Lab notes

✅ Common Characteristics of Enterobacteriaceae:

1. Gram-negative rods
   – They appear pink under Gram stain.

2. Facultative anaerobes
   – Can grow with or without oxygen.

3. Ferment glucose
   – All members can break down glucose (sugar), often producing acid or gas.

4. Oxidase-negative
   – Helps tell them apart from other Gram-negative bacteria like Pseudomonas.

5. Reduce nitrates to nitrites
   – Used in lab tests to identify them.

6. Catalase-positive
   – They have the catalase enzyme that breaks down hydrogen peroxide.

7. Most are motile with peritrichous flagella
   – Except some like Klebsiella which are non-motile.

8. Found in the intestines (enteric)
   – Normal flora or opportunistic pathogens.

Key Point: Glucose Fermentation in Enterics

• Most Enterobacteriaceae (enteric Gram-negative rods) can ferment glucose to acid.

• This is a shared trait used in many identification tests like:

• TSI (Triple Sugar Iron) slant

• Phenol red glucose broth

Post-Lab Key Point: Pseudomonas aeruginosa (PA) – The Exception

🔹 While most Gram-negative rods studied in the Enteric lab were Enterobacteriaceae, one important exception was:

• 
  

• Pseudomonas aeruginosa (PA) 

❌ Why PA is NOT considered enteric:

• Trait	Enterics (e.g., E. coli)	Pseudomonas aeruginosa
  Gram-negative rod	✅	✅
  Facultative anaerobe	✅	❌ (Strict aerobe)
  Glucose fermentation	✅	❌ (does not ferment glucose)
  Oxidase test	❌ (negative)	✅ (positive)
  Nitrate reduction	✅	✅
  Lives in GI tract	✅ (normal flora)	❌ (environmental bacterium)

  
  

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Lab Report Answer – Phenol Red Glucose Test with Pseudomonas aeruginosa:

• Although Pseudomonas aeruginosa (PA) is a Gram-negative rod, it is not considered an enteric bacterium because it does not ferment glucose to acid. In the phenol red glucose assay, PA did not cause a color change in the broth. The medium remained red or pink, indicating a negative result for glucose fermentation.

  This contrasts with enteric bacteria like E. coli, which do ferment glucose, causing the phenol red to turn yellow due to acid production.

💡 Summary for Lab Report:

• Pseudomonas aeruginosa is a Gram-negative rod, but it is not part of the Enterobacteriaceae family. It is a strict aerobe, oxidase positive, and does not ferment glucose. Instead of living in the gut, PA is commonly found in soil and water and is known for its role in opportunistic infections, especially in hospital settings. 

🧪 Phenol Red Glucose Test & Coliforms Summary

🔬 Pseudomonas aeruginosa (PA):

• • Although it's a Gram-negative rod, PA does not ferment glucose.

  • In the phenol red glucose broth, the media stays pink → this means no acid was produced = negative result.

  • This is one reason why PA is not considered enteric.

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🦠 Coliforms (Subgroup of Enterobacteriaceae):

• • Coliforms are lactose-fermenting enteric bacteria.

  • They live in the intestines and are often used as indicators of water contamination.

  • Common coliforms include:

    • Escherichia coli (E. coli)

    • Enterobacter aerogenes

  • They can ferment lactose and sometimes sucrose, producing acid and sometimes gas.

  ---

📝 Summary Sentence for Lab Report:

• Although Pseudomonas aeruginosa is a Gram-negative rod, it is not classified as an enteric because it does not ferment glucose to acid, as shown by the phenol red glucose test staying pink. In contrast, coliforms such as E. coli are enteric bacteria that do ferment glucose and lactose, turning the media yellow due to acid production.

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🔬 How to detect glucose fermentation?

• The media contain glucose and a pH indicator (like phenol red).

• If bacteria ferment glucose → acid is produced → pH drops.

• Color change = Positive:

  • Phenol red turns yellow (acidic)

  • TSI slant turns yellow (acidic butt and/or slant)

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🧠 Important Reminder:

• All enterics ferment glucose → that’s a key trait for identification.

• Non-fermenters like Pseudomonas aeruginosa do not produce acid → no color change (TSI stays red; phenol red stays pink)

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📝 Sample Quiz-Style Question:

Q: What is a shared metabolic trait among all Enterobacteriaceae?
A: They all ferment glucose to produce acid.

