Bacterial and Archael Cell Growth - Chapter 7

Stages of bacterial growth curve

Lag phase, exponential (log) phase, stationary phase, death phase, and long-term stationary phase

What happens during lag phase

Cell synthesizes new components to replenish spent materials or adapt to a new medium/conditions; length varies, sometimes very short or absent

What happens during exponential (log) phase

Rate of growth and division is constant and maximal; population is most uniform in chemical and physical properties

What happens during stationary phase

Population growth ceases and total viable cell number remains constant; active cells stop reproducing or reproduction is balanced by death rate; caused by nutrient limitation, limited oxygen, toxic waste accumulation, or reaching critical population density

What happens during death phase

Number of viable cells declines exponentially at a constant rate due to detrimental environmental changes like nutrient deprivation and toxic waste buildup

What happens during long-term stationary phase

Population continually evolves through successive waves of genetically distinct variants; natural selection occurs

Growth rate constant (k)

The number of generations per unit time; k = (log Nt - log N0)/0.301t

Generation (doubling) time

The time required for a population to double in size; g = 1/k

Steps of binary fission

Chromosome replication and partitioning, followed by cytokinesis (septation)

Where does bacterial DNA replication take place

At the origin of replication, proceeding in both directions around the chromosome until reaching the terminus (located opposite the origin)

Origin of replication

The single starting point on a bacterial chromosome where DNA replication begins; origins move to opposite ends of the cell during replication

End result of binary fission

Two genetically identical daughter cells

What happens during cytokinesis in bacteria

Septation - formation of a cross wall between two daughter cells - via site selection, assembly of the Z ring (FtsZ), assembly of cell wall-synthesizing machinery, and constriction/septum formation

Role of FtsZ

Tubulin homologue that polymerizes to form the Z ring (a contractile ring) at the division site, driving septum formation during cytokinesis

Role of the MinCDE system

Oscillates from pole to pole in E. coli; high MinC concentration at the poles prevents Z ring formation there, limiting Z ring formation to the cell center

Bacterial ribosome composition

70S ribosomes composed of a 30S and 50S subunit

Proteins involved in bacterial cell shape and elongation

MreB (actin homologue, positions peptidoglycan synthesis machinery for rod shape/elongation), FtsZ (division ring), CreS/crescentin (curved shape)

Bacterial chromosome partitioning proteins

ParA, ParB, and the parS region on the chromosome

Difference between bacterial and archaeal cell cycles

Bacterial cycles have three phases (growth, chromosome replication/partitioning, cytokinesis); archaeal cycles (e.g., Sulfolobus) resemble a eukaryotic mitotic cycle with G1, S phase (DNA replication), G2, chromosome segregation, and cytokinesis

Archaeal chromosome segregation proteins

SegA and SegB, similar in function to bacterial ParA/ParB (SegA is structurally similar to ParA; SegB is structurally unique but functionally similar to ParB)

Bacterial and archaeal species used as cell cycle examples

E. coli (bacteria) and Sulfolobus (archaea)

Extremophiles

Microorganisms that grow under harsh conditions that would kill most other organisms

Types of extremophiles

Halophiles/extreme halophiles, psychrophiles/psychrotrophs, mesophiles, thermophiles/hyperthermophiles, acidophiles, alkaliphiles, barotolerant/barophilic organisms

Psychrophile temperature range and environment

Grow between 0°C and 20°C; found in cold environments like polar regions and the deep sea

Adaptations of psychrophiles

Enzymes/transport systems/protein synthesis machinery function well at low temp; membranes have high unsaturated fatty acid content to stay semifluid when cold; accumulate compatible solutes and synthesize antifreeze proteins to lower cytosol freezing point

Adaptations of thermophiles

Proteins stabilized by more H-bonds, more proline, and chaperones; DNA stabilized by nucleoid-associated proteins and reverse DNA gyrase; membranes stabilized by more saturated/branched/higher molecular weight lipids or ether linkages (in archaea)

How non-extremophiles adapt to unfavorable conditions

Morphological changes like endospore formation, entering the stationary phase, using cellular components as nutrients, activating stress response genes/proteins, entering a viable-but-nonculturable state, or forming persister cells

Biofilms

Complex, slime-enclosed communities of attached (sessile) microorganisms

What biofilms do

Protect microbes from harmful agents like UV light and antibiotics; allow metabolic exchange, DNA uptake, and communication among attached organisms

What biofilms are comprised of

A slimy extracellular matrix (EPS) made of various polymers, released by microbes after reversible attachment to a conditioned surface

How biofilms function

Microbes reversibly attach to a conditioned surface, release the EPS matrix, and form a heterogeneous community with differences in metabolic activity and location; cells communicate via quorum sensing and can slough off to spread

Quorum sensing

A density-dependent form of cell-cell communication in bacterial populations using small diffusible signaling molecules (autoinducers)

Why bacteria use quorum sensing

To coordinate population-wide behaviors once a critical population density is reached, such as converting to a competent state, bioluminescence, virulence factor production, and DNA uptake for antibiotic resistance genes

Autoinducer

A small signaling molecule (peptide or other) that increases in concentration as a microbial population grows and, once at high enough levels, diffuses into cells to induce expression of target genes

Autoinducers used in quorum sensing

Autoinducing peptides (AIPs), first observed in Gram-positive bacteria; N-acylhomoserine lactone (AHL), which diffuses across the plasma membrane and is cell-density dependent