Microbiology III: Gram Positive vs Negative Bacteria

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Questions and Answers

What cellular structures differ between gram positive and gram negative bacteria?

Gram positive bacteria have a thick peptidoglycan cell wall with a single internal plasma membrane, lacking LPS but containing teichoic and lipoteichoic acid. Gram negative bacteria have a thinner peptidoglycan cell wall with an outer membrane and an inner plasma membrane, containing LPS.

How do gram positive and gram negative bacteria differ in staining?

Gram positive bacteria stain purple, while gram negative bacteria stain pink/red.

What features of bacteria do antimicrobial agents often target?

They inhibit cell wall synthesis or target bacterial protein synthesis by targeting the 30S or 50S subunits unique to bacterial ribosomes.

What are the 3 main ways bacteria are classified?

<p>By their growth patterns (clusters vs. chains), biochemical analysis, and examination via grams stain (positive or negative) and shape (cocci, bacilli, diplococci, comma-shaped rods).</p> Signup and view all the answers

What are different growth media for bacteria?

<p>Non-selective media includes blood agar and chocolate agar. Selective media includes MacConkey agar, which has bile salts that inhibit the growth of gram positive bacteria and allows lactose-fermenting gram negative bacteria to appear pink/red.</p> Signup and view all the answers

What are different types of hemolysis?

<p>Alpha is partial hemolysis (brown streaks), beta is complete hemolysis (gold blobs), and gamma is no hemolysis (white blobs).</p> Signup and view all the answers

What are different growth patterns on Mac agar?

<p>Gram negative bacteria grow on Mac agar; gram positive do not. Bacteria that ferment lactose appear pink while those that do not appear tan/yellow.</p> Signup and view all the answers

What does a positive and negative coagulase test look like?

<p>Positive appears as small blue flecks, while negative appears as a large blue blob.</p> Signup and view all the answers

What does a positive and negative cytochrome oxidase test look like?

<p>Positive results in a dark color on the q-tip, while negative shows no dark color.</p> Signup and view all the answers

What does a positive and negative antibiotic response test look like?

<p>Antibiotic resistant bacteria grow over the antibiotic spot, while antibiotic sensitive bacteria grow around it and do not reach up to it.</p> Signup and view all the answers

What are adherence factors common in gram negative and positive bacteria?

<p>Gram negative bacteria have pili and fibrae; gram positive bacteria have MSCRAMM.</p> Signup and view all the answers

What are examples of endotoxins?

<p>Only gram negative bacteria have LPS, which causes an inflammatory response when these cells are lysed.</p> Signup and view all the answers

What are examples of exotoxins?

<p>Exotoxins are produced and secreted by both gram negative and positive bacteria. They can be cytolytic toxins, enterotoxins, or superantigens.</p> Signup and view all the answers

Which bacteria release superantigen exotoxins?

<p>Staphylococcus aureus and Streptococcus pyogenes.</p> Signup and view all the answers

What is the type III secretion system (T3SS)?

<p>It is a virulence mechanism for traversing gram negative cell walls to inject toxins into host cells.</p> Signup and view all the answers

What does a positive and negative catalase test look like?

<p>Positive causes bubbles to form, while negative does not.</p> Signup and view all the answers

Flashcards

Gram-positive bacteria

A type of bacteria with a thick peptidoglycan cell wall and a single inner membrane.

Gram-negative bacteria

A type of bacteria with a thinner peptidoglycan wall and an outer and inner membrane (dual membrane).

Lipopolysaccharide (LPS)

A complex molecule found in the outer membrane of gram-negative bacteria, triggering inflammatory responses upon cell lysis.

Gram stain

A staining technique differentiating bacteria based on their cell wall structure.

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Non-selective media

A growth medium supporting a wide range of bacterial growth.

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Selective media

A growth medium hindering the growth of specific bacterial groups, such as gram-positive bacteria.

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Alpha hemolysis

A type of hemolysis where blood is partially broken down, showing brown streaks around colonies.