When we talk about Group D Streptococci, particularly Enterococcus species (like Enterococcus faecalis and Enterococcus faecium), they belong to:

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🧪 Group D Streptococci (Enterococci)

✅ Key Features:

• Gram-positive cocci (in pairs or short chains)

• Bile-tolerant: Can grow in bile and hydrolyze esculin → this is why they are bile esculin positive

• Salt-tolerant: Can grow in 6.5% NaCl, unlike most Streptococci

• Facultative anaerobes: Can grow with or without oxygen

Group D Streptococci – Bile Esculin Agar (BEA Test)

✅ What is Group D?

• • Includes Enterococcus faecalis, Enterococcus faecium, and some non-enterococcal streptococci

  • They are bile-tolerant (can grow in bile)

  • They produce the enzyme esculinase that breaks down esculin

  ---

🧬 BEA Test (Bile Esculin Agar)

• Feature	Description
  Selective	Contains bile salts → inhibits non-group D Streptococci
  Differential	Contains esculin → tests for esculin hydrolysis

  ---

🔍 How It Works:

• 1. If the organism can grow in bile, it survives on the media.

  2. If it produces the enzyme esculinase, it hydrolyzes esculin into esculetin.

  3. Esculetin reacts with ferric citrate in the medium → forms a black precipitate.

  ---

🧾 Interpretation:

• Result	Meaning	Example Organism
  Black media	Positive – can grow in bile & hydrolyze esculin	Enterococcus faecalis
  No color change	Negative – cannot hydrolyze esculin	Streptococcus pyogenes

  ---

🧠 Summary:

• • Group D = bile + esculin (BEA test positive)

  • These bacteria have esculinase, tolerate bile, and form black colonies on BEA

  • Common in hospital-acquired infections, especially UTIs and wounds 

  • 
    
    
    

Group D and BEA Media

• Group D bacteria (e.g., Enterococcus faecalis, EF) can tolerate bile and hydrolyze esculin, which is tested using BEA (Bile Esculin Agar).

• A positive result turns the media dark brown/black, showing the presence of the enzyme esculinase.

• EF (Enterococcus faecalis) is Group D Streptococcus. When grown on BEA (Bile Esculin Agar):

  • It grew: meaning it's bile-tolerant.

  • It hydrolyzed esculin: meaning it has the esculinase enzyme.

  • This reaction causes the media to change color from tan to dark brown/black, which is a positive result.

  So, the black color on BEA confirms:

1. The organism can tolerate bile (selective part).

2. It can break down esculin into esculetin, which reacts with iron to form a dark color (differential part).

• On the BEA (Bile Esculin Agar) plate, Enterococcus faecalis (EF) both grew and hydrolyzed esculin. This means EF is bile-tolerant and produces the enzyme esculinase. As a result, the media changed color from light tan to dark brown/black. This is a positive BEA result. The black color forms when esculetin (from broken-down esculin) reacts with iron in the media. This confirms EF is part of Group D Streptococci.

pathogenic cocci

 include both Staphylococci and Streptococci,  th

They are responsible for a wide range of infections.

Here’s a simple breakdown to remember:

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🦠 Pathogenic Cocci = Staphylococci + Streptococci

Group	Shape & Arrangement	Gram Reaction	Key Enzymes	Common Infections
Staphylococcus	Cocci in clusters	Gram-positive	Catalase +, Coagulase (S. aureus = +)	Skin infections, boils, MRSA, food poisoning, toxic shock
Streptococcus	Cocci in chains	Gram-positive	Catalase –, Hemolysis on blood agar	Strep throat, scarlet fever, pneumonia, rheumatic fever
Enterococcus	Cocci in pairs/chains	Gram-positive	Catalase –, Bile esculin +	UTIs, endocarditis, wound infections
Neisseria	Diplococci (pairs)	Gram-negative	Oxidase +	Gonorrhea (N. gonorrhoeae), meningitis (N. meningitidis)

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🧬 Why they matter:

• These cocci are all spherical bacteria that differ in enzymes, hemolysis, and disease patterns.

• Staph and Strep are both Gram-positive, but Neisseria is a unique Gram-negative coccus.

