Chapter 23: Gram-Positive Bacteria – Traits, Types & Importance

Actinobacteria, many of which are filamentous and form hyphae and mycelia, are indispensable ecologically for mineralization in soil and medically as prolific producers of secondary metabolites. Approximately two-thirds of all prescribed antibiotics, including streptomycin and tetracycline, originate from the genus Streptomyces, which uses these small molecules, potentially as communication signals, to regulate a complex life cycle involving non-heat-resistant exospores. Other key Actinobacteria include Mycobacterium, known for its unusually slow growth and high-lipid, acid-fast cell wall constructed of mycolic acids, which is highly impenetrable to antimicrobial agents. Additionally, Corynebacterium and Arthrobacter utilize snapping division, leading to characteristic palisade cell arrangements, while Frankia species form symbiotic root nodules (actinorhizae) to perform efficient nitrogen fixation in nonleguminous plants. The Class Bacilli within the Firmicutes includes aerobic endospore-formers such as Bacillus, an important model organism known for forming resilient endospores, biofilms, and fruiting bodies, alongside major pathogens like B. anthracis. This class also contains facultatively anaerobic cocci like Staphylococcus (S. aureus), characterized by irregular clusters and a significant collection of virulence factors, and the non-sporing, fermentative lactic acid bacteria (Lactobacillus, Streptococcus, Enterococcus) crucial for producing fermented foods and dairy products like yogurt and cheese. The Class Clostridia consists of strictly anaerobic, endospore-forming rods, which include toxin-producing pathogens (C. botulinum, C. tetani) that often ferment amino acids via the Stickland reaction and utilize a sodium motive force for survival. Finally, the relatively new Class Negativicutes highlights ongoing taxonomic revisions, encompassing genetically Firmicutes members that possess inner and outer membranes (diderm) and stain Gram-negative, a structural feature hypothesized to be the ancestral state of the phylum.