Chapter 9 Micro MAIN

Microbial Growth

Increase in number of microbial cells through cell division, primarily by binary fission

Generation Time (Doubling Time)

The time it takes for a population to double in size

Binary Fission

Most common form of bacterial reproduction involving simple cell division

Steps of Binary Fission

1. Cell growth and increase in components 2. DNA replication 3. Cytokinesis (division of cytoplasm) 4. Septum formation and separation of daughter cells

FtsZ Protein

Protein that directs cytokinesis by forming the Z ring (divisome)

Z Ring

Structure formed by FtsZ that guides septum formation during cell division

Divisome

Protein complex that activates peptidoglycan production and septum formation

Generation Time Examples

E. coli = 20 min; Staphylococcus aureus = 30 min; Bacillus subtilis = 120 min; Mycobacterium tuberculosis = 15–20 hours

Exponential Growth

Population growth where cells divide at a constant rate, doubling each generation

Population Growth Formula

Nₙ = N₀ × 2ⁿ (Nₙ = final cells, N₀ = initial cells, n = generations)

Example Population Calculation

1 cell after 16 generations = 65,536 cells

Closed Culture

Culture with limited nutrients showing predictable growth phases

Open Culture

Culture with continuous nutrient supply and waste removal (used in industrial microbiology)

Lag Phase

Cells adjust to environment; no population increase

Log (Exponential) Phase

Rapid cell division; cell replication > cell death

Stationary Phase

Nutrients depleted; replication = death; endospores may form

Death Phase

Cells die; replication < death; endospores persist

Microscopic Cell Count

Direct counting using microscope; cannot distinguish live vs dead

Coulter Counter

Measures electrical resistance changes to count cells; cannot distinguish live vs dead

Optical Density (Turbidity)

Measures cloudiness using spectrophotometer; includes live and dead cells

Fluorescent Staining

Uses dyes to differentiate live (intact) and dead (damaged) cells

Viable Cell Count

Counts only living cells using colony-forming units (CFU/ml)

CFU/ml

Colony-forming units per milliliter; measure of viable cells

Serial Dilution

Stepwise dilution used to obtain countable colony numbers

Countable Plate Range

30–300 CFU/ml (statistically accurate range)

TFTC

Too Few To Count (<30 colonies)

TNTC

Too Numerous To Count (>300 colonies)

Most Probable Number (MPN)

Statistical method for estimating low bacterial counts (<30 CFU/ml)

MPN Method

Uses multiple dilutions and replicates; growth recorded as positive/negative and compared to reference table

Budding

Asymmetrical cell division where a new cell forms from a parent

Fragmentation

Reproduction where cells break into fragments that grow into new cells

Biofilm

Microbial community attached to a surface embedded in extracellular polymeric substance (EPS)

Biofilm Formation Steps

1. Attachment 2. Irreversible attachment (sessile) 3. Growth/division 4. EPS production 5. Secondary colonization and dispersion

Extracellular Polymeric Substance (EPS)

Protective matrix produced by biofilms

Quorum Sensing

Cell-to-cell communication using signaling molecules based on population density

Autoinducers

Small signaling molecules used in quorum sensing

Quorum Sensing Molecules

Gram-negative: N-acyl homoserine lactones; Gram-positive: short peptides

Biofilm Health Risks

Increased antibiotic resistance due to inactive cells, EPS barrier, and plasmid sharing

Environmental Factors Affecting Growth

Oxygen, pH, temperature, osmotic pressure, barometric pressure

Oxygen Requirement

Classification of microbes based on oxygen use

Obligate Aerobe

Requires oxygen (e.g., Pseudomonas aeruginosa, Micrococcus luteus)

Obligate Anaerobe

Cannot tolerate oxygen (e.g., Clostridium botulinum, Bacteroides spp.)

Facultative Anaerobe

Can grow with or without oxygen (e.g., E. coli, Staphylococcus aureus)

Aerotolerant Anaerobe

Does not use oxygen but tolerates it (e.g., Lactobacillus spp.)

Microaerophile

Requires low oxygen levels (e.g., Campylobacter spp., Neisseria gonorrhoeae)

Capnophile

Requires high CO₂ levels for growth

Fluid Thioglycolate Medium (FTM)

Medium with oxygen gradient used to determine oxygen requirements

pH Effect on Growth

Affects protein function and enzyme activity

Minimum pH

Lowest pH for microbial growth

Optimum pH

pH where growth is best

Maximum pH

Highest pH for growth

Neutrophiles

Prefer pH ~7

Acidophiles

Prefer pH <5.5

Alkaliphiles

Prefer pH 8–10.5

Temperature Effect on Growth

Influences enzyme activity and membrane fluidity

Minimum Temperature

Lowest temperature for growth

Optimum Temperature

Best temperature for growth

Maximum Temperature

Highest temperature for growth

Psychrophiles

Grow at <0°C

Psychrotrophs

Grow at 4–20°C

Mesophiles

Grow at 20–40°C

Thermophiles

Grow at 50–80°C

Hyperthermophiles

Grow at 80–110°C; some survive >121°C

Osmotic Pressure

Effect of solute concentration on cell growth

Halophiles

Require high salt concentrations (e.g., ocean microbes)

Halotolerant

Can tolerate high salt but do not require it (e.g., Staphylococcus aureus)

Barometric Pressure

Pressure effects; high-pressure microbes found deep in oceans

Fastidious Organisms

Require specific nutrients to grow

Culture Media

Nutrient mixtures used to grow microorganisms

All-purpose Media

Supports growth of many organisms

Enriched Media

Contains extra nutrients for fastidious organisms (e.g., blood agar)

Selective Media

Encourages growth of certain microbes while inhibiting others

Differential Media

Distinguishes microbes based on biochemical traits

Defined Media

Exact chemical composition is known

Enrichment Media

Enhances growth of specific microbes from mixed samples

Alpha Hemolysis

Partial RBC breakdown causing green/brown color (e.g., Streptococcus pneumoniae)

Beta Hemolysis

Complete RBC lysis creating clear zone (e.g., Streptococcus pyogenes)

Gamma Hemolysis

No hemolysis (e.g., Enterococcus faecalis)

Mannitol Salt Agar (MSA)

Selective and differential media for Staphylococci based on salt tolerance and fermentation

MacConkey Agar

Selective for Gram-negative bacteria and differentiates lactose fermenters