---

🧪 Key Lab Tests to Differentiate:

Test	Staphylococcus	Streptococcus	Neisseria
Catalase	Positive	Negative	Variable (not used here)
Coagulase	S. aureus = +	N/A	N/A
Hemolysis	Some Staph show beta	Alpha, beta, gamma types	N/A
Gram stain	Purple clusters	Purple chains	Pink diplococci
Oxidase	Negative	Negative	Positive

🔬 Gram-Positive Cocci vs. Gram-Negative Rods (Enterics)

Feature	Gram-Positive Cocci	Gram-Negative Rods (Enterics)
Cell Wall	Thick peptidoglycan	Thin peptidoglycan + outer membrane
Outer Membrane	❌ None	✅ Present (contains LPS/endotoxin)
Shape	Spherical (cocci)	Rod-shaped (bacilli)
Gram Stain Color	Purple/blue	Pink/red
Examples	Staphylococcus, Streptococcus	E. coli, Enterobacter, Proteus

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🖼️ Simple Cartoon-Style Visualization:

scss
  [Gram-Positive Bacteria]
   🟣 Cocci (round)
   🔹 Thick peptidoglycan wall
   ❌ No outer membrane
   ✅ Retains crystal violet → PURPLE

  [Gram-Negative Bacteria]
   🔴 Rods (bacilli)
   🔹 Thin peptidoglycan
   🔹 PLUS an outer membrane with LPS
   🧼 Loses crystal violet → takes up safranin → PINK

---

🧠 Quick Reminder:

• The difference in wall structure explains why:

  • Gram-positive holds onto crystal violet stain → purple

  • Gram-negative loses it after alcohol wash and takes up safranin → pink

✅ 1. Staphylococci (clusters of Gram-positive cocci)

• Key species:

  • Staphylococcus aureus (most pathogenic)

  • Staphylococcus epidermidis (less pathogenic, normal skin flora)

• Lab tests:

  Test	S. aureus	S. epidermidis
  Catalase	Positive (bubbles)	Positive
  Coagulase	Positive (clots plasma)	Negative
  Mannitol Salt Agar (MSA)	Ferments mannitol (yellow color)	No fermentation (pink)

• Common infections caused by S. aureus:

  • Skin infections (boils, impetigo)

  • Wound infections

  • Toxic shock syndrome

  • MRSA (antibiotic-resistant)

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✅ 2. Streptococci (chains of Gram-positive cocci)

• Key species:

  • Streptococcus pyogenes (Group A)

  • Streptococcus agalactiae (Group B)

  • Streptococcus pneumoniae

• Lab tests:

  Test	Streptococcus spp.
  Catalase	Negative (no bubbles)
  Hemolysis on Blood Agar	Varies by species (see below)

• Types of Hemolysis:

  Hemolysis Type	Appearance on Blood Agar	Example
  Beta	Clear zone (complete lysis)	S. pyogenes
  Alpha	Greenish zone (partial lysis)	S. pneumoniae
  Gamma	No hemolysis	Enterococcus faecalis

• Common infections caused by Streptococcus pyogenes:

  • Strep throat

  • Scarlet fever

  • Rheumatic fever

  • Skin infections (cellulitis)

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🧠 How to Tell Staph from Strep in Lab:

Test	Staph	Strep
Catalase	Positive	Negative
Gram stain	Gram-positive cocci in clusters	Gram-positive cocci in chains
Coagulase	S. aureus = positive	Not applicable

✅ Know how to:

• Interpret catalase, coagulase, hemolysis, and oxidase results

• Distinguish Staph vs Strep and identify Neisseria’s unique traits

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🔹 2. Enteric Bacteria (Enterobacteriaceae)

Feature	Description
Gram Reaction	Gram-negative rods
Shape	Bacilli
Oxygen Use	Facultative anaerobes
Oxidase Test	Negative
Catalase Test	Positive
Ferments Glucose	Yes (A/A or K/A on TSI)
Lactose Fermentation	Variable – tested on MacConkey agar
Motility	Most are motile (exceptions: Klebsiella, Shigella)

✅ Know how to interpret:

• MacConkey agar (lactose fermentation = pink colonies)

• TSI slants (A/A = all sugars, K/A = glucose only, H₂S = black)

• Citrate, Urease, Indole, Motility

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🔹 3. Control of Microorganisms Lab (UV & Antibiotics)

• UV Radiation: Damages DNA → less bacterial growth with more exposure

• Antibiotic Sensitivity: Kirby-Bauer disk test; measure zones of inhibition

  • Sensitive = clear zone

  • Resistant = little or no zone

✅ Be able to:

• Match antibiotics to results (e.g., which are broad-spectrum?)

