Growth of Bacteria in Food

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Growth of Bacteria in Food

• FT 3103
• Food Microbiology

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At 1100, we place a single bacterium in a
bottle. It's so small you'd need a microscope to
see it.
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In one minute it grows to twice its original
size and divides in half, reproducing itself, so
at 1101 there are two bacteria in the bottle.
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The bacteria continue growing and dividing,
doubling their numbers every minute, so by 1102
there are four, and by 1103 there are
eight.(they're not getting smaller we're just
zooming out so we can see them all. Each
bacterium is as big as the original one was.)
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At the end of five minutes, there are 32 bacteria
where there used to be just one ... but even all
together they're still so small they can't be
seen without a microscope.
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The bacteria keep doubling their numbers every
minute, until 1200, when the bottle fills up.
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When the bottle has half full of bacteria?
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That's right! Since the bottle filled up at
1200, it must have been half full just a minute
before. At 1145 we could just barely see the
bacteria, and at 1130 we still needed a
microscope!
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The bacteria's growth over time can be graphed
like this -gtThis is called "exponential"
growth. Many people have trouble understanding
exponential growth, because we're used to things
growing "linearly" -- the same amount from one
day to the next, like hair or grass or
fingernails.We expect the bottle to fill with
bacteria as if we were filling it with water from
a faucet. But any time living things are allowed
to reproduce freely, their numbers increase
exponentially, not linearly. And when people talk
about "steady growth," they mean exponential
growth then, too.
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If you were one of the bacteria, when do you
suppose you'd start to worry about
overcrowding?Would that leave you enough time to
do anything about it?
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Now imagine that just before 1200, we bring in
three more bottles.

• If we can help the bacteria to spill over into
  the other bottles, they'll have four times as
  much space as they've ever had before!
• How much time after 1200 do you think this will
  give them?
• 3 hours?30 minutes?15 minutes?2 minutes?
• 1 minute?

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At 1201
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At 1202
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Bacterias life

• Bacteria are all around us. Given good growing
  conditions,
• a bacterium grows slightly in size or length,
• a new cell wall grows through the center forming
  two daughter cells, each with the same genetic
  material as the parent cell.
• If the environment is optimum, the two daughter
  cells may divide into four in 20 minutes. Oh my!
  1, 2, 4, 8, 16, 32, 64...
• Then why isn't the earth covered with bacteria?
• The primary reason may be that conditions are
  rarely optimum.
• Scientists who study bacteria try to create the
  optimum environment in the lab culture medium
  with the necessary energy source, nutrients, pH,
  and temperature, in which bacteria grow
  predictably.

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Microbial Growth

• refers to the growth of a population (or an
  increase in the number of cells),
• not to an increase in the size of the individual
  cell.
• Cell division leads to the growth of cells in the
  population.

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Two Types of Asexual Reproduction in Microbes

• 1.)   Binary Fission
• 2.)   Budding

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1.)   Binary Fission

• Bacterial reproduction occurs through fission, a
  primitive form of cell division that does not
  employ a spindle fiber apparatus.
• A spindle fiber apparatus made of protein
  filaments is responsible for moving the
  chromosomes around during cell division (mitosis
  meiosis) in most eukaryotic cells. Bacteria do
  not have these structures.
• The bacterial cell doubles in size and replicates
  its chromosome.
• Following DNA replication, the two chromosomes
  attach to separate sites on the plasma membrane,
  and the cell wall is laid down between them,
  producing two daughter cells.

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2.)   Budding

• A few bacteria and some eukaryotes (including
  yeasts) may also replicate by budding,
• forming a bubble-like growth that enlarges and
  separates from the parent cell.

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Phases of Growth

• A microbial lab culture typically passes through
  4 distinct, sequential phases of growth that form
  the standard bacterial growth curve
• 1. Lag Phase
• 2. Log Phase
• 3. Stationary Phase
• 4. Death Phase

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Phases of Growth

• LAG PHASE Growth is slow at first, while the
  "bugs" acclimate to the food and nutrients in
  their new habitat.
• LOG PHASE Once the metabolic machinery is
  running, they start multiplying exponentially,
  doubling in number every few minutes.
• STATIONARY PHASE As more and more bugs are
  competing for dwindling food and nutrients,
  booming growth stops and the number of bacteria
  stabilizes.
• DEATH PHASE Toxic waste products build up, food
  is depleted and the bugs begin to die.

