Bacteria are everywhere!  (Pages 324-333)

Note: not all bacteria cause disease.  Some are helpful.  We use them to make yogurt, to give wine its subtle flavors, to make hard sausages and pickles. 

Some estimates state that humans have as many bacteria living in them as they have human cells (you are 50% bacteria).  These bacteria reside on the skin and in your gut.  They do not live in your blood.  We also need them for proper digestion and to help defend us against disease causing bacteria.  For example: one kind of Staphylococcus (epidermis) helps protect us against one of our most nasty pathogens Staphylococcus aureus.  Some researchers suggest that bacteria in our gut may produce many vitamins, may compete with dangerous bacteria for living space, and may breakdown some environmental poisons.

Bacteria need very little to survive.  Water (in the form of moisture), an energy source (like sugar), and a few salts are enough for them to make everything else they need to grow and reproduce.  [We can't even do that!]

Bacteria multiply rapidly.  They use a process called binary fission (dividing in two).  This is similar to the cell division our cells do (mitosis).  Because bacteria have no nucleus, they can divide a lot faster.  (Some can divide every 20 minutes in a food-rich environment).  Their numbers increase exponentially: 1 begets 2 begets 4 begets 8 begets 16 begets 32 begets 64 begets 128 begets 256 begets 512 begets 1024 begets 2048 begets 4096 begets 8192 begets 16,384 begets 32,768 begets 65,536 begets 131,072.  If we assume that an average bacteria can divide every hour, in 18 hours we have over 100,000 bacteria starting from only 1.

We can count colonies (the pile of bacteria in one spot) to count the number of bacteria in a sample.  Bacteria don't swim well across an agar plate.  Therefore, all the bacteria produced from a single original bacteria show up on the plate as a spot or mound. 

[Agar is a derivative of seaweed that gels like gelatin, but is inedible to most bacteria.  You can use gelatin to grow bacteria, but they may eat it and turn it all to liquid slush.]

Nutrient Agar contains Glucose (a source of carbon, hydrogen, and oxygen (and energy)), tryptone ( a source of amino acids, sulfur, and nitrogen), and some salts containing additional sulfur and phosphorus (CHOPSN - 6 needed chemical elements for life)

Manitol-Salt agar is SELECTIVE for Staphlococcus by the addition of table salt.  Few other bacteria survive or grow in high salt concentrations (hence the reason for salting food as a preservative).  M-S agar is also DIFFERENTIAL as only the deadly Staph. aureus will turn the medium yellow by fermenting mannitol.

We swabbed our oral cavities (mouths) then transferred the swab to an Agar slant. 

After 2 days we counted the colonies and recorded the results.

Initial Characterization of Bacterial Cultures.

Some bacteria form distinctive growth patterns and/or have unique metabolic characteristics.  These can be used to identify bacteria by growing them on selective or differential media and observing the colonies, or lack thereof, that grow.

Colony (colonies)- growth of microorganisms, which is visible to the naked eye, on solid culture media.  The colony (theoretically) results from the reproduction of a single cell.

Medium (media) - the material, solid or liquid, that supports growth/reproduction of microorganisms

Agar - a dried polysaccharide extract of red algae used as a solidifying agent in microbiological media.  It has no nutritional value.

Trypticase Soy Agar - A general-purpose medium for growing a large variety of microorganisms.  Contains digests of casein and soybean meal, a small amount NaCl and agar.  The growth on the Trypticase Soy Agar portion of the plate (straw colored) should be greater than that on the Mannitol Salt because it does not contain any inhibiting or selective chemicals.  Note these are possibilities ONLY, not true identification of the bacterial forming your colonies.

Selective medium - incorporates chemical compounds which permit the growth of specific groups of microorganisms while inhibiting the growth of other types.

Differential medium - incorporates substances that produce visible changes permitting differentiation (sometimes identification) of particular microorganisms.

For Example: Mannitol Salt Agar - contains 7.5% NaCl - inhibits growth of most bacteria except for the staphylococci - also contains the sugar mannitol and the pH indicator phenol red - Mannitol fermenting staphylococci produce a yellow zone around the colonies - usually indicative of Staphylococcus aureus.  Staphylococcus epidermidis usually does not ferment mannitol and therefore there is no color change.  This is a selective and differential medium.  Any growth that occurs on the Mannitol Salt portion of your dish probably belongs to the genus Staphylococcus.  If the area around the colonies on the Mannitol Salt Agar is yellow, the organisms might be a Staphylococcus aureus.  This organism can cause serious disease if it grows in some parts of your body (lungs).  However, it is usually present and harmless on your skin and in your nasal cavity.

Initial Observations and Possible (not positive) Identification

Dry, whitish colonies may be Bacillus sp.

Feathery white growth may also be Bacillus sp.  This organism is normally found in soil and dirt.

Creamy white colonies may be yeast.  This is carried naturally by some people.

Yellow to orange colonies may be Flavobacterium - also a normal soil and water organism.

Fuzzy growth with or without black "dots" is mold or fungus, a normal soil and air organism.

Light yellow or whitish small colonies may belong to the genus Staphylococcus or Micrococcus.  Both are normal skin flora.

Microscopic demonstrations of some of the above organisms will be available.  Record your results.  Answer the following questions

What kind of bacteria may be present?

What other kinds of microbes do you find?

What is the significance of the sampling procedures?

What additional tests are necessary for positive identification of these bacteria?

For example: To be more definitive in identifying the bacteria we start with microscope observations (for bacterial shape) and differential staining, such as the Gram Stain, which divides bacterial into two broad groups.  However, please note that this does not provide positive identification of individual types.

Gram Positive

          - Rods

                   Bacillus

                   Clostridium

                   Mycobacterium

                   Lactobacillus

                   Corynebacterium

          - Cocci

                   Streptococcus

                   Aerococcus

                   Micrococcus

                   Staphylococcus

Gram negative

          - Rods

                   Alcaligenes

                   Pseudomonas

                   Citrobacter

                   Enterobacter

                   Serratia

                   Proteus

                   Salmonella

                   Shigella

                   Klebsiella

                   Escheria coli

          - Cocci

                   Acinetobacter

                   Moraxella (Branhamella)

                   Neisseria

Gram Variable – Branhamella and Neisseria

Gram non-reactive – various spirochaetes