• Explain how UV affects spore-formers vs non-spore-formers

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🔹 4. Immunity Lab (Serology & Immune Response)

• Antigen-antibody interactions: Color change or clumping = positive reaction

• Know the difference between:

  • Innate vs Adaptive Immunity

  • B cells (make antibodies) vs T cells (kill infected cells)

✅ Be familiar with:

• ELISA-style test principles

• Humoral vs Cell-mediated responses

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❌ Not on Midterm:

• Epidemiology (disease tracking, graphs, R₀) — no need to study that this time!

Group	Gram Reaction	Shape	Key Tests	Notable Species
Staphylococcus	Gram-positive	Clusters of cocci	Catalase +, Coagulase (S. aureus is +)	S. aureus
Streptococcus	Gram-positive	Chains of cocci	Catalase –, Hemolysis on blood agar	S. pyogenes
Enterococcus	Gram-positive	Chains or pairs	Catalase –, Bile Esculin +	E. faecalis
Neisseria	Gram-negative	Diplococci (pairs)	Oxidase +	N. gonorrhoeae, N. meningitidis

 

Pathogenic cocci, especially Staphylococci and Enterococci, are major causes of nosocomial (hospital-acquired) and opportunistic infections — especially through contaminated medical devices or implants.

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🏥 Pathogenic Cocci in Nosocomial Infections

✅ 1. Staphylococcus aureus

• Gram-positive cocci in clusters

• Coagulase positive, catalase positive

• Common in:

  • Surgical site infections

  • Catheter-related bloodstream infections (CRBSIs)

  • Ventilator-associated pneumonia

  • Infections on prosthetic joints, heart valves

• MRSA (methicillin-resistant S. aureus) = a major resistant strain in hospitals

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✅ 2. Staphylococcus epidermidis

• Normal skin flora → opportunistic when it gets into bloodstream

• Coagulase negative, catalase positive

• Forms biofilms on:

  • IV lines

  • Urinary catheters

  • Artificial heart valves and joints

• Common in immunocompromised patients

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✅ 3. Enterococcus faecalis/faecium

• Gram-positive cocci in chains

• Tolerates bile salts → Bile esculin test positive

• Common in:

  • UTIs from catheters

  • Wound infections

  • Endocarditis

• Some strains are VRE (Vancomycin-resistant Enterococcus) – a hospital concern

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🧠 Summary:

Bacteria	Infection Type	Common Sources
Staphylococcus aureus	Surgical wounds, implants, pneumonia	IV lines, catheters, surgery
Staph. epidermidis	Biofilm-related on devices	Catheters, shunts, prosthetics
Enterococcus spp.	UTIs, endocarditis, bloodstream	Catheters, GI tract, open wounds

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💡 Remember: These infections are not from the environment — they are opportunistic and often arise from the patient's own skin or gut flora, especially when medical equipment gives them access.

🦠 Pathogenic Cocci ("Coxi") – Infections Overview

✅ 1. Device-Associated (Nosocomial) Infections

These happen later on, after contamination from skin flora (often during surgery or catheter insertion). The bacteria form biofilms and become harder to treat.

Bacteria	Infection Type	Common Source
Staphylococcus aureus	Wound infections, sepsis, endocarditis	Catheters, IV lines, implants
Staph. epidermidis	Biofilm-based infections (subtle but persistent)	Central lines, prosthetics
Enterococcus faecalis	UTIs, bloodstream infections, endocarditis	Foley catheters, surgery

🔎 These bacteria are opportunistic — they don’t usually cause disease unless they get inside the body.

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✅ 2. Common Community Infections (Non-hospital)

These are often direct infections (from other people or respiratory droplets) and include things like:

Bacteria	Community-Acquired Infection
Streptococcus pyogenes	Strep throat, scarlet fever, impetigo
Streptococcus pneumoniae	Pneumonia, ear infections, sinusitis
Neisseria meningitidis	Meningitis (spread by saliva, cough)

These infections are more typical respiratory or skin-related, and are not usually associated with implants or catheters.

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🧠 Key Concept for Your Midterm:

• Cocci can cause both serious hospital infections (via medical devices or surgery)
  AND

• Common community infections like strep throat and pneumonia.

📌 Always connect the source of infection (community vs. hospital) with the type of cocci and their lab ID traits (e.g., catalase, hemolysis, Gram stain).

