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No matter how many times it is demonstrated, it’s still difficult to envision bacteria as social creatures with the ability to communicate. These simple, tiny, single-celled prokaryotes use a signaling system called ‘quorum sensing” to alter their behavior to suit the size of their population. What this means is that the bacteria actually ‘know” how many of them are present at that particular time. This “knowledge” is carried in small molecules that the lead bacteria release and the other bacteria then pick up by diffusion through their cell membrane. Previous research has shown that bacteria use quorum sensing in a number of different ways. Some bacteria use it to monitor population size in a host organism. Once they get to a certain number, they release disease-causing chemicals and overwhelm their hosts’ immune system.

The latest study involves using chemicals that mimic quorum sensing in order to control the growth of pathogenic bacteria such as salmonella or shingella. Many of these strains are resistant to most antibiotics, but when given the quorum sensing chemical mimics the bacteria stay “tame” and do not give off their pathogenic chemicals, thus allowing the body’s own immune system to wipe out the bacteria.

Use the results of the controlled experiment below, which shows the growth of shingella bacteria on agar plates with the growth of shingella bacteria on agar plates with various additives to the media (all plates have different components), as well as the information in the initial passage to answer questions 4-6 below.

Each plate is initially swabbed with 1000 colonies of shingella bacteria. All plates are incubated at 37 degrees Celsius.

Key to Data Table:

NA=Normal Agar

S=Shingella Antibiotic added to medium

Q=Quorum sensing mimic chemical added to medium

H=Human anti-shingella antibodies added to medium

$$ \text{Number of Colonies (in thousands)} $$

     Media Plate         0 Hours 6 Hours 12 Hours 18 Hours 24 Hours 48 Hours
1) $NA$ 1 5 25 75 100 350
2) $NA + S$ 1 2 7 15 35 100
3) $NA + Q$ 1 1.5 1.5 1.75 2 2
4) $NA + H$ 1 1.5 2.5 10 25 75
5) $NA + S + H$ 1 1 1.8 2.6 2.8 3.3
6) $NA + Q + H $ 1 1 1.5 .75 .01 .005

What do the results of plate 4 show about the effectiveness of human antibodies against shingella?


The antibiotic is more effective at killing the bacteria than the human antibodies.


The human antibodies are more effective at killing the bacteria than the quorum sensing chemical mimics.


Human antibodies become less effective at killing bacteria when used in combination with the antibiotic.


The shingella bacteria become resistant to the antibodies, much the same way they did to the antibiotic.

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