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NOTE: Unless otherwise stated, the Gram Stain was used to visualize the microbes in the micrographs...

The 3 major shapes of bacteria.

Rod or bacilli

Vibrio or comma-shaped

Cocci or completely round


Bacteria are usually one of  three shapes: round (cocci), rod (bacilli), or wavy (spirochetes/spirillae/vibrio). Genera that are examples of round or spherical bacteria are Staphylococcus  and Streptococcus . Escherichia coli and Bacillus are rod-shaped bacteria. Treponema are spirochetes and Spirillum are wavy shaped-bacteria.

All images are at 1000 X magnification.

Klebsiella bacteria on a positive capsule stain

Some bacteria, such as the Klebsiella pneumoniae bacteria shown in these 1000 X images, produce capsules. Capsules are made of sticky substances such as polysaccharides that are secreted by the bacteria and adhere to the cell. Capsules inhibit phagocytosis in the infected host and thus enhance survival of the bacteria. Anthony's method, a negative staining technique, was used to make the capsules visible in these images. Utilizing crystal violet and copper sulfate, Anthony's method results in a dark purple background and purple rod-shaped Klebsiella bacteria surrounded by white (refractile) capsules (CV-Capsule Staining used)....

Campylobacter sp. are slender Gram-negative vibroid cells, possessing one or more helical turns. Campylobacter fetus are shown here at 1000 X magnification, stained with the Gram stain. They appear as tiny red comma-shaped organisms. Sometimes two Campylobacter appear in a "V" pattern resembling seagull wings. Pathogenic species in the genus Campylobacter include C. fetus, which causes spontaneous abortion in domestic animals, and C. jejuni, a cause of foodborne intestinal disease. It is not possible to distinguish different genera of Campylobacter from one another by a Gram stain. Biochemical tests must be run for complete identification. 
Campylobacter bacteria exhibiting a spiral shape and 2 spirals = "gull wing" shape
Metachromatic granules are stored phosphate regions that occur in some species of bacteria. When stained with methylene blue, like the 1000 X images shown here, metachromatic granules take up the dye more intensely than the rest of the rod-shaped bacterium. Thus a row of irregularly shaped circles (metachromatic granules) appear close together because they are contained within one bacterium. The cell wall (outer border) of the bacterium is not readily seen. 

The diagnostic significance of metachromatic granules arises from the fact that the causative agent of diptheria, Corynebacterium diptheriae , characteristically produces metachromatic granules. Nonpathogens that resemble Corynebacterium diptheriae in appearance are termed "diptheroids". The arrangement of the bacteria provide a clue to distinguish the pathogen Corynebacterium diptheriae from the nonpathogenic diptheroids. C. diptheriae bacteria form 90 degree angles to one another, forming "X's", "Y's", or "Chinese letters". In contrast, diptheroids are often in parallel arrangements called "palisades". Although not every bacteria in the field of view will be in one of these arrangements, many can be seen. 

This is a photo of Corynebacterium diptheriae, the pathogen that causes diptheria and produces "Chinese letter" arrangements.


Corynebacterium xerosis,  a nonpathogenic diptheroid with metachromatic granules that occurs in palisades. This image contains a magnified view of bacteria in a palisade, with several of the bacteria outlined for clarity.
Clostridium tetani  is a pathogenic bacterium that causes tetanus or "lockjaw". This slide shows Clostridium tetani bacteria that are producing spores at 1000 X magnification. Stained with methylene blue, the vegetative rods (bacilli) are dark blue in color and the spores, which do not take up the dye as readily, are light blue. Clostridium tetani is known for the "tennis racket" shape it has due to its terminal spores.
Escherichia coli are very small Gram-negative rods, as seen here at 1000 X magnification. Escherichia coli is often called a "cocobacillus" because its rod is almost "round."  E. coli is a normal bacterial inhabitant of the large intestine of vertebrates, including humans. It is beneficial to humans because it produces vitamins we need and aids in digestion of food. However, some strains of E. coli are pathogenic. For example, E.coli O157:H7 has been implicated in foodborne disease, causing bloody diarrhea. 