🧪 Practice Questions: Pathogenic Cocci

✅ Multiple Choice (Choose the BEST answer)

1. Which of the following organisms is most likely to form a biofilm on a catheter and cause a delayed bloodstream infection in hospitalized patients?
A. Streptococcus pyogenes
B. Staphylococcus epidermidis
C. Neisseria meningitidis
D. Enterobacter aerogenes

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2. A throat swab from a child with fever and white patches on the tonsils tests positive for beta-hemolytic colonies. Which organism is most likely responsible?
A. Staphylococcus epidermidis
B. Enterococcus faecalis
C. Streptococcus pyogenes
D. Neisseria gonorrhoeae

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3. Which of the following is a Gram-negative diplococcus that causes meningitis and is oxidase positive?
A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Neisseria meningitidis
D. Proteus vulgaris

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✅ True or False

4. Staphylococcus aureus is coagulase positive and often causes skin infections.
5. Biofilm-forming cocci are more likely to cause community-acquired respiratory infections than nosocomial infections.
6. Neisseria meningitidis can spread by respiratory droplets and cause outbreaks in college dorms.
7. Streptococcus pyogenes is catalase positive and typically forms clusters under the microscope.

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📝 Answers:

1. B – Staphylococcus epidermidis
2. C – Streptococcus pyogenes
3. C – Neisseria meningitidis
4. ✅ True
5. ❌ False (Biofilm-formers are more common in hospital/nosocomial infections)
6. ✅ True
7. ❌ False (S. pyogenes is catalase negative and forms chains)

Question 1:

Which of the following is NOT a common trait of all Enterobacteriaceae?

A. Gram-negative rods
B. Ferment glucose
C. Oxidase positive
D. Facultative anaerobes

🔹 Match the bacterium in Column A with its correct description in Column B.

Column A – Bacteria

1. Staphylococcus aureus

2. Streptococcus pyogenes

3. Staphylococcus epidermidis

4. Neisseria meningitidis

5. Enterococcus faecalis

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Column B – Descriptions

A. Gram-negative diplococcus, causes meningitis, oxidase positive
B. Forms biofilms on catheters and implants, coagulase negative
C. Causes strep throat, catalase negative, beta-hemolytic
D. UTI and wound infections, bile esculin positive, gamma hemolytic
E. Coagulase positive, causes skin and surgical wound infections

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📝 Your Matching Answers:

A	B
1	___
2	___
3	___
4	___
5	___

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✅ Answer Key:

A	B
1	E — S. aureus = coagulase +, skin/wound infections
2	C — S. pyogenes = strep throat, beta hemolysis
3	B — S. epidermidis = biofilms on medical devices
4	A — N. meningitidis = Gram-negative diplococcus, meningitis
5	D — E. faecalis = UTI, bile esculin +, gamma hemolysis

You answered C. Oxidase positive — and that’s correct.

👉 Explanation:
Enterobacteriaceae are:

• Gram-negative rods ✅

• Ferment glucose ✅

• Facultative anaerobes ✅

• Oxidase-negative ✅ ← That’s why oxidase-positive is not correct for them.

What color does E. coli appear on MacConkey agar?
A. Colorless
B. Pink
C. Blue
D. Green

1. It produces acid from lactose.

2. The acid lowers the pH, turning the colony pink/red.

3. This is a positive result for lactose fermentation.

A/A (yellow slant/yellow butt) – ferments glucose, lactose, and/or sucrose; often with gas.

K/A (red slant/yellow butt) – ferments only glucose; no lactose/sucrose.

K/A + H₂S (yellow butt with black precipitate) – glucose fermentation plus hydrogen sulfide production (e.g., Proteus).

K/K (red/red) – no sugar fermentation; peptone deamination causes alkaline reaction, typical of non-fermenters like Pseudomonas brunofuga.adv.br+10

• Left tube – Bright pink, indicating a positive urease test (e.g., Proteus vulgaris). The urea is hydrolyzed into ammonia, raising the pH and revealing the pink color from phenol red biologynotesonline.com+5microbiologytechinfo.blogspot.com+5microbiologyinfo.com+5.

• Right tube – Yellow or peach, indicating a negative urease test (e.g., E. coli). The medium remains acidic/neutral with no urease activity .

