BACILLUS: CHARACTERISTICS, TYPES, CLASSIFICATION, EXAMPLES - BIOLOGY - 2025

The rods are rod - shaped bacteria. The other forms of bacteria are cocci, which are spherical in shape (coinciding with their name), and curved rods. All bacteria are prokaryotic organisms, lacking nuclei, and organelles such as mitochondria and chloroplasts.

The environments that bacilli inhabit are diverse. They include soil, water, organic matter, animals (skin, mouth, vaginal and intestinal tract), and many more places. However, of all known bacterial species, only a small percentage (about 1%) are causative agents of disease in humans.

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characteristics

In a similar way to the rest of the bacteria, the bacilli are unicellular organisms that lack a nuclear membrane.

A typical bacillus is 3 µm long and 1 µm wide, although they can be shorter or much longer. They can be thin or thick, with sharp ends, or curved and blunt.

Bacilli can appear singly, in pairs (diplobacilli), in chains (streptobacilli), in long filaments, or branched.

The genome consists of a circular chromosome that is a DNA molecule that condenses to give rise to a visible mass, called a nucleoid. They have one copy of each gene, thus being haploid. In addition, they have small amounts of extra-chromosomal DNA, circular in shape, called plasmids.

The bacilli can be Gram positive or Gram negative. This means that when the Gram stain is done, Gram positive bacteria turn purple and Gram negative bacteria turn red. This differential staining is due to differences in the structure of the cell wall of Gram positive and Gram negative bacteria.

Bacilli are not a monophyletic group, having a close common ancestor, but rather different groups belong within the domain of Eubacteria.

Types of bacilli according to Gram stain

Gram positive bacteria have a thick cell wall, 250 Å, surrounding the plasma membrane. Gram negative bacteria have a thin, 30 Å cell wall surrounded by an outer membrane.

The cell wall is a rigid layer that allows bacteria to live in an environment whose salt concentration is lower than that of its intracellular fluid (hypotonic environment).

Peptidoglycan is the component of the cell wall. It is organized in very thin sheets made up of derivatives of sugars: N-acetyl-glucosamine and N-acetyl-muramic. In the sheet, the chains of sugar derivatives are connected to each other by peptide bridges. These bridges are what give the cell wall its characteristic rigidity.

In Gram positive bacteria, an intermediate peptide bridge is formed that connects tetra-peptides, which in turn are covalently linked to chains of sugar derivatives. In Gram negative bacteria, tetrapeptides are linked directly to the chains of sugar derivatives by covalent bonds.

In Gram positive bacteria, peptidoglycan represents 90% of the cell wall. In Gram-negative bacteria, peptidoglycan accounts for 10% of the wall. The rest is outer membrane.

Classification of bacilli

The science that is responsible for identifying and classifying organisms is taxonomy. Bacteria, which include bacilli, cocci, and curved rods, are classified according to their metabolism, their enzymes, etc.

Classical taxonomy takes into account morphology (colony size and shape, Gram stain), mobility (by flagella; sliding; nonmotile), nutrition, and physiology (phototroph; chemoorganotroph; chemolithotroph; relationship with the oxygen and temperature), and other factors such as cellular inclusions and pathogenicity.

Molecular taxonomy consists of the analysis of the molecules that make up the cell. The main methods used are DNA: DNA hybridization, ribotyping and lipid analysis. The bacilli do not form a taxonomic group, but belong to various phyla, orders, classes and genera of bacteria.

Bacteria can be classified by phylogenetic analysis, which determines the evolutionary relationships between organisms. Currently, in a routine way, ribosomal RNA sequences are obtained, which are then analyzed by different methods, generating phylogenetic trees.

In the field of microbial taxonomy, the Bergey manual of systematic bacteriology and the treatise The prokaryotes are the most important references.

Examples of bacilli

Phylum Enteric Proteobacteria

Most are mobile, by flagella, on the surface. They are facultative aerobic, and can ferment glucose and other sugars. The best known member of this group is Escherichia coli, but there are also other genera that are well known because they are pathogenic to humans, such as Salmonella, Shigella, and Yersinia.

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They have a rare cell wall structure, which has lipids called mycolic acids. This makes the acid-fast test positive. They can form filaments. Fragmentation is the way to spread. Leprosy and tuberculosis in humans are caused by M. leprae and M. tuberculosis, respectively.

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They are obligate anaerobic. They form endospores resistant to heat and chemical agents. Some examples are C. tetani, which is the causative agent of tetanus, C. botulinum, which is the causative agent of botulism, and C. perfringens, which is a causative agent of diarrhea from ingestion of food.

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They are facultative anaerobic. They form endospores. They are Gram positive and Gram negative. They are generally mobile by flagella on the surface. Some examples are B. anthracis, which is the causative agent of anthrax, and B. subtilis, which is used by the pharmaceutical industry for the biosynthesis of bacitracin.

Bacilli and the microbiome

The term microbiome was first used by Nobel laureate Joshua Lederberg. The microbiome refers to the microbial diversity (pathogens, commensals, symbiotics, among others) that occupies a certain habitat or ecosystem. The composition and abundance of the microbiome differ between habitats in the global ecosystem.

Bacilli are part of the abundance of microbial cells present in various habitats. For example, the soil has 10,000 microorganisms in a 1cm ³, while glacial ice has 10,000 microorganisms in the same volume. Another example is the human mouth, which has 570 bacilli per mL of saliva.

References

1. Bagdi, ML 2009. Microbiology and biochemistry. Maglan, Delhi.
2. Barton, LL 2005. Structural and functional relationships in prokaryotes. Springer, New York.
3. Bauman, BW 2012. Microbiology with diseases by body system. Pearson, Boston.
4. Black, JG 2008. Microbiology: principles and exploration. Wiley, New York.
5. Burton, GRW, Engelkirk, PG 1998. Microbiology for the health Sciences. Lippincott, Philadelphia.
6. Desalle, R., Perkins, S. 2015. Welcome to the microbiome. Yale University Press, New Haven.
7. Madigan, MT, Martinko, JM, Parker, J. 2004. Brock: biology of microorganisms. Pearson, Madrid.
8. Saleem, M. 2015. Microbiome community ecology: fundamentals and applications. Springer, New York.
9. Talaro, KP, Talaro, A. 2002. Foundations in microbiology. McGraw-Hill, New York.
10. Tortora, GJ, Funke, BR, Case, CL 2010. Microbiology: an introduction. Benjamin Cummings, San Francisco.