Bacteria

A large group of unicellular and simple multicellular organisms, lacking chlorophyll, which multiply through cell fission.

Until recently, all bacteria were grouped into a single kingdom of prokaryotes, Monera, which included both eubacteria and archaebacteria. Eubacteria are distinguished by having very strong cell walls containing peptidoglycan. Archaebacteria lack peptidoglycan in their cell walls and their genes are more similar to those found in eukaryotes than are those of eubacteria. The differences are so great that most biologists now agree that archaebacteria and eubacteria should be assigned to separate kingdoms. In the new taxonomic scheme, eubacteria, including cyanobacteria, make up the Kingdom Monera, while archaebacteria, redesignated archaea, comprise the Kingdom Archaea (see life, classification).

Shapes of bacteria

The shape of bacterial cells is of fundamental importance in the classification and identification of bacteria. The majority of bacterial cells come in three basic shapes: round, rod shaped, or spiral. However, they display a remarkable variety of forms when viewed microscopically:

1. Round (spherical) bacteria are referred to as cocci (singular: coccus).
2. Elongated or rod-shaped cells are known as bacilli (singular: bacillus).
3. Ovoid cells are something in between cocci and bacilli. These are known as coccobacilli (singular coccobacillus).
4. Spiral shaped cells can be one of two types: either rigid called spirilla (singular spirillum) or flexible called spirochaetes (singular spirochaete). Spiral-shaped bacteria are distinguished by their length, the number and size of the spirals, and direction of the coil. Short segments or incomplete spirals are common, as the comma-shaped Vibrios. The spirochetes of syphilis are typical spiral bacteria. Diseases caused by spirochaetes include the following: syphilis, yaws, leptosporosis, and Lyme disease.
5. Square bacteria are flat and box-like, but can vary in their angular shape.
6. There are also fungal bacteria, known as actinomycetes, which grow like fine threads, called hyphae (singular hypha). A mass or group of these is known as 'mycelium'. One example is Actinomyces scabies, which resembles fungal mycelia. Specialized reproductive elements produce conidia (functioning similar to spores) that are eventually released into the air.