✅ In summary:

• Positive = Bright pink

• Negative = Peach/yellow

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🦠 Enterobacteriaceae – Common Characteristics

All members of the Enterobacteriaceae family share these traits:

Trait	Description
Gram Reaction	Gram-negative rods (pink under Gram stain)
Oxygen Requirement	Facultative anaerobes (grow with or without O₂)
Glucose Fermentation	All ferment glucose to produce acid
Catalase Test	Catalase positive (bubbles with H₂O₂)
Oxidase Test	Oxidase negative
Motility	Most are motile (except Klebsiella, Shigella)
Nitrate Reduction	Reduce nitrate → nitrite
Spore Formation	Non-spore forming

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🧪 Common Tests for Enteric Bacteria

1. MacConkey Agar

Feature	Description
Purpose	Isolates Gram-negative bacteria and tests lactose fermentation
Selective?	✅ Yes – Inhibits Gram-positive bacteria with bile salts and crystal violet
Differential?	✅ Yes – Differentiates lactose fermenters from non-fermenters
Positive result	Pink/red colonies = lactose fermenter (E. coli, Enterobacter)
Negative result	Colorless colonies = non-lactose fermenter (Proteus, Serratia)

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2. TSI Slant (Triple Sugar Iron Agar)

Feature	Description
Purpose	Tests for fermentation of glucose, lactose, sucrose, gas, and H₂S
Selective?	❌ No
Differential?	✅ Yes

🔍 Interpretation of Results

Result	What It Means
A/A (yellow/yellow)	Ferments glucose + lactose and/or sucrose (E. coli, Enterobacter)
K/A (red/yellow)	Only glucose fermented (Proteus vulgaris, Shigella)
Black precipitate	H₂S production (e.g., Proteus vulgaris)
Cracks or lifting agar	Gas production (CO₂ release during fermentation)
K/K (red/red)	No sugar fermentation; peptone used aerobically (Pseudomonas)

✅ How colors change:

• Yellow (A) = Acid (from sugar fermentation)

• Red (K) = Alkaline (from protein use in aerobic slant)

• Black = H₂S reacting with iron salts to form FeS

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Example Summary for TSI:

• E. coli: A/A + gas

• Enterobacter aerogenes: A/A + gas

• Proteus vulgaris: K/A + H₂S

• Serratia marcescens: A/A or K/A (strain dependent), no H₂S 

📌 Bullet Point Summary – Enteric Gram-Negative Rods

• Belong to the Enterobacteriaceae family

• Gram-negative rods, facultative anaerobes

• Many are part of normal gut flora

  • Examples: E. coli, Enterobacter, Proteus

• Can become opportunistic pathogens

  • Cause UTIs, wound infections, sepsis (especially in hospital settings)

• Some strains cause intestinal diseases

  • Example: pathogenic E. coli (EHEC), Salmonella, Shigella

• Spread via contaminated food, water, or fecal-oral route

• Identified using selective/differential media (e.g., EMB, MacConkey)

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🎴 Flashcard-Style Questions & Answers

Q: What are enteric gram-negative rods?
A: Rod-shaped, gram-negative bacteria that live in the intestines. Many are part of normal flora.

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Q: Are all enteric bacteria harmful?
A: No. Most are harmless in the gut but can cause disease if they spread elsewhere.

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Q: What is an example of a normal enteric bacterium?
A: E. coli is a common example found in healthy intestines.

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Q: What is an example of a disease-causing enteric bacterium?
A: E. coli (EHEC), Salmonella, or Shigella can cause diarrhea or food poisoning.

---

Q: What is an opportunistic infection caused by enterics?
A: UTIs from E. coli or Proteus in catheterized patients. 

Practice

1.      1. What is the typical shape and Gram reaction of Enterobacteriaceae?

Answer: Gram-negative rods

2.      2. Name one enteric bacterium that is a normal part of intestinal flora.

Answer: E. coli

3.      3. What type of infection can enteric bacteria cause outside the intestine?

Answer: Opportunistic infections like UTIs and wound infections

4.      4. Which media are commonly used to identify enteric bacteria?

Answer: EMB agar, MacConkey agar

5.      5. What is an example of a pathogenic strain of E. coli?

Answer: EHEC (Enterohemorrhagic E. coli)

6.      6. How are intestinal diseases from enterics usually transmitted?

