Infectious DiseasesUnit 2 Lesson 6 plan

Do Now

You are sick and two different bacteria could be causing your symptoms. • Both types of bacteria are sensitive to different

antibiotics.

• How would you find out which antibiotic to use?

What’s growing in your nose?

1. Lets make some observations from the plates!

+ or ?

Next step: antibiotic susceptibility –

Your teacher will re-plate Staph. Aureus for antibiotic susceptibility by streaking it as a ‘lawn’.

Take a single colonyif possible

Streak in two directionsTo fill the plate

Place disks impregnatedWith antibiotic on the plate

Switching Gears Slightly

Case Study

Objective: Finding the microbes that make us sick

We need to isolate the pathogen to treat the disease

Why?

Any time you see a question with a pink box round it, answer it in your notebook.If the question doesn’t have a pink box round it, it is to help you learn.

Carlos went to a picnic yesterday. It was so hot he went to swim in the pond, even though his cousin told him the water was dirty and the flag alerted him not to swim in the water.

Case Study: Love that dirty water!

Later that afternoon…

Carlos went to the hospital. He is experiencing explosive diarrhea and he’s feeling really sick

What will the doctor do first in order to find out what is making Carlos sick?

If Carlos has caught cholera ….

Sack et al., 2004Daily Star, Bangladesh April

2009

Vibrio cholera

Rice water stool

1. Cholera is transmitted by water contaminated with feces containing vibrio cholera.

2. If you drink the contaminated water the bacteria infect your gut and suck the water out of your intestine.

3. This causes a severe watery diarrhea.

4. If you sample the stool it will have millions of vibrio cholera in it.

If Carlos has caught typhoid….

Salmonella Typhi

1. Typhoid is transmitted by water contaminated with feces containing Salmonella typhi.

2. In this case, you drink the contaminated water the bacteria perforate your gut wall and are taken up cells of the immune system.

3. This causes a severe bloody diarrhea.

4. But very few bacteria remain in the gut.

Where might the Doctors take a sample from in this case?

If Carlos has caught typhoid

There are also very few bacteria in the blood, so it is very hard to diagnose typhoid by finding the bacteria.

These are the options:

1. Grow bacteria from the blood (very few bacteria, so need lots of blood and days of culture)

2. Grow bacteria from the bone marrow where the immune cells end up (very painful to take a sample)

3. Look in the stool for any antibodies that have been made to Salmonella Typhi. This takes 2 weeks, and you might be dead by then.

4. Also, this assay only shows that a person has been infected with Salmonella typhi at some point; not necessarily this point.

OK, we’ve managed to get a sample containing bacteria

How do we know for sure what kind it is?

What might be the first thing you would do?

Try to get a look at it with a microscope?

Bacteria come in different shapes

Spheres (cocci) Rods Spirilla

Vibrio cholera (cholera) Salmonella typhi (typhoid)

What structures do they share?

Do they differ in any way that you can see?

Are the cholera and the typhoid bacteria rods, cocci or

spirilla?

Remember the two types of cell wall:

Gram positive

A thick outside wallprotects the bacteria – itwill take up blue dye

Gram negativeThe wall is thinner and in themiddle of two membranesIt doesn’t take up dyewill stay pink in color

Can we use gram stain to distinguish between them?Hint – focus on the cell wall

Vibrio cholera Salmonella typhimurium

Vibrio and salmonella – gram positive or gram negative?

Vibrio cholera Salmonella typhimurium

How could we diagnose whether Carlos has cholera or

typhoid?

Charlie was at the picnic with Carlos. He went swimming too, and ate a hamburger

Now Charlie has stomach cramps and diarrhea too. Not as bad as Carlos (yet) but he heard what

happened to his cousin, so he went to the doctor too.

Case Two: Love that greasy burger!

Design a strategy to make a diagnosis

Let’s say it could be one of two species of Salmonella

Food poisoning Typhoid

What shape? Gram positive or Gram negative?

What next?

Remember the Gram stain marks the cell wall

All gram negative bacteria have the LPS proteinThe immune system recognizes the LPS as a foreign invaderThe LPS protein is different in different bacteriaThe immune system makes different antibodies to different LPS proteins.

Where would we find the different antibodies?

Gram negativeThe wall is thinner and in theMiddle of two membranesIt doesn’t take up dyeSo easily

Wrap up

• Give some examples of diseases we still don’t understand today. Could any of them be infectious?

• How would we find out?

• What if the infectious agent only causes disease in some people – like Typhoid Mary?