Bacterial Diversity: Cyanobacteria and Gram +
Chapter 18 Part 1: Bacterial Diversity - Cyanobacteria and Gram +
Chapter Overview
Bacterial Diversity at a Glance
Bacteria vary widely in cell structure and metabolism.
Bacterial species include heterotrophs, lithotrophs, and phototrophs.
Bacteria have evolved an array of life forms that colonize every habitat on Earth, including humans.
Cyanobacteria: Oxygenic phototrophs
Gram-positive: Firmicutes, Tenericutes, and Actinobacteria
Gram-negative: Proteobacteria, Spirochetes, Acidobacteria, Bacteroidetes, Chlorobi
PVC Superphylum: Planctomycetes, Verrucomicrobia, Chlamydiae
Bacterial Diversity at a Glance
Analogy: Touring all the countries of a continent in one day to survey bacterial diversity.
Like countries, bacterial taxa have complex traits and histories, often-contested borders.
Common Traits of Bacteria:
Overview of shared major traits to explore differences among bacteria.
Bacteria: Common Traits and Diverging Phylogeny
Common Features:
Central apparatus for gene expression includes RNA polymerase, ribosomal RNAs, translation factors (selective targets for antibiotics).
Most bacterial cells have cell walls made of peptidoglycan (another antibiotic target).
Phylogeny:
A phylum consists of bacteria sharing a common ancestor that diverged early based on small-subunit ribosomal RNA (SSU rRNA) sequences.
Emerging Clades: Newly discovered kinds of bacteria with unexpected traits.
Deep-Branching Taxa: Lineages that diverged early, at or before well-known phyla, based on genome sequences.
Phylogenetic Tree:
Based on comparison of rRNA and ribosomal protein sequences, rooted in Archaea and Eukarya.
Well-Studied Bacterial Phyla
Key phyla include:
Cyanobacteria
Firmicutes, Tenericutes, and Actinobacteria (Gram-positive bacteria)
Proteobacteria (Gram-negative)
Deep-branching Gram-negatives
PVC Superphylum: Planctomycetes, Verrucomicrobia, Chlamydiae
Deep-Branching Gram-Negative Microbes
Characteristics:
Diverged from Proteobacteria, displaying diverse metabolism and morphology.
Bacteroidetes and Chlorobi:
Obligate anaerobes, e.g., Bacteroides (ferment complex carbohydrates, forms part of gut microbiome).
Characteristics:
Gram-negative, nonsporulating rods, gliding motility.
Aerobic/fermentative (saccharolytic); obligate anaerobic fermenters producing acetate and succinate.
Typically commensals aiding digestion; synthesize sphingolipids.
Chlorobi: Anaerobic “green sulfur” phototrophs.
Use H2S as electron donor (oxidizing to SO4–).
Extracellular S0 deposits, can be thermophilic; pigments (bacteriochlorophyll) stored in chlorosomes.
Efficient at low light intensities; inhabit anoxic, sulfidic environments.
Other Groups:
Nitrospirae: Oxidize nitrite (NO2–) to nitrate (NO3–).
Acidobacteria: Soil bacteria, involved in nitrogen cycling (up to 50% of soil community).
Fusobacteria: Includes human pathogens (e.g., septicemia, skin ulcers).
Deep-Branching Thermophiles
Key Groups:
Aquificae and Thermotogae: Most extreme bacterial hyperthermophiles.
Aquifex pyrophilus:
Flagellated rod, microaerophilic, hydrogen chemolithotroph.
Thermocrinis ruber:
Forms filamentous mats in hydrothermal vents; grows optimally at 80°C, known for ‘pink streamers’ of filamentous cells.
Thermotoga maritima:
Forms sheath-like envelope, Gram-negative, nonsporulating; fermentative anaerobes with extensive archaeal genes transfer.
Characteristics of Deep-Branching Thermophiles:
Exhibit fast growth rates, high mutation rates, and may show an accelerated molecular clock, potentially appearing to diverge earlier than they did.
Deep-Branching Thermophilic Phototrophs
Chloroflexi:
Characteristics: Photoheterotrophic, moderately thermophilic.
Contains photosynthetic apparatus within chlorosomes, forms filamentous mats associated with thermophilic cyanobacteria; appears red/yellow due to accessory pigments.