Answer: Fecal-oral route, contaminated food or water

🔬 Lab Review: Control of Microorganisms (UV & Antibiotics) + Immunity

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✅ 1. Control of Microorganisms – UV & Antibiotics

🔹 UV Radiation

• Purpose: Tests how ultraviolet (UV) light affects bacterial growth.

• Mechanism: UV causes DNA damage (thymine dimers), which can kill or mutate cells.

• Key Factors:

  • Time of exposure: Longer = more damage

  • Wavelength: UV-C (around 254 nm) is most lethal

  • Limitations: Only works on surfaces, not deep into materials or liquids

• Expected Results:

  • Less growth where UV was exposed longer

  • Endospore-forming bacteria (e.g., Bacillus) may survive better than non-spore formers (e.g., E. coli)

🔹 Antibiotic Sensitivity (Kirby-Bauer Disk Diffusion Test)

• Purpose: Determines which antibiotics are effective against specific bacteria

• How it works:

  • Paper disks with antibiotics are placed on an agar plate spread with bacteria.

  • After incubation, look for zones of inhibition (clear areas around disks).

• Interpretation:

  • Large zone = bacteria is sensitive

  • Small or no zone = bacteria is resistant

• Measured in mm, compared to a standard chart

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✅ 2. Immunity Lab

🔹 Serological Testing – Antigen-Antibody Reactions

• Purpose: Detect specific antigens or antibodies in a sample

• Examples: ELISA test, rapid strep test, COVID-19 antigen tests

• Reaction: A visible change (like a color shift or clumping) shows a positive result

🔹 Types of Immunity (overview)

• Innate immunity: First line of defense (skin, phagocytes, inflammation)

• Adaptive immunity: Specific response (B cells, T cells, antibodies)

• Humoral immunity: B cells → plasma cells → produce antibodies

• Cell-mediated immunity: T cells attack infected cells

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🧠 Midterm Prep Tip:

Be able to:

• Interpret a UV exposure plate

• Read zones of inhibition and explain resistance/sensitivity

• Recognize the difference between innate vs adaptive immunity

• Understand the role of B cells, T cells, antibodies, and antigens 

What's gonna happen if you forget the substrate, or what's gonna happen if you forget the indicator, or what might happen if you forget the inhibitor like being very comfortable with how that might interfere with these tests.

ChatGPT said:

Great question! Understanding what happens if a key ingredient is missing will help you prepare for your lab midterm and also make sense of how media works.

Here’s a breakdown of what could go wrong if you forget each component:

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🔴 1. If You Forget the Substrate (e.g., lactose, mannitol):

• What is the substrate?
  It’s the thing the bacteria metabolize (like sugar).

• What happens if it’s missing?
  ➤ Fermentation can’t occur, even if the bacteria are capable.
  ➤ There will be no acid production, so...
  ➤ No color change in the indicator.
  ➤ You can’t tell if it’s a fermenter or not → False negative result.

🧪 Example: If lactose is missing from EMB or MacConkey, E. coli won’t show the green sheen or pink color — it might look like a non-fermenter!

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🟣 2. If You Forget the Indicator (e.g., phenol red, eosin Y/methylene blue, neutral red):

• What is the indicator?
  It changes color when pH shifts (from acid or base).

• What happens if it’s missing?
  ➤ The medium might look normal, and bacteria still grow, but...
  ➤ You can’t see the results.
  ➤ You’ll miss fermentation reactions because there's no color change to show pH change.
  ➤ Bacteria may ferment sugars, but you won't know.

🧪 Example: If methylene blue and eosin are left out of EMB, you’ll get bacterial growth—but no dark purple or green sheen, so you can’t tell if fermentation happened.

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🟡 3. If You Forget the Inhibitor (e.g., bile salts, dyes, high salt):

• What is the inhibitor?
  It suppresses unwanted bacteria, like gram-positives, so you only grow the target organisms.

• What happens if it’s missing?
  ➤ Unwanted bacteria (like gram-positives) will grow!
  ➤ You’ll get mixed growth, which interferes with interpretation.
  ➤ The media is no longer selective, and the results can be confusing.

🧪 Example: If bile salts are missing from MacConkey, gram-positive bacteria (like Staphylococcus) might grow, making it harder to tell which colonies are which.

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✅ Summary Table

Missing Component	Effect on Media
Substrate	Bacteria can't ferment → No acid → No color change → False neg.
Indicator	No visible result → Can't tell if fermentation happened
Inhibitor	Non-target organisms grow → Media loses selectivity → Confusing results