Gram Positive Bacteria Notes

Phylum Actinobacteriota

• Chemoorganoheterotrophs, some use atmospheric H2 as an electron donor.

• Produce antibiotics, anticancer, antihelminthic, and immunosuppressive drugs.

• Complex life cycle, most are non-motile.

• Form filaments called hyphae, divided by septae into long cells.

Life Cycle of Actinobacteria

• Hyphae form a branching network, growing into the substrate to form a substrate mycelium.

• Aerial mycelium forms, giving a fuzzy appearance.

• Secondary metabolites form, some medically useful.

• Aerial hyphae septate to form chains of exospores.

• Spores are produced in response to nutrient deprivation and chemical signaling; spores withstand desiccation but not heat.

Order Actinomycetales

• Includes genera: Actinomyces, Bifidobacteria, Arthrobacter, and Micrococcus.

Genus Actinomyces

• Straight or slightly curved rods and slender filaments with branching.

• Facultative or strict anaerobes, normal inhabitants of oral mucosa.

• $A. bovis$ causes cattle lumpy jaw; $A. israelii$ causes human lumpy jaw.

Genus Bifidobacterium

• Important in human gut microflora, sold as probiotic agents.

• Nonmotile, nonsporing, anaerobic Gram-positive rods of varied shapes.

• $B. bifidum$ are GI tract inhabitants.

Genus Micrococcus

• Aerobic, catalase-positive cocci in pairs, tetrads, or irregular clusters.

• Nonmotile, form yellow, orange, or red colonies.

• Common in soil, water, and on mammalian skin.

Genus Arthrobacter

• Aerobic, catalase-positive rods with respiratory metabolism, found in soil.

• Resistant to desiccation and nutrient deprivation, can degrade some herbicides and pesticides.

Order Mycobacteriales

• Includes human pathogens with genera: Mycobacterium, Corynebacterium, Nocardia, and Rhodococcus.

• Differ by diderm cell envelope (mycomembrane external to peptidoglycan).

Family Mycobacteriaceae

• Straight or slightly curved rods that branch to form filaments, fragmenting into rods and coccoid bodies.

• Aerobic and catalase positive, grow slowly.

Mycobacterial Cell Walls

• Mycomembrane constructed of mycolic acids, which are complex fatty acids that are hydrophobic and impenetrable to antibiotics.

• Require porins for entry into the cell wall, also possesses a type VII secretion system.

• Lipid content makes cell wall acid-fast.

Important Species of Mycobacterium

• $M. bovis$ (reclassified to $M. tuberculosis$): tuberculosis in cattle and humans. $M. leprae$: leprosy. $M. avium$ complex (MAC): various diseases in compromised individuals.

Genus Corynebacterium

• Aerobic and facultative, catalase positive, straight to slightly curved rods.

• Two-layered cell walls result in snapping division, leading to palisade arrangement of cells.

• $C. diphtheriae$ causes diphtheria in humans.

Nocardia

• Develop a substrate mycelium that readily breaks into rods and coccoid fragments, strict aerobes found worldwide.

• Degrade many molecules; some are opportunistic pathogens.

Genus Rhodococcus

• Widely distributed that degrade petroleum hydrocarbons, detergents, PCBs, and pesticides.

Order of Micromonosporales, Family Micromonosporaceae

• Often called Actinoplanetes, spore-forming soil organisms that form a true mycelium.

• Extensive substrate mycelium, hyphae are highly colored.

• Spores formed within sporangium, with flagellated mobility.

• Micromonospora spp. degrade chitin and cellulose and produce antibiotics like gentamicin. Actinoplanetes grow in soil habitats, decompose plant and animal material, source of acarbose.

Genus Frankia

• Forms multilocular sporangia with clusters of non-motile spores.

• Symbiotic association within the roots of non-leguminous plants, microaerophilic and fix atmospheric nitrogen.

Order Propionibacteriales, Genus Propionibacterium

• Pleomorphic, nonmotile, nonsporing rods that ferment lactate and sugars to produce propionic and acetic acids.

• Used to produce Swiss cheese; found in the digestive tract and on skin.

• Cutibacterium acnes is abundant on human skin and contributes to acne vulgaris.

Order Streptomycetales

• Important source of antibiotics, with Streptomyces being the most important genus. Often called streptomycetes, true multicellularity.

• Exospores germinate to form substrate hyphae, then aerial hyphae upon nutrient limitation. Stressed substrate hyphae produce antibiotics. Aerial hyphae curl and septate into chains of spores.

Genus Streptomyces

• Ecologically and medically important, produces geosmin (moist earth odor).

• Important in mineralization, secreting exoenzymes.

• Produces streptomycin.

Streptomyces Genome

• Linear chromosome, largest bacterial genomes.

• Biosynthetic gene clusters (operons) encode antibiotics.

