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Bacterial Cell Size

• Spherical bacteria average 0.5-2.0 µm in diameter.
• Rod-shaped or filamentous bacteria are 1-10 µm long and 0.25-1.0 µm in diameter.
• E. coli is a bacillus, approximately 1.1 to 1.5 µm wide and 2.0 to 6.0 µm long.
• Spirochaetes can reach 500 µm in length.
• Oscillatoria, a cyanobacterium, is about 7 µm in diameter.
• Epulosiscium fishelsoni is visible to the naked eye, measuring 600 µm long by 80 µm in diameter.
• Mycoplasmas are a group of bacteria with individuals measuring around 0.25 µm; formerly known as pleuropneumonia-like organisms (PPLO).
• Mycoplasma gallicepticum is considered one of the world's smallest bacteria, sized approximately 200 to 300 nm.
• Thiomargarita namibiensis, a gram-negative Proteobacterium, is the world's largest bacteria, found off the coast of Namibia.
  • It typically measures 0.1-0.3 mm (100-300 µm) across, but can reach up to 0.75 mm (750 μm).
• Eukaryotic cells (plant and animal) average 10 to 50 µm in diameter.

Bacterial Cell Shape

• Basic bacterial shapes are coccus (spherical), bacillus (rod-shaped), as well as spiral (twisted).
• Pleomorphic bacteria can take on several shapes.

Spherical (Coccus)

• Cocci are round cells, sometimes flattened when adjacent to each other.
• Cocci can occur singly (micro or monococci), in pairs (diplococcic), in groups of four (tetracocci), in chains (streptococci), or in irregular bunches (Staphylococci).

Rod-Shaped or Cylindrical (Bacillus)

• Bacilli usually occur singly but may be found in pairs (diplobacilli) or chains (streptobacilli).

Spiral or Spirilla

• Spirilla are curved bacteria that range from gently curved to corkscrew-like.
• Spirilla are rigid and motile, while spirochetes are long, slender, and flexible.

Arrangement of Cocci

• Cocci bacteria can exist singly, in pairs as diplococci, in groups of four as tetrads, in chains as streptococci, in clusters as staphylococci or in cubes of eight cells known as sarcinae.
• Cocci can be oval, elongated, or flattened, remaining attached after cell division, useful for identification purposes.

Diplococci

• Arranged in pairs: Streptococcus pneumoniae, Moraxella catarrhalis, Neisseria gonorrhoeae.

Streptococci

• Arranged in chains as the cells divide in one plane; Streptococcus pyogenes, Streptococcus agalactiae.

Tetrads

• Arranged in packets of four cells as cells divide in two planes; Aerococcus, Pediococcus and Tetragenococcus.

Sarcinae

• Arranged in a cuboidal manner made up of regular cell divisions in three planes; Sarcina ventriculi, Sarcina ureae.

Staphylococci

• Arranged in grape-like clusters formed by irregular cell divisions in three planes; Staphylococcus aureus.

Arrangement of Bacilli

• Rod-shaped bacteria are called 'bacillus' (plural: bacilli).

Diplobacilli

• Appear in pairs after division
  • Bacillus cereus

Streptobacilli

• Arranged in chains as the cells divide in one plane; Streptobacillus moniliformis.

Coccobacilli

• The bacilli bend at division points, creating a palisade arrangement resembling a picket fence; Corynebacterium diphtheriae.

Arrangement of Spiral Bacteria

• Spirilla are curved bacteria ranging from gently curved to corkscrew-like.
• Many spirilla are rigid and can move, whereas spirochetes are long, slender, and flexible.

Vibrio

• Comma-shaped bacteria with less than one complete turn or twist; Vibrio cholerae.

Spirilla

• Rigid spiral structure, resembling spirochetes, lacking outer sheath and endoflagella, but have typical bacterial flagella; Campylobacter jejuni, Helicobacter pylori, Spirillum winogradskyi.

Spirochetes

• Helical and flexible bodies moving with axial filaments beneath a flexible external sheath, lacking typical bacterial flagella; Leptospira species (Leptospira interrogans), Treponema pallidum, Borrelia recurrentis.

Other Shapes and Arrangements of Bacteria

Filamentous Bacteria

• Very long, thin, filament-shaped bacteria, some forming branching filaments known as 'mycelium'; Candidatus Savagella.

Star-Shaped Bacteria

• Stella.

Rectangular Bacteria

• Haloarcula spp (H. vallismortis, H. marismortui).

Pleomorphic Bacteria

• Lack a characteristic shape and can change shape in pure cultures; Mycoplasma pneumoniae, M. genitalium.

Beneficial and Deleterious Effects of Microorganisms

Useful Effects of Microorganisms

• Bacteria are used in the production of curd, cheese, vinegar, lactic acid, alcohol and even in the ripening of tea leaves.
• Fungi are useful: yeast for bakery and alcohol, antibiotics, some mushrooms for vitamins and organic acids.
• Algae are useful: Chlorella produce protein and vitamin, red algae (agar) / brown algae (Iodine and potassium).
• Protozoa are useful: help in the final degradation of waste and sewage and some are symbiotic in nature.
• Viruses are useful: scientific research such as biotechnology, genetic engineering and gene therapy.

Harmful effects of microorganisms:

• Bacteria: Cause diseases such as typhoid, diarrhea, and cholera.
• Fungi: Cause plant and animal diseases such as rust diseases in plants, fruit rot in apple, red rot in sugar cane, and ring worm disease.
• Algae: Algal bloom causes poisonous effects and death of aquatic organisms.
• Protozoa: Cause Amoebic dysentery, pyorrhoea and sleeping sickness
• Virus: Cause small fox, common cold, influenza, herpes, hepatitis, polio and rabies

Applications in Microbiology

• Biotechnology applies biology to solve practical problems and produce useful products economically.

Food Production

• Yeast used in bread making, production of beer from fermenting cereal grains / other fermented drinks, fermentation of milk to produce yogurt, cheese, and butter milk, use probiotics- bacteria addition to fermented milk reported to protect against intestinal infections / bowel cancer.

Bioremediation

• Use of microorganisms for degradation of environmental pollutants / cleanup of polluted environment.
• Degradation of dangerous chemicals such as PCBs, DDT and trichloroethylene.
• Bacteria usage for oil spills and radioactive waste treatment.

Commercially valuable products derived from bacteria

• Produced faster and cheaper rate than synthesized in factories.
• Cellulose used in sterol, Hydroxylbutyric acid used in disposable diapers and plastics
• Ethanol added to gasoline.

Genetic Engineering

• Introduction of genes from one organism into an unrelated organism to develop new properties such as the production of interferon, insulin, human growth hormone, blood clotting factors / enzymes dissolving blood clots.
• Microorganisms are modified to produce vaccines against rabies, gonorrhea, herpes, leprosy, malaria, and hepatitis.
• Bacteria are used to genetically engineer plants resistant to insect attacks / viral diseases and produce large amounts of β-carotene.
• Bacteria are used to transfer antibody eliciting genes into bananas for diarrheal disease resistance.
• Viruses deliver genes into humans to correct conditions like cystic fibrosis, heart disease, and cancer.

Genomics

• The study of bacterial DNA provides information for an organism's characteristics.