Eubacteria and Microbial Diversity

• Topic is microbial diversity focusing on Eubacteria with references to archaea and other classifications.

Diversity of Microbes in Each Domain

• Domain Eukarya includes microscopic algae, fungi, protozoa, and helminths.
• Domain Archaea consists of archaeabacteria.
• Domain Bacteria includes eubacteria, or "true" bacteria.
• Gram-positive and Gram-negative subgroups exist within the Bacteria domain, with Gram-negative bacteria also known as proteobacteria.
• Topic 6 provides detail on archaea and eubacteria.

Gram Positive Bacteria

• Historically divided into two main phyla based on the ratio of guanine and cytosine (G+C) bases in their DNA.
• Firmicutes have a low G+C content (35-55%).
• Non-sporulating Firmicutes group includes notable genera such as Staphylococcus, Micrococcus, Streptococcus, and Lactobacillus.
• Endospore-forming Firmicutes include Bacillus and Clostridium.
• Actinobacteria have a high G+C content (>65%).
• Rod-shaped Actinobacteria include Corynebacterium and Propionibacterium.
• Acid-fast Actinobacteria include Mycobacterium.
• Filamentous Actinobacteria include Streptomyces and Actinomyces.
• DNA sequence analysis offers a more accurate classification method.

Staphylococcus (Staph. spp.)

• Gram-positive organisms.
• Occur in single form, in pairs or clusters.
• Catalase positive which helps distinguish it from streptococci.
• Common inhabitants of the human body.
• Two primary species: Staph. epidermidis and Staph. aureus.
• Staph. aureus is a significant human pathogen that produces various virulence factors like enterotoxin and coagulase.
• Staph. aureus excretes a range of toxins (34+) that promote its colonization.
• Excreted toxins are Alpha toxin, Coagulase, Dnase, Exfoliative toxins, Hyaluronidase, Enterotoxins, Leukocidin, and TSS toxins.
• Methicillin-resistant Staph. aureus (Staph. aureus) is an opportunistic pathogen.
• Healthcare-associated (nosocomial) infections are common in patients with lowered resistance.
• Antibiotics have been the most effective treatment historically, but extensive use causes antibiotic resistance.
• Combination therapy, using more than one antibiotic, combats resistance.

Streptococcus (Strep. spp.)

• Gram-positive organisms.
• Occur in pairs or chains.
• Catalase negative, distinguishing them from staphylococci.
• Grouped by cell wall surface antigens (Lancefield typing).
• Transmitted mainly from the throat in respiratory secretions.
• Groups A, B, and D are most significant in causing infectious diseases as well as their ability to cause haemolysis.
• Some more significant diseases can result, to include Scarlet fever, Rheumatic fever, Invasive bacteraemia and Necrotising fasciitis.
• Main excreted toxins are Hyaluronidase, Streptolysin , Pyrogenic toxins and Streptokinase.

Compare and Contrast Staph. aureus and Strep. pyogenes

• Staphylococcus aureus
  • Gram-positive
  • Facultative anaerobe.
  • Catalase tests positive.
  • Grape-like clusters.
• Strep. pyogenes
  • Is Gram-positive
  • Fermentative
  • Catalase tests negative
  • Long chains.
• Both can infect the skin and be invasive.
• Staph. aureus is notorious for antibiotic resistance.
• StRep. pyogenes is sensitive to penicillin.
• Staph. aureus infects the digestive tract, lungs, and urinary tract, etc.
• Strep. pyogenes infects the respiratory tract, causing "strep throat" and scarlet fever.

Non-sporulating Firmicutes: LAB (Lactic Acid Bacteria)

• Central to the production of many fermented foods.
• Gram-positive rods and cocci that produce lactic acid as the major or sole fermentation product.
• Examples include: Leuconostoc, Lactobacillus, Enterococcus, Bifidobacterium, Aerococcus, Pediococcus, Carnobacterium, Streptococcus, and Lactococcus.
• There are two primary LAB Fermentation Pathways:
  • Homolactic fermentation produces lactic acid as the main product
  • Heterolactic fermentation produces mixed end-products, including ethyl alcohol and acetic acid.