Neisseria gonorrhoeae is the pathogenic bacterium that causes gonorrhea. This slide shows Neisseria gonorrhoeae colonies growing on blood agar. N. gonorrhoeae are identified as "Gram-negative intracellular diplococci". The next slide shows uninfected white blood cells stained with the Gram stain. 
N. gonorrhoeae present in a urethral or vaginal discharge stained with the Gram stain at 1000X magnification. The bacteria have been phagocytized by white blood cells (neutrophils). The nuclei of the white blood cells are the large red circles, while the bacteria are the small red circles that tend to be in pairs (diplococci) and Gram-negative.

 Micrococcus sp. also present as diplococci.  These bacteria are GRAM positive however and may also show in tetrads.  NOTE that different species of Micrococcus present with pigmented colonies on agar.  M. roseus is red-orange while M. luteus is yellow. 

Some other Genera produce pigmented rose colonies as well. Serratia marcescens, a GRAM-negative rod has a temperature dependant pigment that presents best at 25 degrees C; it shows a white colony when grown at 37 C.

Both Micrococcus sp. and Sarcina sp.are Gram-positive cocci that more than one plane. Sarcina remain attached in packets of eight cells, forming a cube. However, often a two-dimensional image such as those shown here, reveals only the four cells (tetrad) in the plane nearest our vision. Another four bacteria are stacked behind the four we can see. 
Bacillus sp. & Clostridium sp produce ENDOSPORES:
These images show bacteria that are producing endospores. They are stained with the Schaeffer-Fulton stain, which uses malachite green as the primary dye and safranin as the counter stain. The endospores are a light green color. The vegetative rods absorb the red safranin dye.  Two medically important genera of bacteria of bacteria that produce spores are Clostridium sp. (anaerobic) and Bacillus sp. (aerobic).  This is a photo of Clostridium tetani.
Green endospores... Because the endospores in these images occur at the end of the rod, they are designated as "terminal" spores. Endospores that occur in the middle of a rod are termed "central", while endospores located between the middle and end of a rod are termed "subterminal".  Endospores occur by themselves after the death of the vegetative rod to which they were attached.

Spirillum are wavy shaped-bacteria.

Spiral bacteria have one or more twists in them.  They are usually one of three shapes:  commas (vibrio/vibrios), rigid wavy shapes (spirillum/spirallae) or flexible corkscrews (spirochete/spirochetes).  Spirillum volutans and Rhodospirillum rubrum are nonpathogenic spirillae. 

Treponema pallidum is the pathogenic spirochete that causes syphilis.

Bacteria in the genus Staphylococcus are Gram-positive cocci that arrange themselves in grape-like clusters.  A well-known pathogenic Staphylococcus is Staphylococcus aureus, which causes infections such as meningitis, pneumonia, toxic shock syndrome, food poisoning, impetigo, osteomyelitis, and carbuncles. Staphylococcus epidermidis is part of the normal flora of the human skin but can be an opportunistic pathogen in a compromised host. It is not possible to distinguish Staphylococcus aureus from Staphylococcus epidermidis in a Gram stain. Both organisms look like the images shown below.

Bacteria in the genus Streptococcus sp. are Gram-positive cocci that arrange themselves in chains. Pathogenic Streptococcus include Streptococcus pyogenes, the cause of "Strep throat", and Streptococcus pneumoniae, which, as its name suggests, may cause pneumonia. Streptococcus lactis  is a nonpathogen that is used in food production. Species of Streptococcus are differentiated by biochemical tests, not by their appearance on a Gram stain.
Streptococcus sp.
Slender red rods of Mycobacterium tuberculosis are  pathogenic bacteria are present in a sputum specimen from a patient with tuberculosis. Other non-acid fast bacteria are also visible. Mycobacterium are said to be "acid-fast" because they retain the red primary dye (carbolfuchsin) even when exposed to acid-alcohol. Most other bacteria lose the carbolfuchsin dye and then absorb the methylene blue counterstain, resulting in blue non-acid-fast organisms and red acid-fast stain organisms.