Firmicutes: Characteristics, Morphology And Classification

Firmicutes is one of the phyla within which Bacteria are classified. This phylum comprises three classes (Bacilli, Clostridia and Erysipelotrichia), 26 families and 223 genera, thus constituting the main bacterial phylum.

The bacteria classified in this phylum have a common evolutionary history. All have a rigid cell wall , from which the name Firmicutes derives (in Latin firmus   means firm and complexion refers to the skin or cell wall). 

All phylum bacteria contain muramic acid in the cell wall and some contain teitoic acid. Most are gram-positive with the exception of the families Veillonellaceae and Syntrophomonadaceae, which are gram-negative.

Firmicutes bacteria are phenotypically diverse, being able to be spherical, straight, curved cells, helical bars or filaments, with or without flagella, with or without heat-resistant endospores.

They can be aerobic, facultative or strict anaerobic. Some are thermophilic and / or halophilic. Some of them are chemoorganotrophs and others are anoxic photoheterotrophs. Most grow at a neutral pH, but some are acidophilic or alkaline. They have a DNA content that is generally less than 50%.

Bacteria classified in the phylum Firmicutes form an important part of the human intestinal flora, together with those classified in the phylum Bacteriodetes. Recent studies have identified that the intestinal microbiota of obese human beings and animal models is associated with a greater abundance of Firmicutes, in relation to Bacteriodetes.

General characteristics


The phylogeny of the Firmicutes has been constantly revised and re-studied incorporating more and more data and new methods that allow proposing new evolutionary hypotheses.

Recent studies based on analysis of small subunits of nucleotide sequences of ribosomal RNA have generated a phylogenetic framework in which the Firmicutes phylum is made up of three classes (Basilli, Clostridia and Erysipelotrichia), 26 families and 223 genera.

In earlier classifications, this phylum included an additional class called Mollicutes, which differs from the rest of the Firmicutes bacteria in that it lacks rigid cell walls, lacks peptidoglycans and muramic acid, is flexible, and is highly pleomorphic cells.

This group has been removed from Firmicutes in more recent studies with alternative markers. As a result, the Mollicutes class was elevated to the Tenericute edge.

The family Erysipelotrichaceae, previously classified as Mollicutes, which forms a gram-positive cell wall, remains in Firmicutes as a new class called Erysipelotrichia, with a single order (Erysipelotrichales) that includes a single family (Erysipelotrichaceae). Recent analyzes corroborate the separation between the Basilli and Clostridia classes.


Class Clostridia

The Clostridia class is a phenotypically, physiologically and ecologically diverse polyphyletic clade. It contains both gram-positive and gram-negative bacteria and most genera are spore-forming, with exceptions such as Clostridium .

The class Clostridia is composed of the orders Clostridiales, Halanaerobiales, and Thermoanaerobacterales. Bacteria belonging to the order Clostridiales are strict anaerobes, those of the order Halanaerobiales are halophilic and those of the order Thermoanaerobacteriales are thermophilic.

Some species of Clostridia are of great importance to industry, as they are used to produce solvents as the final product of the fermentation process . 

Others produce toxins, such as Clostridium botulinum , also known as BoTox, to paralyze the muscles of the face to reduce skin wrinkles.

Erysipelotrichia class

This class is composed of bacteria with the shape of fine thin, straight or slightly curved rods, with a tendency to form long filaments. They are immobile, with a gram-positive cell wall, that do not produce endospores.

They are aerobic, facultative anaerobic. They are chemoorganotrophic. They have a fermentative respiratory metabolism. This group includes some pathogenic species for mammals and birds, among them Erysipelothrix rhusiopathiae and Bulleidia extructa .

Basilli class

Bacteria of the Basilli class generally form a gram-positive cell wall. They may or may not form endospores. Most are aerobic or microaerophilic, while only some are facultative anaerobes. The Bacilli Class is composed of the orders Bacillales and Lactobacillales.

Most of the genera of the order Bacillales form endospores, although there are exceptions. In addition, they have gram-positive staining of the cell wall in young cells. Most of the genera have menaquinone 7 (MK-7).

Long chain menaquinones, from MK-6 to MK-11, have been reported in the thermoactinomycetaceae family. Menaquinones are also known as vitamin K 2

The number of side chains is indicated in the particular name of each menaquinone (eg MK-6 has six molecular units attached to the nucleus of the molecule formed by a quinone ring).

The genus bacillales include highly pathogenic bacteria such as Bacillus anthracis , which causes anthrax. Another important species is Bacillus subtilis , one of the organisms used as a model in research to understand topics ranging from cell differentiation to iron storage and DNA replication.

Bacteria of the order Lactobacillales can be rod or coccal shaped, have gram-positive cell wall staining, do not form endospores, and are facultative anaerobes and are mostly catalase negative.

This order includes beneficial bacteria such as Lactobacillus , important components of the gastrointestinal flora in humans and other animals, and used artisanal and industrially for the production of yogurt and other fermented foods.

Gastrointestinal microbiota

The gastrointestinal microbiota plays an important role in maintaining human health by providing energy, nutrients, and immune protection. In healthy adults, more than 1250 species of bacteria have been identified, mainly belonging to the phyla Bacteroidetes, Firmicutes and Actinobacteria.

In general terms, the Firmicutes / Bacteroidetes relationship is considered of great importance in the composition of the human intestinal microbiota. In obese people a high proportion of Firmicutes has been identified, while in those with low weight an inverse relationship has been reported.

The composition of this microbiota evolves throughout life, from early childhood to old age. Thus, the Firmicutes / Bacteroidetes ratio increases from birth to adulthood and is further altered with advanced age.

Diet has also been identified to have a significant effect on the gastrointestinal microbiota. The Firmicutes / Bacteroidetes ratio is higher in people with diets based on animal protein and saturated fat than in those with diets high in vegetables and fiber.


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