Mutualism & Attenuation

• Lecture 17

Mutualism

• A reciprocal benefit accrues to both partners
• An example is Buchnera aphidicola and aphids

Buchnera aphidicola

• A gram-negative organism
• It has a 617kb genome, while E. coli has ~4.6 Mb
• Lives inside aphid cells within bacteriocytes (mycetocytes)
• Transmitted vertically from mother to daughter
• Obligate mutualists
• Aphids evolved together with Buchnera for millions of years
• ~75% of the Buchnera genome has been lost
• Aphids provide Buchnera with amino acids it cannot make
• Aphids cannot make Trp
• Buchnera must synthesize & provide Trp for the aphid

Attenuation

• A proposed mechanism of control in some bacterial operons
• Results in premature transcription termination
• Relies on the fact that transcription and translation proceed simultaneously in bacteria
• If Region 2 of the RNA pairs with Region 3 of the RNA a nonterminating stem loop forms and transcription continues
• If Region 3 of the RNA pairs with Region 4 of the RNA the terminating stem loop forms and transcription is terminated
• Translation of the leader peptide affects which region pairs

Cooperation

• A reciprocal benefit accrues to both partners
• Examples include where cellulomonas provides nitrogen fixers like azotobacter with glucose, and where chromatium supplies desulfovibrio with hydrogen sulfide

Commensalism

• One symbiont (the commensal) benefits, while the other host isn't harmed or helped
• An example is Staphylococcus epidermidis which grows on the skin and consumes human waste like oils, water, salts, and dead skin cells, normally without impacting human health

Predation

• One organism preys on another
• Bdellovibrio is a gram-negative bacterium that preys on other organisms

Bdellovibrio

• It enters the prey's periplasmic space
• And feeds on the cytoplasmic contents

Parasitism

• The parasite benefits, while the host is usually harmed

Amensalism

• The adverse effect that one organism has on another
• The Streptomyces spp. produce many different antibiotics
• Because of producing antibiotics, they are heavily studied in the hunt for novel therapeutics

Competition

• Two organisms try to acquire the same resource, like location or nutrient

Human Microbiota

• Microbes that live in or on humans
• Often commensals or mutualists
• Human body contains ~10^13 human cells and ~ten times more microbial cells
• Most microbes are found in the large intestine (~10^14 or 99%) , then on the skin (~10^12 or 1%), then the mouth (10^10 or .01%)
• Human microbiota provides nutrients for the host
• It also protects the body from invasion by harmful bacteria

Biofilm

• An encased aggregation of bacteria
• Biofilms are composed of polysaccharide, protein, and extracellular DNA
• In Staphylococcus aureus biofilm formation, cells attach to a surface
• The cells multiply to a confluent "lawn" of cells on the surface
• An "exodus" phase occurs and some cells leave
• Biofilm then matures into towers of cells
• Superbugs are largely shared among individuals and over 99% of their genes are bacterial
• Humans lack CAZymes (carbohydrate active enzymes); gut microbes supply the human body with energy from dietary polysaccharides through CAZymes that break down carbs
• Gut microbiota shapes intestinal immune responses during health and disease
• The human microbiota protects the body from invasion of harmful bacteria

C. difficile

• A Gram (+), spore-forming anaerobe
• Colonizes and infects people who have been treated with antibiotics
• A large problem in hospital and healthcare

C. difficile pathogenesis

• Disruption of colonic microflora leads to exposure to C. difficile spores, germination, and colonization
• This results in toxin production, and damage to colonic mucosa, inflammation, & diarrhea

C. difficile Pathogenicity Locus

• Includes: tcdR, tcdB, tcdE, tcdA, and tcdC

Sporulation Pathway

• Is activated by desiccation, cell density and starvation
• The stages of germination are: Stage I, Stage II, and Outgrowth

Treatment Options

• Vancomycin
• Difficid (Fidaxomicin)
• Zinplava (Bezlotoxumab)
• Toxoid Vaccine (failed Phase III clinical trial)
• Probiotics (hit or miss)
• Fecal Replacements (very successful)