Examples of Microbes
Staphylococcus epidermidis-
Lives on skin (microflora)-Usually non-motile, but interacts with environment for colonization
Klebsiella aerogenes- Gut microbiota
Rhizobium leguminosarum-Nitrogen-fixing bacteria in pea root nodules
Uses motility and taxis to locate plant roots
Classification of Flagellum
-polar
-Lophotrichous
-Peritrichous
-Periplasmic endoflagella in spirochetes
Chemotaxis:
ability of bacteria to move along a concentration gradient toward a chemical attractant or away from a repellent
Chemoeffectors
Attractants: chemicals that draw bacteria toward them (e.g., nutrients like sugars or amino acids)
Repellents: chemicals that push bacteria away (e.g., toxins or harmful compounds)
Chemoreceptors
sense chemoeffectors. Responses can vary: one chemical may be an attractant for some bacteria and a repellent for others
A chemical’s effect can switch depending on conditions
How is bacterial chemotaxis behavior measured?
Capillary assay: bacteria swim into a tube with attractant or avoid a repellent
Soft agar (swarm) plates: low-percentage agar shows outward swarming for attractants or reduced movement for repellents
Microscopy tracking: observes individual cell movement, runs vs. tumbles, and population patterns
how bacteria senses concentration
Scenario 1: Detects spatial difference (between cell ends).
Scenario 2: Detects temporal difference (over time while moving).
Chemotactic Response
Signal Transduction (ST): phosphorylation
Signal Adaptation (SA): involves methylation
Sensory Transduction and
Adaptation
Sensory transduction: is the ability to detect and respond to temporal changes in stimuli
Sensory adaptation: The ability to adjust sensitivity so the bacterium can respond to new changes in the same stimulus
* Chemotaxis involves both of these parallel processes
chemotaxis is there is an attractant
if there is an attractant then there is smooth swimming , therefore the TCS does not get activated by phosphorylation.
Methylation of chemoreceptor
controls adaptive response
Two-component system
(phospho-
relay) carries out the orders
Concentrations recorded by
changing methylation and
phosphorylation levels
What happens to the CheA–CheY system when a repellent is present?
MCP senses repellent → CheA activity increases.
More CheY-P produced → flagella rotate CW frequently → bacterium tumbles more to escape.
What happens to the CheA–CheY system when no attractant or repellent is present?
MCP interacts with CheA normally.
CheA autophosphorylates at baseline rate → CheY-P at steady-state levels.
Flagella switch occasionally CW → normal tumbling and swimming.
What happens to the CheA–CheY system when an attractant is bound and no repellent is present?
MCP undergoes conformational change.
Signal reaches CheA, but it does not autophosphorylate.
CheY-P levels drop → flagella rotate CCW → bacterium swims straight toward attractant
Key Players
MCPs (chemoreceptors) – sense attractants/repellents.
CheA – kinase that phosphorylates proteins (CheY, CheB).
CheY-P – binds flagella → CW rotation → tumble.
CheB-P – methylesterase that removes methyl groups from MCPs → lowers sensitivity.
CheR – methyltransferase that adds methyl groups to MCPs → increases sensitivity.
Che R
adds methyls to MCP
Che B-P
removes methyls from MCP
High methylation levels
MCP more likeley to activate Che A
Low methylation
MCP less likely to activate Che A
Methylated
Methylated = tumbling
unMethylated
running
Signal gain
means bacteria can detect very small environmental changes because receptor clusters and CheA amplify the signal into a big motor response.