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Standard Bacterial Growth Curve
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Factors Affecting the Growth of Microbes

• extrinsic factors
• temperature
• oxygen requirements
• interaction between microbes
• intrinsic factors
• pH
• Aw
• nutrition
• Eh
• biological structures

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Extrinsic Factors

• Temperature bacteria can be classified as
• psychrophiles (cold-loving) 15 C to 20 C some
  can grow at 0 C.
• mesophiles - grow best between 25 C and 40 C
  human body temp is 37 C.
• thermophiles (heat-loving) 50 C to 60 C found
  in compost heaps and in boiling hot springs.

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Extrinsic Factors

• Oxygen Requirements
• strict or obligate anaerobes oxygen kills the
  bacteria ex. Clostridium tetani
• strict or obligate aerobes lack of oxygen kills
  the bacteria ex. Pserdomonas
• facultative anaerobes can shift their
  metabolism (anaerobic if oxygen is absent or
  aerobic if oxygen is present) ex. E. coli,
  Staphylococcus
• aerotolerant the bacteria dont use oxygen, but
  oxygen doesnt harm them ex. Lactobacillus
• microaerophiles like low oxygen concentrations
  and higher carbon dioxide concentrations ex.
  Campylobacter

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Extrinsic Factors

• Interaction between Microbes
• Interactions are both positive and negative. 
• Dominant microbes tend to be the best competitors
  for nutrients in a food,
• some others produce inhibitory products,
• other produce stimulatory products which
  stimulate other species in the food to grow....

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Intrinsic Factors

• pH
• acidophiles (acid-loving) grow best at a pH of
  1 to 5.4 Ex. Lactobacilllus (ferments milk)
• neutrophiles exist from pH to 5.4 to 8.5 most
  bacteria that cause human disease are in this
  category.
• alkaliphiles (base loving) exist from pH to 7.0
  to 11.5 ex. Vibrio cholerae (causes cholera)

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Intrinsic Factors

• Aw
• Most microbes require high Aw for growth the
  Aw should be above 0.9.             
• Aw  (vapor pressure of substance in
  solution)/(vapor pressure of water)
• Water activity is affected by many things 
  solutes, surfaces, and ice.  All these things
  lower Aw  and when the Aw  gets below the minimum
  for growth for each microbe, growth will not
  occur.  This is one of the oldest methods of food
  preservation  drying, salting.     
• Effect of Aw  on growth affects BOTH lag phase
  and growth rate.
• When Aw  gets below the minimum for
  growth....cells begin to DIE due to osmotic
  damage which can be brought about by 
• damage to cell membranes,
• damage to cell membrane proteins,
• damage to cytoplasmic enzymes. 
• Foods are classified by aw  
• very high moisture foods (0.95-0.99)
• high moisture foods (0.90-0.95)
• intermediate moisture foods (0.61-0.9)
• low moisture foods (lt0.61). 

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Intrinsic Factors

• Nutrition - Nutrients needed by microorganisms
  include
• Carbon carbon containing compounds are needed
  as
• an energy source (ex. glucose)
• for building blocks.
• Nitrogen
• needed for amino acids and nucleotides
• some can synthesize all 20 amino acids
• others have to have some provided in their
  medium.
• Sulfur needed for amino acids, coenzymes,
• Phosphorus needed for ATP, phospholipids, and
  nucleotides
• Vitamins
• a vitamin is an organic substance that an
  organism requires in small amounts and that is
  typically used as a coenzyme
• many bacteria make their own, but some are
  required in the medium
• microbes living in the human intestine
  manufacture vitamin K, needed for blood clotting,
  and some of the B vitamins, thus benefiting their
  host.
• Certain trace elements ex. copper, iron, zinc,
  sodium, chloride, potassium, calcium, etc.
• often serve as cofactors in enzymatic reactions.

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Intrinsic Factors

• Redox Potential (Eh)
• The redox potential (Eh) of a medium is a measure
  of the medium's overall ability to reduce or
  oxidize referenced to the hydrogen-electrode. 
• The major thing here is that aerobes need a more
  oxidizing environment and
• anaerobes tend to require a more reducing
  environment, particularly the obligate anaerobic
  Clostridium species. 

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• The D value or Decimal Reduction Time (DRT),
• The time required to kill 90 of the
  microorganisms in a sample at a specified
  temperature. Only 10 or 1/10 of the original
  number of microbes survives the Decimal Reduction
  Time.

Used in canning industry.
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When bacterial populations are heated or treated
antimicrobial chemicals, they usually die at a
constant rate.
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Presented by

• Ms. Krissda T. ID 4518709
• Mr. Surat P. ID 451