Lecture 6 - bacterial persistence and surviving an environment in flux Flashcards
Environments that bacteria can grow in
Bacteria can pretty much grow in any environment (soil, hot, dry, cold, ice, within and on us)
oxygen is one of the most important ______ factors in both medical and environmental microbiology
Abiotic
Abiotic factors refer to non-living physical and chemical elements in the ecosystem.
Aerobic environment
Aerobic = high oxygen (atmospheric, 21%)
Aerobes prefer oxic environments
Microaerobic environment
Microaerobic = trace oxygen (2-10% or less)
Microaerophiles prefer hypoxic environments
Anaerobic environment
Anaerobic = no oxygen (strictly 0%)
Anaerobes prefer anoxic environments
Facultative = change metabolism under oxia (they can adapt)
Host associated environments
internal or bacteria dense environments are anaerobic
Soils and oceans as environments
decreasing oxygen with increasing depth (oxygen can’t diffuse as far and because of bacterial activity)
Eutrophication
High local activity of aerobes can lead to oxygen minimum zones, eutrophication (Eutrophication is when the environment becomes enriched with nutrients. The microbes may use up all the oxygen in the water, leaving none for other marine life. This results in the death of many aquatic organisms such as fish, which need the oxygen in the water to live.)
Flexibility in terms of microbes means
the ability to adapt to a wide range of environments
What environment does most microbes inhabit?
Relatively few microbes are lucky enough to live in nutrient-rich eutrophic environments. Rather, most inhabit oligotrophic environments - ones in which nutrient levels are low and some live in feast-and-famine environments.
Higher eukaryotes in terms of flexibility
Higher eukaryotes (which are hosts for example) = inflexible Obligately heterotrophic (e.g. sugars, amino acids, fats) Obligately aerobic (oxygen for respiration) Ferments transiently and products must be recycled
Bacteria in terms of flexibility
Bacteria = flexible
Diverse organic and inorganic electron donors
Oxygen and anaerobic electron acceptors (facultative anaerobe)
Fermentation as terminal, sustained mode of energy generation (even when there is limited processes for respiration to occur)
E. coli in terms of flexibility
Numerous substrates for energy generation
Can use greater than 6 alternatives to oxygen for respiration
Can live on trace gases (H2) if no nutrients are available
Flexiblity allows ATP generation when environment changes
Bacteria as recyclers
Bacteria are the ultimate recyclers which is part of their flexibility, there is a vast amount of carbon and energy sources that they harness - plant tissue, animal tissue, excretory products - end products, cells of dead bacteria, antibiotics, plastics, heavy metals
Bacteria have diverse _____ _____ for breaking down several polymers into monomers
digestive enzymes
Bacteria digestive enzymes are typically
Typically bound-in or associated with cell surface to prevent losses to competitors
Releases monomers or shorter oligomers that conserved pathways can use such as glucose, amino acids, fatty acids
Polymers and monomers
Polymers are molecules made of monomers such as proteins (monomer =amino acid), DNA (monomer = nucleotides, fats (monomers = glycerol)
What happens if there is not food for bacteria?
Bacteria are specialists at growing slowly or not at all - persistence
Still metabolically active
Different to true dormancy i.e. being an endospore (endospore is not only resistant to a variety of stressors but are also metabolically dormant)
Most environments are too nutrient limited to reflect batch culture
Persistence most likely normal state of bacteria in environment
In reality in the environment, bacteria can enter a long term stationary phase which is also known as long term resistance where the bacteria that have survived the death phase have now adapted to growing slowly/if not at all and in this case there is very little cell death but what we can have is spontaneous mutations occur that can create lots of different kinds of strains
R type strategist examples
e.g. E.coli, B.subtilis
R type strategist
Fast growing (TD= minutes)
May use food wastefully
Last ditch strategies during stress: sporulation, “bust and boom”
Dynamic persistence of small subpopulation
Overproduce and those that survive are those that are likely to have entered the persistent state
R type strategist TD
Fast growing (TD= minutes)
K type strategist examples
e.g. M.tuberculosis, soil bacteria
K type strategist
e.g. M.tuberculosis, soil bacteria
Slow growing (TD= days)
Uses food efficiently therefore there is less waste
Survives stress for months-years with less drastic metabolic changes
Bulk of population persists
K type strategist TD
Slow growing (TD= days)
Gram negative vs gram positive
Gram negative cell wall has two plasma membranes separated by a periplasmic space, gram positive has one inner membrane and peptidoglycan layer that is thick
current antibiotics
are only effective when a bacterium is growing (log phase)
e.g. cell wall synthesis inhibitors and protein synthesis inhibitors
Starvation responses induce ….
