Staphylococcus Species Characteristics Quiz

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Questions and Answers

What indicates an antibiotic's effectiveness in relation to a bacterial strain?

  • Formation of a clear zone around the antibiotic disk (correct)
  • Thickness of the agar used
  • Presence of a color change in the agar
  • Growth density of bacteria in the agar

Which factor is NOT listed as influencing the size of an inhibition zone?

  • Concentration of the antibiotic
  • Agar depth
  • Diffusion rate of the antibiotic
  • Size of the bacteria (correct)

How is the size of the zone of inhibition measured?

  • Measuring the diameter in millimeters with a ruler or caliper (correct)
  • Measuring the thickness of the agar layer
  • Using a stopwatch to time bacterial growth
  • Estimating by visual inspection

What classification corresponds to a small or no zone of inhibition?

<p>Resistant (B)</p> Signup and view all the answers

What does a susceptible reaction indicate for a bacterium?

<p>The bacterium is inhibited or killed by the antibiotic (D)</p> Signup and view all the answers

Which statement correctly describes an intermediate reaction?

<p>The bacterium may require higher concentrations for inhibition (C)</p> Signup and view all the answers

Which species is indicated to be capable of mannitol fermentation?

<p>S.aureus (Sa) (B), S.saprophyticus (Ss) (C)</p> Signup and view all the answers

What is the result of the coagulase test for S.epidermidis (Se)?

<p>Negative (B)</p> Signup and view all the answers

What characteristic of Staphylococcus sp. is indicated by a Gram stain reaction?

<p>They have a thick peptidoglycan layer and stain purple. (C)</p> Signup and view all the answers

What is the morphology and arrangement of Staphylococcus sp.?

<p>Coccus shaped and arranged in clusters. (B)</p> Signup and view all the answers

What is the selective agent in MSA agar that favors the growth of Staphylococcus sp.?

<p>High salt concentration (7.5%). (B)</p> Signup and view all the answers

What does a positive DNase test indicate?

<p>There is a clear zone around the bacterial growth after incubation. (C)</p> Signup and view all the answers

What is the purpose of the coagulase test?

<p>To identify whether the organism can clot fibrinogen. (B)</p> Signup and view all the answers

In the Kirby-Bauer method, what is measured to evaluate the effectiveness of antibiotics?

<p>The zones of inhibition around antibiotic disks. (A)</p> Signup and view all the answers

What indicates that a Staphylococcus species ferments mannitol on MSA agar?

<p>The medium turns yellow due to acid production. (B)</p> Signup and view all the answers

What type of reaction is associated with the DNase enzyme in the DNase test?

<p>It breaks down DNA into nucleotides. (B)</p> Signup and view all the answers

Flashcards

Antibiotic Inhibition Zone

The clear area around an antibiotic disk on agar where bacteria cannot grow, indicating antibiotic effectiveness.

Measurement of Zone Size

The diameter of the inhibition zone is measured in millimeters using a ruler or caliper.

Susceptible Bacteria

Bacteria that are inhibited or killed by the antibiotic at the tested concentration.

Resistant Bacteria

Bacteria that survive and grow in the presence of the antibiotic, indicating ineffectiveness.

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Intermediate Bacteria

Bacteria that show a variable response to the antibiotic, requiring interpretation for treatment.

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Factors Influencing Inhibition Zone Size

Includes antibiotic concentration, diffusion rate, inoculum size, temperature, growth rate, and agar depth.

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Mannitol Fermentation (S.aureus)

Positive result indicates the ability of S.aureus to ferment mannitol, turning MSA plate yellow.

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Coagulase Test Result (S.epidermidis)

S.epidermidis is negative for coagulase, indicating it does not clot blood.

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Staphylococcus sp

Gram-positive bacteria with a thick peptidoglycan layer, round-shaped, forming clusters.

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MSA agar

Mannitol Salt Agar used to select and differentiate Staphylococcus species.

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Catalase-positive

Staphylococcus species produce catalase, breaking down hydrogen peroxide into water and oxygen.

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DNase test

Tests for DNase enzymes that degrade DNA; positive result shows clearing around growth.

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Coagulase test

Detects coagulase enzyme; positive result means clot formation from fibrinogen conversion.

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Kirby-Bauer method

Test to measure antibiotic effectiveness by observing zones of inhibition on agar plates.

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Selective Agent in MSA

High salt concentration (7.5%) selects for Staphylococcus by inhibiting most other bacteria.

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Differential Agent in MSA

Mannitol; species that ferment it turn the medium yellow indicating acid production.

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Study Notes

Staphylococcus Species Characteristics

  • Gram Stain: Gram-positive (purple stain) due to a thick peptidoglycan layer in the cell wall.
  • Morphology: Coccus (round) shape, arranged in clusters (grape-like).
  • Catalase: Catalase-positive, breaking down hydrogen peroxide (Hâ‚‚Oâ‚‚) into water and oxygen (observable as bubbling).

MSA Agar (Selective and Differential)

  • Selective agent: High salt concentration (7.5%) inhibits most bacteria, except Staphylococcus species.
  • Differential agent: Mannitol; species fermenting mannitol (e.g., S. aureus) change the pH indicator, turning the medium yellow. Non-fermenters (e.g., S. epidermidis) remain pink.

DNase and Coagulase Tests

  • DNase Substrate: DNA
  • DNase Reaction: Enzyme degrades DNA.
  • DNase Result: Positive – clear zone around growth, indicates DNA degradation. Negative – no clearing, cloudy growth.
  • Coagulase Substrate: Fibrinogen in plasma
  • Coagulase Reaction: Enzyme converts fibrinogen to fibrin, causing clotting.
  • Coagulase Result: Positive – clotting, indicates coagulase production (e.g., S. aureus). Negative – no clotting.

Kirby-Bauer Method

  • A method to determine antibiotic effectiveness against bacteria.
  • Steps: Inoculate agar, place antibiotic disks, incubate, measure zones of inhibition, and interpret results based on size (susceptible, intermediate, or resistant).

Antibiotic Inhibition Zone

  • Formation: Antibiotic diffuses, preventing bacterial growth around the disk, creating a clear zone.
  • Measurement: Diameter of the clear zone in millimeters (mm).
  • Interpretation: Zones are compared to standard charts to classify as susceptible, intermediate, or resistant.

Factors Influencing Inhibition Zone Size

  • Antibiotic concentration (higher = larger zone).
  • Antibiotic diffusion rate.
  • Inoculum size.
  • Temperature.
  • Incubation time.
  • Bacterial growth rate.
  • Agar depth.

Antibiotic Susceptibility Categories

  • Susceptible (S): Bacteria are killed or inhibited by the tested antibiotic concentration.
  • Resistant (R): Bacteria survive and grow in presence of antibiotic.
  • Intermediate (I): Response is between susceptible and resistant, requiring further consideration.

Staphylococcus Species Differentiation (Key Characteristics)

  • S. aureus: Mannitol fermentation (yellow MSA), coagulase-positive, DNase-positive.
  • S. epidermidis: Mannitol fermentation negative (pink MSA), coagulase-negative, DNase-negative.
  • S. saprophyticus: Mannitol fermentation (yellow MSA), coagulase-negative, DNase-negative. (Note: corrected information from figure).

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