Cytoplasmic Membrane Structure
Bacterial Cell Layering:
Cytoplasmic membrane
Peptidoglycan
Outer membrane
Deep-Branching Thermophiles with Unique Resistance
Phylum Deinococcus-Thermus:
Contains ornithine in peptidoglycan cross-bridges.
Thermus species (e.g., T. aquaticus):
Moderate thermophiles; heterotrophs.
Deinococcus radiodurans:
Not thermophilic; resistant to extremely high radiation and desiccation, featuring a thick cell wall.
Radiation and desiccation-resistant, carotenoids give reddish-pink coloration, possesses an efficient DNA repair mechanism, and unique DNA arrangement for recombination.
Emerging Clades
Discovery:
New bacteria identified through innovative screening methods for unexpected traits (e.g., ultrasmall bacteria discovered that can pass through a 0.2 µm filter).
Case Study: Ultra-small bacteria discovered in an aquifer in Rifle, Colorado; possess around 50 ribosomes and a cell wall surrounded by an S-layer.
Cyanobacteria: Oxygenic Phototrophs
Contributes all oxygen gas in Earth's atmosphere, with origins linked to an ancient cyanobacterium that evolved into plant chloroplasts.
Phylum: Cyanobacteria (Cyanophyta) receives its name from blue phycocyanin pigments.
Appearance: Commonly green due to absorption from chlorophylls a and b.
Cyanobacterial Cell Structure
Unique Features:
Oxygenic phototropic prokaryotes; morphologically diverse with chlorophyll and accessory pigments.
Photosynthesis occurs in thylakoids; CO2 fixation in carboxysomes; buoyancy maintained through gas vesicles.
Specialized cells (heterocysts) for nitrogen fixation.
Types of Cyanobacteria
Single-Celled Cyanobacteria:
Includes Synechococcus and Prochlorococcus (most abundant oceanic phototrophs) and Microcystis (produces harmful toxins).
Filamentous Cyanobacteria:
Example genera: Oscillatoria, Nostoc, Anabaena (forms akinetes, resistant spores).
Colonial and Multicellular Cyanobacteria
Cyanobacteria can form colonies or multicellular organisms.
Examples include Gloeocapsa and Merismopedia forming cellular aggregates; Pleurocapsa utilizes baeocytes (multiple fission for reproduction).
Multicellularity: Cyanobacterial filaments represent multicellular organisms via chains formed through serial cell division and differentiation into specialized cell types.
Cyanobacterial Communities
Engage in mutualistic associations with animals, plants, fungi, and protists—sponges on coral reefs utilize cyanobacteria for nourishment.
Participate in multilayered microbial mats in environments like salt marshes.
Gram-Positive Bacteria
Three Distinct Phylogenetic Branches:
Firmicutes: “Low-GC” species.
Actinobacteria: “High-GC” species.
Tenericutes: No cell wall (also called mollicutes).
Firmicutes - Bacillales
Genus Bacillus:
One of the first classified bacterial genera.
Large rod-shaped cells, e.g., B. subtilis (model organism) and B. anthracis (pathogen).
Vegetative cells develop endospores during stress, germinating under favorable conditions.
Bacillus thuringiensis:
An effective insecticide against caterpillars; the sporulating cell produces delta endotoxin crystals.
Firmicutes – Clostridiales
Genus Clostridium:
Characterized by endospore-forming cells that swell to form a drumstick appearance.
Notable species: C. botulinum (produces Botox), C. tetani, C. difficile.
Metabacterium polyspora:
Grows in guinea pig GI tract, forms multiple endospores, limited binary fission.
Clostridiales: Epulopiscium fishelsoni
Notable for being huge and giving live birth, with internal offspring developing before cell release.
Non-Spore-Forming Firmicutes – LAB and Listeria
Includes non-spore-forming rods and cocci; notable genera include Lactococcus, Lactobacillus (food producers), and Listeria monocytogenes.
Listeria monocytogenes:
Causes severe gastroenteritis and can progress to the nervous system by penetrating macrophages and using an actin propulsion system.
Non-Spore-Forming Firmicutes – Staphylococci & Streptococci
Staphylococcus:
Facultative anaerobes, cocci in clusters (e.g. S. epidermidis, S. aureus).
Streptococcus:
Aerotolerant, cocci in chains (e.g. S. pneumoniae, S. pyogenes).