protective responses to some antibiotics
Some antibiotics need…
Some antibiotics need energy to be taken up by bacteria, stationary vs log
- Adding nutrients can recover activity
However since 1940s its known that some log phase bacteria survive in clinic
Persisters
Persisters are bacterial cells identified by their ability to survive exposure to an antibiotic (often multiple antibiotics) even though they do not harbour antibiotic resistance genes. They are non-growing cells that are though to be dormant. Dormancy is central to the hypotheses regatding their antibiotic-tolerant phenotype; in dormant cells, the target of an antibiotic is unavailable to inactive and would be unaffected by the antibiotic. Persisters are of considerable interest for several reasons, including their potential to cause recurrent infections after major antibiotic treatment
Antibiotic resistance
Antibiotic resistance = heritable genetic changes allow bacteria to grow
Mutate their genes or acquire enzymes but there are changes in DNA therefore are heritable
Antibiotic persistance
Antibiotic persistance = non-heritable physiological changes allow survival of a few slow or non-growing cells
Something happens during the course of treatment, most of them die off but there is a dynamic subpopulation of cells that grow very slowly can then regrow back to the beginning point when conditions become favourable again
What keeps the resistance genes in a population and contributes to the drug-resistant bacteria strains is …
exposure to the drug
What makes treatment time longer in terms of antibiotics?
inability of antibiotics to kill non-growing cells makes treatment time longer
Persistence can lead to
resistance
Wiping out populations with antibiotics …
Unlike environmental persistence, although we have the R and K type selection in the environment and population level strategies, when we just wipe out populations with antibiotics it is environmental catastrophe, their responses tend to look a lot more like an R type selection response regardless of the organism
Environmental persistence features
Response to fluctuating food and nutrient supplies
Different population-level strategies (r/K)
Adaptation for long-term survival
More time to respond
Antibiotic persistance features
Response to an environmental catastrophe (antibiotic killing)
Similar population-level strategies
Subpopulation of cells stochastically (randomly) preadapted
Less time to response
responses to stress (3)
stringent response, SOS response, Toxin-antitoxin systems
methods of persistance (5)
mutations, stationary phase, antibiotic exposure, vacuoles and biofilms
responses to stress purpose
Evidence suggests that in some cases, small sub sets of cells in a population of growing cells spontaneously be come dormant persister cells, even when nutrients are readily available. These persisters can be thought of as the population’s attempt to prepare for future possible starvation conditions. There is also increasing evidence that persisters arise in response to various triggers, with starvation probably being the most im portant.
Stringent response
shift in the expression the genes to a survival based response
Mediated by molecule (p)ppGpp
Stops expensive biosynthetic processes
ppGpp is a signaling molecule that functions in a regulatory network called the stringent response. ppGpp is often referred to as an alarmone because it is synthesized in response to many stressors and regulates the activity of numerous genes and proteins so that the cell can survive.
SOS response
stress induces DNA repair, also has survival features
Both repair and survival responses
Mediated by protein RecA
Toxin-antitoxin systems
Balancing act with diverse cellular functions
Disruption of antitoxin leads to growth inhibition by toxin
The toxin when functioning disrupts normal cellular functions, causing growth arrest and the persister phenotype
internal, nonsecreted toxin and a cognate antitoxin that prevents the toxin from exerting its effects on the cell. The toxin, when functioning, disrupts normal cellular functions, causing growth arrest and the persister phenotype.
Hypoxia in terms of persistance and example
gradual depletion of oxygen leads to a stationary phase, slowed growth and persistent phenotypes
M.tuberculosis frequently encounters hypoxia in host and intracellular environments
Macrophage phaglysosome
Granuloma; immune structures forming wall
Biofilms
Slimy matrix of various bacteria produced polymers
Local depletion of nutrients/oxygen leads to persister cell formation
Physical protection from environment combined with antibiotic persistence
A mature biofilm is a complex, dynamic community of microorganisms. It exhibits considerable heterogeneity due to differences in the metabolic activity of microbes at various locations within the biofilm; some are persister cells
While in the biofilm, microbes are protected from numerous harmful agents such s UV light and antibiotics
All forms of persistance cause a shift to form of
energy metabolism that permits survival without growth