Pathogenic Streptomycetes

• $S. scabies$ causes scab disease in potatoes and beets; $S. somaliensis$ and $S. sudanensis$ cause actinomycetoma in humans.

Phylum Firmicutes, Class Bacilli

• Aerobic Endospore-Forming Bacteria: Bacillales and Clostridiales. Endospores are resistant survival strategy.

Order Bacillales

• Genus Bacillus: Endospore-forming, chemoheterotrophic rods with peritrichous flagella. Bacillus subtilis is well-studied, facultative anaerobe, non-pathogenic, soil-dwelling.

• Bacillus Species Produce antibiotics: bacitracin, gramicidin, and polymyxin. $B. cereus$ causes food poisoning, $B. anthracis$ causes anthrax, $B. thuringiensis$ produces Bt insecticide.

• Genus Sporosarcina: Only known endospore-forming bacterium with coccoid shape, S. Ureae is tolerant of alkaline conditions.

• Genus Thermoactinomyces: True endospore former. Thermophilic, found in high-temperature environments. $T. vulgaris$ causes farmer’s lung.

Order Staphylococcus

• Facultative anaerobic, nonmotile cocci, catalase positive and oxidase negative.

• Associated with warm-blooded animals in skin and mucous membranes, cause many human diseases.

• Staphylococcus aureus: Virulence factors evade the immune system such as Protein A and coagulase, and produces toxins like β-hemolysin.

Order Lactobacillales

• Streptococcus, Enterococcus, Lactococcus, Lactobacillus, and Leuconostoc are all genera in the order Lactobacillales.

• Lactic acid bacteria produce lactic acid as their main fermentation product.

• Morphologically diverse, nonmotile, fermentative, lack cytochromes, facultative anaerobes.

Lactobacillus

• Include rods and some coccobacilli, lack catalase and cytochromes, produce lactic acid.

• Homolactic and heterolactic fermentation pathways, used in the food industry.

Family Leuconostocaceae

• Genus Leuconoostoc: Facultative, Gram-positive cocci, heterolactic fermentation, involved in food spoilage and tolerate high sugar concentrations.

Families Enterococcacee and Streptococcaceae

• Pairs or chains, strictly fermentative (homolactic fermentation), aerotolerant and anaerobic.

• Enterococcus faecalis: Normal residents of the intestinal tracts of humans and many animals. Opportunistic pathogen with horizontal transfer of antibiotic-resistance genes.

• Lactococcus lactis is used in the production of buttermilk and cheese

Genus Streptococcus

• Facultatively anaerobic and catalase negative. Species identification through Lancefield grouping system.

• alpha (a)—hemolysis: green zone around colony on blood agar, due to incomplete hemolysis.

• beta (ß)-hemolysis: clear zone around colony on blood agar due to complete lysis of red blood cells.

• Gamma hemolysis: no lysis of red blood cells.

• Three Groups of Streptococci:

  • Pyogenic streptococci: $S. pyogenes$ causes streptococcal sore throat and skin infections

  • Oral streptococci: $S. mutans$ causes dental caries.

  • Other streptococci: $S. pneumoniae$ associated with pneumonia and otitis media.

Family Listeriaceae

• $L. monocytogenes$ is a facultative intracellular pathogen that causes listeriosis, a food-borne infection.

Order Mycoplasmatales

• Mycoplasmas: Genomic analysis places them in the class Bacilli. Small genomes lacking a cell wall, pleomorphic.

• Facultative anaerobes with fried-egg appearance.

• Energy Conservation in Ureaplasma urealyticum: hydrolyzes urea to generate an electrochemical gradient.

• Motility in Mycoplasma mobile: Gliding motility powered by ATP hydrolysis.

• $M. pneumoniae$ causes walking pneumonia.

Phylum Firmicutes, Class Clostridia

• Anaerobic Endospore-Forming Bacteria: Clostridia - Clostridium, spp. Anaerobic and form heat-resistant endospores, responsible for food spoilage.

Stickland Reaction

• During anaerobic protein decomposition, clostridia often ferment amino acids via the Stickland reaction.

• Amino acids are oxidized and reduced to produce acetate and ATP along with ammonia, hydrogen sulfide, and fatty acids.

• Important Species of Clostridium:

  • $C. botulinum$ produces botulism toxin.

  • $C. tetani$ causes tetanus

  • $C. difficile$ causes severe gastrointestinal tract disease.

Genera Heliobacterium and Heliophilum

• Anaerobic, phototrophic species with bacteriochlorophyll g.

• Capable of nitrogen fixation.

• Monoderm cell wall, but low peptidoglycan content and they stain Gram-negative.

Phylogenetic Analysis of Firmicutes

• Ancestral Firmicutes were diderm. Genus Veillonella: Anaerobic, chemoheterotrophic cocci that ferment carbohydrates, causing gas production.