Coryneform bacteria are gram-positive, aerobic, nonmotile rods that exhibit irregular, club-shaped, or V-shaped arrangements. Their V-shape results from snapping division, where the inner cell wall layer forms the cross-wall, while the outer layer remains intact until it ruptures on one side, causing the daughter cells to bend away.
The primary genera are Corynebacterium and Arthrobacter. Corynebacterium includes diverse species, ranging from saprophytes to pathogens like Corynebacterium diphtheriae. In contrast, Arthrobacter, primarily found in soil, undergoes a developmental cycle shifting between rod and coccus forms. While some coryneform bacteria also develop coccoid cells, Corynebacterium often has a club-shaped structure, unlike the less distinct Arthrobacter.
Arthrobacter species are highly resilient to desiccation and starvation, despite lacking spores. They exhibit metabolic adaptability, breaking down compounds such as herbicides, caffeine, nicotine, and phenols, making them key players in soil ecosystems.
Propionic acid bacteria (Propionibacterium) were first identified in Swiss cheese, where their fermentation produces CO₂, forming the characteristic holes, and propionic acid, contributing to the cheese’s distinct flavor. These gram-positive anaerobes ferment lactic acid, carbohydrates, and polyhydroxy alcohols, yielding propionic acid, acetic acid, and CO₂.
In Swiss cheese production, Propionibacterium participates in secondary fermentation, utilizing lactate produced by Streptococcus and Lactobacillus during curd formation. As the curd drains, propionic acid bacteria multiply, generating CO₂, which accumulates at weak points, creating the cheese's holes.
Additionally, Propionigenium, an unrelated bacterium, produces propionate through succinate fermentation, highlighting a different but energetically significant metabolic process.
Mycobacteria are common soil bacteria, mostly harmless, but some species in the genus Mycobacterium are notable human pathogens, including Mycobacterium tuberculosis, which causes tuberculosis. These rod-shaped bacteria exhibit acid-fastness due to the presence of mycolic acids—complex lipids that give their cell wall a waxy, hydrophobic nature. Because of this, they do not stain well with Gram stain and require the Ziehl–Neelsen acid-fast stain, where they retain red dye while non-acid-fast bacteria stain blue.
Mycobacteria can appear pleomorphic, sometimes forming branching structures, but unlike actinomycetes, they do not develop true mycelia. They are classified into slow-growing species (e.g., M. tuberculosis, M. avium, M. bovis) and fast-growing species (e.g., M. smegmatis, M. chelonae). Slow growers like M. tuberculosis require extended incubation to form colonies, which are tight, wrinkled, and hydrophobic due to their high lipid content.
Nutritionally, mycobacteria grow aerobically in simple mineral salts media, using ammonium as a nitrogen source and glycerol or acetate as a carbon source. However, M. tuberculosis thrives better in the presence of lipids and fatty acids. Its virulence is associated with cord-like bacterial aggregates caused by a glycolipid called cord factor.
Some mycobacteria produce carotenoid pigments, aiding identification. They can be nonpigmented (M. tuberculosis, M. bovis), photochromogenic (producing pigment in light, e.g., M. parafortuitum), or scotochromogenic (producing pigment even in the dark, e.g., M. gordonae). Carotenoids likely protect these bacteria from oxidative damage caused by singlet oxygen.
Actinomycetes are filamentous, aerobic, gram-positive bacteria commonly found in soil. Many undergo a developmental cycle that includes forming desiccation-resistant spores. Their growth involves filament elongation and branching, creating a mycelium similar to fungi. When nutrients are scarce, aerial hyphae develop and differentiate into spores for survival and dispersal.
The genus Streptomyces, with over 500 species, is the most significant in this group. Streptomyces filaments, typically 0.5–1.0 μm in diameter, grow at their tips and frequently branch, forming a dense mycelium. As colonies mature, aerial filaments called sporophores emerge, producing spores known as conidia. Unlike Bacillus and Clostridium endospores, conidia form through cross-wall division in sporophores.
Species classification is based on variations in aerial filaments and spore structures. Conidia and sporophores are often pigmented, giving colonies distinctive colors. The mature colonies are compact, pigmented, and have a dusty appearance, making Streptomyces colonies easy to identify on agar plates.
Actinobacteria are a diverse group of Gram-positive bacteria, including coryneform, propionic acid bacteria, mycobacteria, and actinomycetes, and are distinguished by their high G+C DNA content.
The coryneform bacteria are aerobic, nonmotile rods with irregular, club-shaped, or V-shaped arrangements due to snapping division.
The main species include pathogenic Corynebacterium diphtheriae, and Arthrobacter — a genus of soil bacteria capable of switching between rod and coccus forms.
Propionic acid bacteria, such as Propionibacterium, are anaerobes that ferment lactic acid, producing carbon dioxide and propionic acid.
Mycobacteria are commonly found in soils; most are harmless, but some, like Mycobacterium tuberculosis, are significant human pathogens.
Mycobacterium tuberculosis is a slow-growing species, but the genus Mycobacterium also includes fast-growing species like Mycobacterium smegmatis.
Their unique features include mycolic acids in their cell walls and the ability to undergo branching or filamentous growth.
Actinomycetes are filamentous and aerobic bacteria, forming mycelia and desiccation-resistant spores.
A diverse genus, Streptomyces, is well-known for its antibiotic production.
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