Sporulating Firmicutes: Bacillus and Clostridium spp.

• Gram-positive bacteria producing endospores.
• Can differentiate into endospores when carbon, nitrogen, or phosphorus is depleted.
• Clostridium: rod-shaped, obligate anaerobes and form spores.
  • Many are medically important, causing tetanus (Clostridium tetani), botulism (Clostridium botulinum), gas gangrene (Clostridium perfringenes), and diarrhoea (Clostridium difficile).
• Bacillus: rod-shaped, facultative anaerobes and form spores.
  • Common in soil, with a few important pathogens such as Bacillus anthracis, causing anthrax, and Bacillus cereus, casing food poisoning.
• Clostridium spp. produce toxins that target the central nervous system (CNS).
  • Endospores are resistant to heat, UV irradiation, solvents, acid, desiccation.
• Clostridium botulinum
  • Botulinum neurotoxin causes flaccid paralysis by preventing the release of acetylcholine at neuromuscular junctions
• Clostridium tetani
  • Tetanus neurotoxin causes rigid (spastic) paralysis by preventing the release of glycine, stopping muscle relaxation, and resulting in "lockjaw".
• Bacillus species have many industrial applications.
  • Used in biotechnology since the early 1900's.
  • Bacillus species secrete a wide range of extracellular enzymes.
  • Serine protease is used in laundry detergents.
  • Cellulase is used in laundry detergents for processing denim.
  • Alpha amylase from B. licheniformis is used in starch processing for food and high fructose syrups.
• Bacillus species are used in agricultural applications.
  • Bacillus thuringiensis produces crystal proteins toxic to insect larvae, acting as a biocontrol agent.
  • The crystal protein gene is cloned into plants to yield innate resistance to insect pests in cotton, corn, soya, etc.
• Bacillus species used for medical applications.
  • Certain Bacillus species produce peptide antibiotics that are not synthesized on ribosomes but via a complex enzyme.
  • They contain D-amino acids that form circular configurations (10-20 amino acids).
  • Not used systemically due to toxicity, but topically for skin infections, cuts, and eye infections.
  • Neosporin contains neomycin, polymyxin, and bacitracin.

Actinobacteria

• Gram-positive bacteria group based on G+C content.
• Actinobacteria have a high G+C content (>65%)
• Mycobacterium tuberculosis causes tuberculosis (TB) in humans and other mammals.
  • Causes a severe, contagious airborne disease.
• The TB bacteria lodge in the lungs to multiply, creating patches and lung cavities with slow growth.
  • Untreated TB can spread to the brain, bone, eyes, liver, kidneys, spine, and skin.
  • Mycobacterium tuberculosis grow on supplemented media like Lowenstein-Jensen. -Mycobacterium tuberculosis are obligate pathogens.
  • Mycobacterium tuberculosis survives in milk and organic materials if not exposed to UV light.
  • Mycobacterium tuberculosis is heat sensitive and is destroyed by pasteurisation.
• Filamentous Actinomycetes - Streptomyces species:
  • Generally associated with soil and decaying plant materials.
• Often highly pigmented "geosmin", are filamentous and spore-forming
  • Extracellular enzymes breakdown organic substrates in the environment.
  • Produce bioactive molecules, considered secondary metabolites
    • Most prolific producers of antibiotics, used clinically.

Summary of Gram positive Bacteria

• Form a distinct group of Eubacteria, separated into two groups by the G+C content of their genome.
• Bacillus and Clostridium are in the low G+C group
• Bacillus can form endospores, highly resistant dissemination forms.
• Most Bacillus are harmless.
• A few species are important to man for industrial applications, pest & infection control agents and as pathogens (Bacillus anthracis and Bacillus cereus)