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Actinobacteria are Gram-positive bacteria distinguished by their high G+C DNA content. This diverse phylum includes coryneform bacteria, propionic acid bacteria, mycobacteria, and actinomycetes. Members exhibit varied morphologies and metabolic capabilities, ranging from aerobic filamentous forms to anaerobic fermenters, making them ecologically significant across soil and clinical environments.

Coryneform bacteria divide through snapping division, where the inner cell wall layer forms the cross-wall while the outer layer remains intact until rupturing on one side. This mechanism causes daughter cells to bend away from each other, creating the characteristic irregular, club-shaped, or V-shaped arrangements visible in microscopy.

Propionic acid bacteria participate in secondary fermentation of Swiss cheese, utilizing lactate produced by Streptococcus and Lactobacillus. As the curd drains, these anaerobes multiply and ferment lactic acid, generating carbon dioxide that accumulates at weak points, creating the cheese's characteristic holes, while also producing propionic acid that contributes to its distinctive flavor.

Mycobacteria contain mycolic acids—complex lipids that create a waxy, hydrophobic cell wall. This composition prevents standard Gram staining, requiring the Ziehl-Neelsen acid-fast stain instead. Acid-fast bacteria retain red dye while non-acid-fast bacteria stain blue, enabling reliable differentiation and identification of mycobacterial species.

Streptomyces is a diverse genus containing over 500 species and is the most significant actinomycete group for antibiotic production. These filamentous, aerobic bacteria form dense mycelia with aerial hyphae that differentiate into pigmented spores called conidia. Their prolific secondary metabolite production makes them invaluable for pharmaceutical applications and soil ecology.

Arthrobacter species are highly resilient to desiccation and starvation despite lacking spores. They exhibit metabolic adaptability, breaking down diverse compounds including herbicides, caffeine, nicotine, and phenols. This metabolic flexibility and stress tolerance make them key players in soil ecosystems, enabling survival through nutrient-poor periods.

Mycobacteria are classified into slow-growing species like Mycobacterium tuberculosis and fast-growing species like Mycobacterium smegmatis. They are further categorized by pigmentation: nonpigmented, photochromogenic (producing pigment in light), or scotochromogenic (producing pigment in darkness). Carotenoid pigments likely protect these bacteria from oxidative damage caused by singlet oxygen.