Gram Staining

There are two types of cell walls in Eubacteria:

– Gram-positive bacteria have a thick peptide layer, retain the crystal violet, and appear purple when viewed under a microscope.

• Gram-negative bacteria have a thin peptide layer and space surrounding it so the cell wall is more protected. They take up the pink stain, and appear pink when viewed under a microscope.

Gram Staining Technique

• Cells on a microscope slide are stained with a purple dye solution called crystal violet.

• The cells are then washed with water.

• An iodine stain solution is applied to the cells.

• The slide is then washed with alcohol.

• Finally, the cells are restained with a pink dye solution called safranin.

Explanation

• Photograph A: E. coli a common gram-negative rod found in the colon – single cell

• Photograph B: Staphylococcus epidermidis a gram-positive cocci found on the skin – clusters or chains

• Photograph C: Bacillus cereus a gram-positive rod found in the soil- chains

Killing Eubacteria

– There are many chemicals that are lethal to bacteria:

• cyanide does a good job — but it’s lethal to the host as well.

– The way antibiotics work:

• Gram positive bacteria tend to be killed by antibiotics such as penicillin and erythromycin which target the cell wall.

• Gram negative bacteria are resistant to these drugs because their cell wall is protected, but are sensitive to streptomycin and tetracycline.

• Broad spectrum antibiotics target either the bacteria’s ability to make proteins or replicate their DNA.

Evolution of Antibiotic Resistance:

• 40-50 years ago, thanks to antibiotics, scientists thought medicine had all but eradicated infectious agents as a major health threat.

• More recently, an upsurge of infectious disease is a problem we have unwittingly created for ourselves b/c:– rapid, frequent, and relatively cheap international travel

allows diseases to leap from continent to continent – Many people have inadequate sanitation and lack of

clean drinking water – We have overused the "miracle drugs“ to treat such

diseases to the point that they lose their potency

• Whenever antibiotics are used, a few of the “enemy” are able to survive the drug.

• Mutations- – Because microbes reproduce so quickly and often some random

mutation eventually will protect against the drug. • When antibiotics are used only when needed and as

directed they usually overwhelm the bugs. • By using antibiotics too often, more resistant

mutants arise. • When patients cut short the full course of drugs, the

resistant strains have a chance to multiply and spread.

Controlling Microbial Growth

Illness CauseWill

AntibioticsWork?

Strep Throat Streptococcus bacteria Yes  

"Common Cold"Over 200 different

rhinovirusesNo  

AIDS - Acquired Immune Deficiency

Syndrome

Human Immunodeficiency Virus (HIV)

No  

"Mono" (Infectious Mononucleosis)

Epstein Barr Virus orCytomegalovirus No  

Common Ear Infection Usually bacteria Usually  

Chlamydia (STD)Chlamydia tachomatis

bacteriaYes  

Herpes Herpes simplex virus No  

How Bacteria Cause Disease

• Metabolizing the host-– Heterotrophic bacteria obtain nutrients by

secreting enzymes that break down organic structures and absorb them

– If the environment is your throat or lungs, this can cause serious problems!

• Ex. – Tuberculosis settles into the lungs and use human tissue

as their nutrients– Propionibacterium acnes causes acne

• Toxins-Some bacteria secrete chemical compounds into their environment which are poisonous to eukaryotic cells (toxic)

– Ex. Diptheria grows in the throat, but the toxins attack the heart, nerve, liver and kidneys

– Food poisoning occurs when humans eat food where bacteria have grown and produced toxins

– One milligram of pure Botulinum toxin is enough to kill 1 million guinea pig

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Controlling Bacteria

3 ways to control bacteria:

1) Heat

• Canning the process of sealing food in airtight cans or jars after killing bacteria

•endospores are killed during this process

• Pasteurization- process of heating milk to kill harmful bacteria

2) Dehydration- removing water from food

•Bacteria can’t grow when H2O is removed

•example: uncooked noodles & cold cereal

3) Freeze- inhibits bacteria

Bacteria can also be Helpful!• Decomposers help recycle nutrients into

the soil for other organisms to grow

• Bacteria grow in the stomach of a cow to break down grass and hay

•Used to treat sewage          Organic waste is consumed by the bacteria, used as nutrients by the bacteria, and is no longer present to produce odors, sludge, pollution, or unsightly mess.

•foods like yogurt, cottage & Swiss cheese, sour cream, buttermilk are made from bacteria that grows in milk

Helpful Bacteria