Community• Group of interacting populations• Succession: a gradual change in a community’s species

composition over time• Two major types

Types of succession• Primary succession:

community development on a new site never before colonized by living things • Example: glacier or volcano

• Secondary succession: community development on a site that has already supported life • After disturbance • Example: hurricane, fire

More about succession• Usually an orderly, predictable process but slow • Community increases in complexity and biodiversity

• Pioneer species very simple

• Most advanced community is the climax community • A disturbance can reset succession to an earlier stage

• Example?

Today’s lab• Observe succession of bacteria communities in different

types of milk • Bacteria reproduce quickly • Milk is highly nutritious with carbohydrates, proteins, and lipids

Stages of milk succession• Psuedomonas and Achromobacter (gram-negative rods) digest

butter fat and give milk a putrid smell • Lactobacillus (gram-posi1ve rod) and Streptococcus (gram-

positive coccus) ferment lactose to lac1c acid and ace1c acid.• Acidity sours milk and converts casein to curd.• Yeast (fungi) thrive in acidic condi1ons and metabolize the acids

into non-acidic compounds.• Bacillus (gram positive) metabolize proteins into ammonia

products and raise the milk’s pH. Spoiled milk odor is very noticeable at this stage.

Design two experiments• First experiment: related to the different types of milk

(whole, skim, etc.) • Look at the nutritional information

• Second experiment: investigate temperature or amount of time/age of milk

• Use the scientific method! • Hypothesis/null hypothesis • Identify independent and dependent variable • Identify control and experimental group • Identify standardized variables (constant)

Test the milk samples• • Make sure you check:

• pH (use Vernier probe)• Color • Odor• Consistency

More about bacteria• Bacteria have three different

shapes: • Spirilla (spiral) • Bacillus (rod) • Coccus (sphere)

Gram stain technique• Distinguishes between gram +

and gram – bacteria • Different cell walls

• Gram +: high in peptidoglycan (sugars and amino acids) • Stains violet

• Gram –: less peptidoglycan and outer lipopolysaccharide layer • Stains pink

Gram stain procedure: 4 steps

1. Primary stain (Crystal violet): stains bacteria

2. Mordant (iodine): sets stain

3. Decolorization (alcohol): gram negative bacteria will lose their color when alcohol is applied to the sample

4. Counterstain (safranin): stains gram negative bacteria

http://www.youtube.com/watch?v=8zd3HPRxx1U

Gram-positive cocci

Gram-negative bacilli

A few additional notes• Only 3 gram staining kits

• We need to share

• Make sure tubing on Bunsen burners is not twisted

• Sign out microscopes • Ask if you need a review of microscope

procedures• Please put scopes away correctly

• Stage down, scan objective • After viewing slides please rinse and

place in bucket to soak • Please wash out all beakers • Have fun!