FIRMICUTAE

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PHYLUM APHRAGMABACTERIA

INTRODUCTION TO THE APHRAGMOBACTERIA

Mycoplasmas are tiny ameba-like cells that live as parasites of many different cells (Figure A&B). As such, they are the smallest cellular organisms on earth with a range in size of 0.3-0.5 µm and a genome of 580-2200 kb. Likely, the parasitic existence allowed them to lose the typical bacterial wall and become among the most reduced of the bacteria. Several mycoplasma diseases afflict humankind. One is Mycoplasma pneumoniae, the causative agent of a disease of the upper and lower respiratory tract. Figure A shows the mycoplasmas attached to the cilia of swine trachea. In the lungs, they can cause an acute type of pneumonia against which antibiotics are only marginally effective. Spiroplasma (Figure B) is an intracellular parasite of arthropods and plants.

A. Mycoplasma attached to cilia in swine respiratory tract (TEM micrograph).

B. Spiroplasma, a plant pathogen (SEM micrograph).

Images taken from:
A: http://biology.kenyon.edu/Microbial_Biorealm/bacteria/gram-positive/mycoplasma/Mhyo_cilia.gif
B: http://www.apsnet.org/education/IllustratedGlossary/PhotosS-V/spiroplasma.jpg

SYNOPTIC DESCRIPTION OF THE APHRAGMOBACTERIA

The following description comes mainly from Margulis and Schwartz (1998), Barnes (1984), Brock et al. (1994), and Tudge (2000).

I. SYNONYMS: Mycoplasmas

II. PHYLUM CHARACTERISTICS:

A. Structure

Cell Form: Irregularly lobed.

Cell Wall: Absent or not rigid; gram-.

Motility: Non-motile.

B. Physiology

O₂ TOLERANCE: Anaerobes or facultative aerobes.

Substrates: Chemoheterotrophs or fermentive heterotrophs. Require sterols which they incorporate into their cell membranes (only Acholeplasma does not).

Products: Variable, among the products are small organic acids (acetic, lactic and formic acids), ethanol, and CO₂.

C. Other: Cells have very small amounts of genetic material; they multiply by binary fission; colonies on solid media have a "fried egg appearance."

D. Ecology: Grow as parasites of plants and animals.

SYSTEMATICS OF THE APHRAGMABACTERIA

This is a modification of Margulis and Schwartz (1998) in which the phylum includes all bacteria that lack walls; however, with that as the sole criterion, they include many taxa (e.g. Ehrlichia) that are rickettsias and belong in the Proteobacteriae. Brock and Madigan (1988) suggest that the mycoplasms are divided into 2 major groups according to their sterol requirement. Bergey's Manual of Systematic Bacteriology, volume 1, section 10 (Holt 1984) treats the mycoplasmas as very different from the other bacteria. Holt (1984) places them into their own division (phylum) called Tenericutes. Unfortunately, he lumps all bacteria without cell walls into the same taxon; therefore, Thermoplasma is treated as a mycoplasma.

Bergey's Manual of Systematic Bacteriology, 2nd edition (Garrity et al. 2001 and 2003) treats the taxa that I include in the Aphragmabacteria as a class of phylum Firmicutes (BXIII). Indeed, they seem to be clostridia without walls.

HIERARCHICAL CLASSIFICATION OF THE APHRAGMABACTERIA

The elevation of this group to phylum level status follows the system of Margulis and Schwartz (1998); however, they place many organisms now recognized as rickettsias in this group. Thus, the organization of the Aphragmabacteria as given here generally follows the system of Garrity et al. (2001 and 2003).

CLASS MOLLICUTI

ORDER MYCOPLASMALES

Mycoplasma, Eperythrozoon, Haemobartonella, Ureaplasma.

ORDER ENTOMOPLASMALES

Entomoplasma, Mesoplasma, Spiroplasma.

ORDER ACHOLEPLASMALES

Acholeplasma.

ORDER ANAEROPLASMALES

Anaeroplasma, Asterolplasma.

ORDER INCERTAE SEDIS (PLACEMENT OF THESE TAXA IS STILL PROBLEMATIC)

Erysilelothrix, Bulleidia, Holdemania, Solobacterium.

This page is maintained by Jack R. Holt. Last revised: 03/17/2008.