2A Intrinsic and Extrinsic Factors Affecting Microbial Growth (1)

Microorganisms

can occur from any level from farm to pork processing

Sublethal injury microbes exposed in stress

won’t grow/manifest colony once you let them in that environment, they can later adapt the stress from the environment

Adapted cells

can adapt from harsh environment

Non-adapted cells

can’t change/adapt from their environment

Holoenzyme

core enzyme + sigma factor

sigma (σ) factor in prokaryotes,

is the subunit of the RNA polymerase that chooses which SPECIFIC gene to transcribe

σ^70 (RpoD) (σA)

• it is also called “housekeeping sigma”

• start RNA transcript, ensures that everything is in orders

σ^19 (Fecl) ferric citrate sigma factor

regulates the fec gene for iron transport (when lack of iron)

σ^24 (RpoE)

• extracytoplasmic/extreme heat stress sigma factor

• temperature change to extreme heat level

σ^28 (RpoF), flagellar sigma factor

threat found and microorganism should run

σ^38 (RpoS)

• starvation/stationary phase sigma factor

• when the environment is lacking of food/nutrient, we slow down the metabolism to prevent loss of food in the environment

σ^54 (RpoN)

• nitrogen-limitation sigma factor

• limit of nitrogen in the environment, thus it activate

σ^32 (RpoH) heat shock sigma factor

• most important sigma factor

• Some of the enzymes that are expressed upon activation are chaperone, proteases and DNA-repair enzymes.

• is only translated at high temperatures

Thermometer RNA

sudden change results to sudden transcribe (constituvely transcribed)

The secondary structure of the transcript at low temperatures

prevents ribosome binding and translation

Reduction in water activity

increases the population doubling time of microorganisms in food.

E. coli K-12

is one of the good e. coli

Microbial adaptation and change

Diseases can come back any time because of (??)

Intrinsic parameters

The characteristics inherent (naturally occuring) to a food system

σ (sigma factor)

• special subunit of protein

• chooses which gene will be transcribed in response to a particular stimulus

Start RNA transcript

ensure that everything is in order

Bacteria

pH 6.0 to 8.0

Yeasts

pH 4.5 to 6.0

Filamentous fungi

3.5 to 4.0

Optimum Growth (OG)

where microrganisms love to grow

High Acid

1.0 - 4.5

Low Acid

4.5 - 7

Microbial growth

is not solely dependent on pH alone

Clostridium Botulinum

most dangerous in alkaline foods

Acidophilic

acid-loving

Neutrophilic

neutral pH loving

Alkaliphilic

loves alkaline environment

↓ pH, acid

↑ pH, alkalinity

pH minima of certain lactobacilli

have been shown to be dependent on the type of acid used in adjusting growth medium pH.

Citric, hydrochloric, phosphoric and tartaric acids

were found to favor growth at lower pH than acetic or lactic acid

pH values around 7.0 (6.6-7.5)

Most microorganisms grow best at (???)

Bacteria

are more fastidious in their relationships to pH than yeasts and molds

Fruits

undergo mold and yeast spoilage because these organisms grow at pH <3.5

Meats and seafoods

have final pH of about 5.6 and above, making them susceptible to both bacterial and fungal spoilage.

Acetic acid, Citric acid, Lactic acid, Propionic acid, Malic acid, Succinic acid, and Tartaric acid

all these acids lower the pH of meat products

The dependence of microbial inactivation and growth on pH

is the basis of the severity of a food process

Yeasts and molds

are more tolerant to pH variations in food

Bacteria

are more exacting when it comes to their requirement of environment pH

 Most microorganisms

have optimal growth at pH near neutrality

Acidification of food

is more antimicrobial at higher temperatures

The efficacy of acidulant

is highly dependent on the pH of the food system

Acidification

increases the susceptibility of microorganisms to other antimicrobials

Hydrogenase catalysis

In E. coli exposed to acid challenge, this mechanism converts and expels internal hydronium ions into hydrogen gas

Reduction in water activity

increases the population doubling time of microorganisms in food.

Meat from fatigued animals

spoils faster than from those of rested ones

AUG

Start codon

Glycogen

If the meat is stressed they release (???), once this is degraded it produces lactic acid environment which means bacterial contamination

Meats

are also able to resist changes in pH than other products like fruits and vegetables

The production of acidic compounds

Some food products are preserved through (??), due to the actions of certain microorganisms

Fermented milks, sauerkraut, pickles, etc

They are best to be at acidic environment

Adverse pH changes

could affect at least 2 aspects of a respiring cell

Enzymes, H20, and nutrients

are in and out in the cell

Near neutrality

internal pH of most microorganisms appears to be (??) despite the changes in environmental pH

5.8

The intracellular pH of S. cerevisiae was found to be (??)

The microorganisms

In general, most of them are neutral

H+ and -OH

Inside the cell (cytoplasmic membrane) the (??) can’t enter there

Near 7 or neutral

internal pH of our microorganisms is (??)

Cl. acetobutylicum

when placed in acidic medium, it reduces butyric acid to butanol

Enterobacter aerogenes

produces acetoin from pyruvic acid

fatty acids increases

↑ Cyclophanes

Hydrogenase

pumps out 2H+ as H2

Proton -pumping respiratory chain complexes

is upregulated (Nuo and Cyo both expels H+)

Amino acid decarboxylase -antiporter pairs upregulated

• glutamate enters

• convert to GABA

• it will exit GABA

Tryptophanase (TnaA) is upregulated

it enters in the internal of E.coli and it will converse to indole and pyruvate

Indole and pyruvate

will release Na+ and enters 2H+

Cations (+)

can only be attach to the cell, hence can’t enter since it is charge particles

AH

whole form of organic acids

A- + H+

In neutral environment it becomes (??)

Lower than pka

pH of our food should be (??) for backward reaction to favor (acidic environment)

pKa

should be greater than the pH of the food

food has high moisture

has lots of water

Water Activity

there’s minimum requirement of water for microorganisms to grow there

Water activity (aw)

is defined by the ratio of water vapor pressure of the food substrate to the vapor pressure of pure water at the same temperature.

Moisture

total water where microorganisms can grow

0.99

maximum water activity

0.6 or 0.5

minimum water activity (minimum amount of water for microorganisms to grow)

Lower water activity

means large amount of solute is dissolved in there

High water activity

means small amount of solute is dissolved there

Higher water activity

lower % w/v

lower water activity

higher % w/v

1.0

most microorganism will grow in this water activity

PHF (potentially hazardous foods)

microorganisms can grow in here faster because of its water activity level

S. Aureus

can grow in water amount of 0.85 and below

Reduced

At any temperature, microbial growth rates decrease when aw is (??)

The general effect of lowering aw below optimum are

• increase in the length of the lag phase

• decrease in the growth rate

• decrease in the size of the final population

Aqueous milieu

microorganisms biochemical reactions occur in an aqueous environment

Microorganisms

can prevent loss of water

The ability of microorganisms to concentrate molecules within the cells

prevents loss of water and allows the cells to extract moisture from the external environments.

Food systems high in proteins

are easily spoiled by microorganisms, particularly by bacteria

Amino acids.

The primary N source utilized by heterotrophic microorganisms

Simpler substrates first

In general microorganisms will assimilate the(??) than the complex ones