ketogluconate also formed from glucose and may crystallize as the calcium salt when limestone is present. Strict aerobic rods, never fermentative. Other sugars and alcohols oxidized by some species.

Lactobacillus metabolism is fermentative even when growing on surface. Oxidize sugars to acids, traces of alcohol and carbon dioxide, but never digest proteins. Thus, they are ideal for producing fermented milk products. In cheese making, they oxidize sugars to acids and the low pH precipitates the casein without destroying its food value. Cells of most species are long slender rods, but some are spherical. Facultative anaerobes; may form chains. No catalase or hydrogen sulfide formed.

Leuconostoc species require rich media because they need many vitamins and amino acids. They require fermentable carbohydrate and produce lactic acid, ethanol, and carbon dioxide. Milk rarely acidified or curdled unless a fermentable sugar and yeast extract is added. Facultative anaerobes. Catalase negative. Produce slimes in sugar factories. Used commercially to produce dextrans. Non-motile spherical cells, usually in pairs or chains.

Micrococcus species are very common in soil and dust. Many of the colored colonies found on agar plates set out in any location are Micrococcus. The colonies are usually pale to yellow or orange, occasionally pink. Spherical cells, non-motile, no spores, aerobic, and catalase present. While the cells are very small, the non-pathogenic nature of these species and biochemical traits make this genus a favorite for beginning microbiology students.

Nocardia often form short filaments which breakup to form cells, much like Arthobacter. Some species are able to use phenol or petroleum and related compounds for carbon and energy sources. Aerobes. A few are pathogens.

Proteus species are common in decaying meat. Known for swarming motility but mechanism is not known. They produce putrefaction odors. Proteus vulgaris produces hydrogen sulfide on TSI slants and digests milk to clear liquid. Isolated from fecal material.

Pseudomonas use a very wide variety of organic compounds for carbon and energy. Large rods. Polar flagella. Strict aerobes. Many species produce greenish yellow to bluish soluble pigments in the medium, some are fluorescent. Common on plants. Some are plant pathogens and a few can grow in human wounds. Leftover milk in discarded cartons is often bluish or greenish due to members of this genus.

Rhodospirillum species have large spiral cells which have very rapid motility with a spinning movement. Will grow on yeast media in the dark microaerophilically with pale white color. In presence of light, all species grow beautiful maroon color deep in medium anaerobically via photosynthesis. Huge populations develop in algae covered sewage ponds because rhodobacteria have red bacteriochlorophylls and use wavelengths of light not used by the green chlorophylls of algae. Wind often parts the green scum of algae exposing large bright red areas of bare water colored by the dense growth of rhodobacteria. Readily isolated from pond muds. No spores.

Selenomonas species are strict anerobes usually found in cattle rumen. Kidney shaped cells bear flagella on concave side causing a flipping motility. You need special equipment to grow them in a carbon dioxide atmosphere. Exposure to air kills them.

Streptococcus species require many growth factors and have fermentative metabolism producing mainly lactic acid. Cells are round or ovoid in pairs or chains and most are non-motile. Facultative anaerobes. Some species are pathogenic, but others are important in food processing such as milk fermenations.

Streptomyces form filaments and might be mistaken for fungi but the cell wall does not contain cellulose which is present in the cell walls of fungi. Streptomyces form chains of aerial spores at the ends of filaments. Many species produce important antibiotics. Usually isolated from soil and decomposing leaves which often have the odor of Streptmyces cultures. Some form purplish pigments. Due to the filamentous nature, colonies may be very difficult to cut apart. Strict aerobes.

Thiobacillus species obtain energy by oxidizing sulfur (some also oxidize iron) producing sulfuric acid and very low pH (about 1). Some can use peptones and other organic materials. They require nitrogen salts and CO2. Most species can't use sugars and other organic compounds. The acid they produce corrodes mining equipment and destroys limestones in concrete. Much highway destruction is caused by thiobacilli. Thiobacilli are used to digest sulfide ores and release copper and other metals. Place some sulfur, soil and limestone in a container for a few weeks and watch the pH drop. Crystals of gypsum (CaSO4) will form. Bacteria produced many important ore deposits.

Salmonella, Escherichia, Proteus, Enterobacter, Alcaligenes, and other genera from the intestine are too similar to discuss here. Some experts would not separate them as different genera, but giving them genera names has been useful in medical work. Special stains and microscopic examination is not very useful for distinguishing them. They are usually identified by fermentation results on a multitude of rare sugars. Specific antisera may be needed to identify pathogenic subspecies. Bacteriophages are sometimes used to distinguish individual strains.

Luminescent bacteria. These have often been lumped together as Photobacterium, but they have too many differences to be placed in one genus. Some are pathogenic and similar to Vibrio. My favorite is Photobacterium phosphoreum because if it has plenty of oxygen and glycerol, it emits enough light to illuminate the faces of campers on dark nights. Some of these populate the light organs of deepsea fishes. Some other luminescent species are [to be completed].