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Beta hemolysis

A type of hemolysis where blood is completely broken down, producing a clear zone around colonies.

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Gamma hemolysis

A type of hemolysis where blood is not broken down, resulting in no change in the blood agar.

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MacConkey Agar

An agar medium selectively favoring the growth of gram-negative bacteria.

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Coagulase Test

A test used to identify bacteria capable of producing coagulase, an enzyme that clots blood.

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Cytochrome Oxidase Test

A test used to identify bacteria containing cytochrome oxidase, an enzyme involved in respiration.

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Antibiotic Response Test

A test determining bacterial susceptibility to a particular antibiotic.

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Pili and Frimbriae

A mechanism used by gram-negative bacteria to attach to host cells.

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MSCRAMM

A mechanism used by gram-positive bacteria to attach to host cells.

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Exotoxin

A secreted toxin produced by both gram-negative and gram-positive bacteria, often composed of two parts: a binding portion (B) and an active portion (A).

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Study Notes

Cellular Structures

  • Gram positive bacteria feature a thick peptidoglycan cell wall and a single inner plasma membrane.
  • Have teichoic and lipoteichoic acids; lack lipopolysaccharides (LPS).
  • Gram negative bacteria possess a thinner peptidoglycan wall with an outer and inner membrane (dual membrane) and contain LPS.

Staining Properties

  • Gram positive bacteria stain purple due to their thick cell wall.
  • Gram negative bacteria stain pink/red, reflecting their thinner wall.

Antimicrobial Targets

  • Antimicrobial agents primarily inhibit cell wall synthesis or disrupt bacterial protein synthesis by targeting the unique 30S or 50S ribosomal subunits.

Bacterial Classification

  • Classification is based on growth patterns (clusters vs. chains), biochemical analysis, and examination via gram stain and cellular shape (e.g., cocci, bacilli).

Growth Media for Bacteria

  • Non-selective media like blood agar and chocolate agar support the growth of a wide range of bacteria.
  • Selective media such as MacConkey agar hinders gram positive bacteria with bile salts; lactose fermenters appear pink/red.

Hemolysis Types

  • Alpha hemolysis: partial hemolysis, presenting brown streaks.
  • Beta hemolysis: complete hemolysis, looks like gold blobs.
  • Gamma hemolysis: no hemolysis, characterized by white blobs.

Mac Agar Growth Patterns

  • Only gram negative bacteria grow on Mac agar; gram positive do not.
  • Lactose fermenters appear pink, whereas non-fermenters appear tan/yellow.

Coagulase Test Results

  • Positive coagulase test: small blue flecks observed.
  • Negative coagulase test: large blue blob evident.

Cytochrome Oxidase Test Results

  • Positive result presents as a dark color on the q-tip.
  • Negative result shows no dark color on the q-tip.

Antibiotic Response Test Results

  • Antibiotic resistant bacteria grow over the antibiotic disk.
  • Sensitive bacteria show growth around the disk without breaching it.

Adherence Factors

  • Gram negative bacteria use pili and frimbriae for adherence.
  • Gram positive bacteria utilize MSCRAMM for attachment.

Endotoxins

  • Endotoxins are associated solely with gram negative bacteria's LPS, leading to inflammatory responses upon cell lysis.

Exotoxins Characteristics

  • Produced and secreted by both gram negative and positive bacteria.
  • Dimeric A-B proteins: B portion binds to the cell, A portion promotes injury once inside.
  • Cytolytic toxins may break down phospholipids or form pores, while enterotoxins cause gastrointestinal symptoms.

Superantigen Exotoxins

  • S. aureus and S. pyogenes are known for releasing superantigen exotoxins, triggering massive cytokine storms.

Type III Secretion System (T3SS)

  • A virulence mechanism enabling gram negative bacteria to inject toxins directly into host cells, bypassing their cell wall.

Catalase Test Results

  • Positive catalase test results in bubble formation.
  • Negative catalase test shows no bubble formation.

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