Gram-Positive Bacteria Flashcards
Gram positive bacteria
Thick peptidoglycan cell wall
No outer membrane
- Firmicutes – Endospore forming (Bacillus) – Nonsporulating (Staphylococcus)
- Mollicutes – No peptidoglycan cell wall (Mycoplasma)
- Actinobacteria – Mycobacterium – Filamentous (Streptomyces)
Spore forming bacilli
- Gram positive rods (Low G+C content)
- Produce endospore – Highly differentiated cells which arise by asymmetric division • Survival – Resistant to heat, chemicals, radiation, dehydration – “Dormant” stage may last for years
- Dispersal – Wind – Water – animal gut
Bacillus anthracis
• Causes anthrax • >80% mortality when untreated • Grows in lymph nodes and lymphatic tissue of lungs • Death from septicaemia, haemorrhagic meningitis
Anthrax vaccination uses anthrax antigens and not the organism
The Life Cycle of an EndosporeForming Bacterium
Endospore structure
- Structurally complex – many layers
- Contains dipicolinic acid complexed with Ca2+ – Dehydrates endospore and stabilises DNA
- Core contains small-acid soluble proteins (SASP) – Bind DNA – protection from damage – Carbon and energy source for germination
Endospore Formation
Structure of the Bacterial Endospore
Key differences between vegatative cells and endospores
Toxic parasporal chrystal in Bacillus thuringiensis
• Parasporal body • BT toxin: toxic to insect larvae – Activated in their guts • Insect resistant GM crops
Nonsporulating Gram positive Bacteria
ex: Staphylococcus – Aerobic, cocci – Resistant to reduced water potential – Tolerate high salt – Many species are pigmented – Staphylococcus aureus (yellow) – Methicillin Resistant Staphylococcus aureus (MRSA)
Cell-wall-less Gram positive bacteria: Mycoplasmas
Mycoplasma
- Lack cell walls – No cell wall visible by EM – Key components of peptidoglycan shown to be absent – Membranes contain sterols and lipoglycans which stabilise the membrane
- Mycoplasma cells are pleomorphic – Cells may be cocci or filaments of various lengths
- Some of the smallest organisms capable of autonomous growth
Some of the smallest organisms capable of autonomous growth • Parasites that inhabit animal and plant hosts • Often defective in several biosynthetic pathways Mycoplasma genitalium – Host associated – Genome 580 kbp (compared to E. coli 4600 kbp) – 470 ORFS (genes)
Actinobacteria: Mycobacterium
- Rod-shaped organisms, can be somewhat pleomorphic
- Obligate aerobe
- Facultative intracellular parasite – Can live within macrophages
- Complex cell wall
- Mycobacterium tuberculosis – Cause of Tuberculosis (TB) – “Discovered” by Robert Koch
- Some species grow very slowly – M. tuberculosis generation time of 15-20h (compared to 20 mins for E. coli)
Filamentous Actinomycetes: Streptomyces
Filamentous, Gram positive bacteria
- Branching filaments – Produce mycelium analogous to mycelium of filamentous fungi
- Produce spores called conidia
- Important secondary metabolites • Strict aerobes that produce many extracellular enzymes
- Genomes are typically quite large (8 Mbp and larger)
Streptomyces Life Cycle
- Vegetative hyphae emerge from germinating spore
- Mycelial growth – Tip extension gives filaments – Few cell walls, many copies of the genome in each “cell” – Elongate and branch on the surface and penetrating the medium
- Nutrient depletion results in a switch to growth of aerial hyphae – complex signalling cascade – Production of a surfactant that coats some emerging filaments this allows them to grow away from the substrate – Substrate mycelia used as nutrient source
- Once aerial hyphal growth stops synchronous, multiple septation to give rise to prespore compartments (one copy of genome)
- Mature and develop characteristic grey pigment
Comparison of endospores and bacterial conidia