Tenericutes
Characterized by the complete loss of cell wall; stabilized by lipoglycans and sterols for osmotic stress protection.
Notable genus: Mycoplasma, responsible for pneumonia and meningitis, identifiable by “fried egg” colony morphology.
Actinomycetes – Streptomyces
An order within Actinobacteria; form multicellular filaments resembling branched fungi.
Streptomyces: Major soil organisms, significant antibiotic producers, have telomeric linear chromosomes, produces earthy-smelling metabolite geosmin.
Actinomycetes – Associations
Many actinomycetes form mutualistic associations with animals, such as sponges and leaf-cutter ants.
Frankia species: Form nitrogen-fixing nodules on alder trees; ants culture Streptomyces for antifungal production.
Nonmycelial Actinobacteria
Mycobacterium:
Thick cell walls with mycolic acids; notable species include M. tuberculosis (causes TB) and M. leprae (causes leprosy).
Irregularly Shaped Actinomycetes
Corynebacterium diphtheriae: Causative agent of diphtheria; divides with a snapping mechanism.
Arthrobacter: Follows unusual cell cycle, switching forms between coccoid and rod shapes.
Chapter 18, Thought Question (1)
Consider the relationship between diversity in bacterial structure and its ecological function/metabolism, using examples of specific organisms discussed in class.
Chapter 18 Part 2: Bacterial Diversity - Gram Negatives
Chapter Overview
Further exploration of bacterial diversity, focusing on:
Cyanobacteria
Gram-positive (Firmicutes, Tenericutes, Actinobacteria)
Gram-negative (Proteobacteria, Spirochetes, Acidobacteria, Bacteroidetes, Chlorobi)
PVC Superphylum (Planctomycetes, Verrucomicrobia, Chlamydiae)
Proteobacteria (Gram-Negative)
Proteobacteria have five major classes, sharing a triple-layered cell envelope comprising an outer membrane, a cell wall (periplasm), and a cell membrane.
They display diverse metabolism, with diversity arising from minor modifications in biochemical modules.
Alphaproteobacteria
Major Characteristics:
Photoheterotrophic; unicellular and structurally diverse.
Oligotrophs adapted to low nutrient concentrations; methylotrophs oxidize single-carbon compounds (e.g. Methylobacterium, Hyphomicrobium).
Alphaproteobacteria – Endosymbionts
Nitrogen Fixers:
Rhizobium and Sinorhizobium:
Rod-shaped bacteria that lose their cell walls inside host plant cells and become bacteroids specialized for nitrogen fixation.
Host cells provide nutrients and produce leghemoglobin that helps maintain anaerobic conditions in infected cells.
Alphaproteobacteria – Agrobacterium and Rickettsias
Agrobacterium: Plant pathogens related to rhizobia (e.g., Ti plasmid).
Wolbachia: Lives within arthropods and impacts reproduction.
Rickettsias:
Obligate intracellular pathogens (OIP) akin to mitochondria genetics, e.g., Rickettsia rickettsii (causes Rocky Mountain spotted fever).
Betaproteobacteria
Major Characteristics:
Photoheterotrophs, e.g., Rhodocyclus; Lithotrophs: oxidize ammonia to nitrite (e.g., Nitrosomonas used in wastewater treatment, Nitrospira).
Pathogens: Bordetella, Neisseria gonorrhoeae (causes gonorrhea), Neisseria meningitidis (causes meningitis).
Gammaproteobacteria
The largest and most diverse class, with rod-shaped bacteria that can grow singly, in chains, or biofilms; capable of rapid fermentation on carbohydrates.
Examples include Escherichia coli (E. coli O157:H7 as a pathogen) and Proteus species (noted for swarming behavior).
Gammaproteobacteria – Aerobic Rods
Notable species include Pseudomonas (flagellated swimmers that may form biofilms) and Legionella pneumophila (grows within macrophages and amoebas).
Deltaproteobacteria
Myxobacteria (Myxococcus xanthus): Soil bacteria that can aggregate into fruiting bodies when starved; glide across surfaces without flagella.
Bdellovibrio species: Predators, penetrating proteobacterial cells using resources for growth, causing host cell disruption.
Epsilonproteobacteria
Microaerophilic helical pathogens like Campylobacter and Helicobacter pylori (causes gastritis and ulcers) neutralize acidity by secreting urease enzyme.