control of microbial growth Flashcards
kinetics of microbial growth, microbial numbers, control microbial growth, environmental factors
macronutrients
required in large amounts and from the buld of the bacterial cell’s biomass. contribute to cell structure and energy metabolism.
micronutrients
requires in smaller amounts but essential to enzymatic activity and cellular processes.
trace elements
required in even smaller amounts but are still vital for specific cellular functions.
microbial growth
- increase in the number of cells
- binary fission: one cell grows full size, duplicates DNA then divides to form two daughter cells.
generation time or doubling time
- time it takes for the population to double in number
- during exponential growth doubling time is constant.
microbial growth kinetics
why does exponential growth stop?
reduction in nutrients, accumulation of waste products
bacterial growth stages
lag phase
intense activity in the cell to prepare for growth, cells are adapting but not growing.
bacterial growth stages
log phase
exponential phase, doubling at a constant rate, doubling time is constant.
stationary phase
period of equlibrium, cell death=cell growth
death phase
cells begin to die, no cell growth
methods to monitor microbial growth
- direct microscopic count: both alive and dead cells
- viable plate count: counts colonies that grow
- turbidity/optical density: not exact count of cell number
direct microscopic count
Uses a hemocytometer viewed under a microscope.
Counts total cells but cannot differentiate live vs. dead without special stains.
Viable Plate Count (CFU/ml)
10-fold serial dilutions of culture.
Plated on agar, incubated, then colonies counted.
Only viable (living) cells counted.
Uses 30-300 colonies for accurate results.
Formula: CFU/ml = (colonies × dilution factor) / volume plated.
Turbidity / Optical Density (OD600)
Measured with a spectrophotometer.
OD ≠ absorbance; bacteria scatter light, not absorb it.
Directly proportional to cell concentration within OD 0.1–1.0 range.
key terms direct microbial growth
Sterilization: Removes all microbes, including spores (e.g., autoclaving).
Disinfection: Destroys most microbes on surfaces (e.g., bleach).
Antisepsis: Destroys most microbes on living tissues (e.g., iodine).
Sanitization: Reduces microbes to safe levels (e.g., dishwashing).
Bactericidal: Kills bacteria.
Bacteriostatic: Inhibits growth without killing.
physical methods of control
heat treatment
Moist Heat (denatures proteins & destroys membranes):
Boiling: Kills most pathogens (not spores).
Autoclaving: 121°C, 15 psi, 15-20 min (sterilizes).
Dry Heat (oxidizes & denatures proteins):
Hot air ovens: 170°C for 2 hours.
Incineration: Destroys biohazards.
Pasteurization (kills pathogens in liquids):
LTLT: 63°C for 30 min.
HTST: 72°C for 15 sec.
physical methods of control
cold treatment
Refrigeration (4°C): Slows growth.
Freezing (-20°C or lower): Stops growth but doesn’t kill.
physical methods of control
radiation
Ionizing (X-rays, gamma): Sterilizes by damaging DNA.
Non-ionizing (UV): Causes thymine dimers in DNA.
physical methods of control
desiccation and osmotic pressure
Drying: Inhibits microbial growth (e.g., dried food).
High salt/sugar: Draws water out of cells (e.g., salted meat).
Chemical Methods of Control
Disinfectants
Alcohols (70%): Denature proteins & disrupt membranes.
Halogens:
Chlorine: Water treatment.
Iodine: Surgical antiseptic.
Phenolics: Disrupt membranes & proteins (e.g., Lysol).
Aldehydes: Cross-link proteins & DNA (e.g., glutaraldehyde).
Chemical methods of control
Antiseptics
Hydrogen peroxide: Produces free radicals.
Alcohol-based sanitizers: Kill bacteria & viruses.
chemicsl methods for control
Antimicrobial Agents
Antibiotics: Target bacteria.
Antifungals: Target fungal structures (e.g., ergosterol).
Antivirals: Inhibit virus replication.
Antibiotics and resistance
Common Antibiotics & Targets
Penicillin, Vancomycin, Bacitracin: Cell wall synthesis.
Daptomycin: Cytoplasmic membrane.
Quinolones: DNA gyrase.
Rifampin, Actinomycin: RNA polymerase.
Puromycin, Streptomycin: Ribosomes.
antibiotics and resistance
Antibiotic Resistance Mechanisms
Intrinsic resistance: Bacteria naturally resistant.
Acquired resistance:
Mutations or gene transfer.
Resistance mechanisms:
Limiting drug uptake.
Modifying the drug target.
Inactivating the drug.
Efflux pumps remove drug.
antibiotics and resistance
Example: MRSA (Methicillin-resistant Staphylococcus aureus)
Methicillin targets penicillin-binding proteins (PBPs) in bacterial cell walls.
MRSA modifies PBPs, so methicillin cannot bind → resistance.
Mechanism: SCCmec gene transfer → mecA gene → modified PBP2a
antibiotics and resistance
Antibiotic Persistence
Some cells (persisters) tolerate antibiotics but are not resistant.
Genetically identical to normal cells.
After treatment, persisters regrow, leading to chronic infections
Environmental Factors Affecting Growth
Temperature Ranges
Psychrophiles: Grow in cold.
Mesophiles: Grow in moderate temperatures (human body).
Thermophiles: Grow in hot.
Hyperthermophiles: Grow in very hot (e.g., deep-sea vents)
Environmental Factors Affecting Growth
pH Preferences
Acidophiles: Prefer acidic environments.
Neutrophiles: Prefer neutral pH.
Alkaliphiles: Prefer alkaline conditions
Environmental Factors Affecting Growth
Oxygen Requirements
Obligate aerobes: Require oxygen.
Facultative anaerobes: Grow with or without oxygen.
Obligate anaerobes: Die in oxygen.
Aerotolerant anaerobes: Ignore oxygen.
Microaerophiles: Need low oxygen.
