Microbiology Laboratory Study Guide: Physiological Tests, Pathogenic Bacteria, Epidemiology, and Immunology

Physiological Characteristics of Bacteria

Overview of Biochemical Tests

Biochemical tests are essential for identifying and differentiating bacterial species based on their metabolic activities. Each test targets a specific enzyme or metabolic pathway, using defined media and reagents to detect characteristic reactions.

• Sugar Fermentation: Detects the ability to ferment specific sugars, producing acid and/or gas.

• IMViC Tests: A series of four tests (Indole, Methyl Red, Voges-Proskauer, Citrate) used to distinguish members of the family Enterobacteriaceae.

• Hydrolysis Tests: Assess the ability to hydrolyze macromolecules such as starch, casein, and lipids.

• Enzyme Production: Tests for enzymes like catalase and oxidase, important for aerobic metabolism.

Summary Table: Major Biochemical Tests

Test Name	Alternate Name	Medium	Enzyme	Control Microorganism	Reagent	Positive Result	Notes
Sugar Fermentation	-	Phenol red sugar broth	Varies	Escherichia coli	-	Yellow (acid); gas bubble	Gas not always present
Mixed Acid Fermentation	Methyl Red (MR)	MR-VP	Formic hydrogenylase	Escherichia coli	Methyl red	Red color	-
Butanediol Fermentation	Voges-Proskauer (VP)	MR-VP	-	Enterobacter aerogenes	Barritt's A & B	Red color (with aeration)	Shake tube periodically
Nitrate Reduction	-	Nitrate broth	Nitratase	Escherichia coli	Nitrite reagents	Red color	No zinc dust used
Starch Hydrolysis	-	Starch agar	Amylase	Bacillus subtilis	Gram's iodine	Clearing around growth	-
Casein Hydrolysis	-	Skim milk agar	Caseinase	Bacillus subtilis	-	Clearing around growth	-
Fat Hydrolysis	-	Spirit blue agar	Lipase	Staphylococcus aureus	-	Clearing and blue precipitate	-
Tryptophan Hydrolysis	Indole Test	Tryptose broth	Tryptophanase	Escherichia coli	Kovac's reagent	Red ring	-
Urea Hydrolysis	-	Urea broth	Urease	Proteus vulgaris	-	Hot pink (fuchsia)	-
Phenylalanine Deamination	-	Phenylalanine agar slant	Phenylalanase	Proteus vulgaris	Ferric chloride	Green slant	-
Hydrogen Sulfide Production	-	SIM medium	Cysteine desulfurase	Proteus vulgaris	-	Black precipitate	H2S reacts with iron in medium
Citrate Utilization	-	Simmon's citrate agar	Citratase	Enterobacter aerogenes	-	Growth and blue color	-
Catalase Production	-	Nutrient agar	Catalase	Staphylococcus aureus	H2O2	Bubbles	-
Oxidase Production	-	TSA or similar	Oxidase	Pseudomonas aeruginosa	Oxidase reagent	Purple color within 2 min	Test on cotton swab

Key Concepts and Examples

• Fermentation Tests help distinguish between bacteria based on their ability to ferment carbohydrates, producing acid and/or gas. For example, Escherichia coli ferments glucose, turning phenol red broth yellow.

• IMViC Tests are especially useful for differentiating Escherichia coli (MR+, VP-) from Enterobacter aerogenes (MR-, VP+).

• Hydrolysis Tests (e.g., starch, casein, fat) reveal the presence of extracellular enzymes that degrade complex substrates.

• Catalase and Oxidase Tests are rapid assays for key respiratory enzymes, distinguishing genera such as Staphylococcus (catalase+) from Streptococcus (catalase-).

Additional info: The IMViC series stands for Indole, Methyl Red, Voges-Proskauer, and Citrate tests, which are classic for identifying enteric bacteria.

Staphylococcus: Characteristics and Identification

General Features

• Gram Reaction and Morphology: Gram-positive cocci, typically found in clusters.

• Family: Staphylococcaceae

• Pyogenic: Capable of forming pus.

• Diseases: Abscesses, boils, wound infections, toxic shock syndrome.

• Habitat: Nasal membranes, hair follicles, skin.

• Catalase Positive: Distinguishes from Streptococcus.

• Nosocomial Infections: Hospital-acquired infections, e.g., MRSA (Methicillin Resistant Staphylococcus aureus).

• Medically Significant Species: S. aureus (most pathogenic), S. epidermidis, S. saprophyticus.

Identification Methods

• Fermentation of Mannitol: Tested on Mannitol Salt Agar (MSA).

• Type of Hemolysis: Beta, alpha, or gamma hemolysis on blood agar.

• Coagulase Production: S. aureus produces coagulase, which clots plasma.

Selective and Differential Media

• Mannitol Salt Agar (MSA):

  • Selective: High NaCl inhibits non-halophiles and Gram-negatives.

  • Differential: Contains mannitol and phenol red; fermentation turns medium yellow.

  • Results: S. aureus ferments mannitol (yellow), S. epidermidis does not (no color change).

• Blood Agar: Used to observe hemolysis patterns.

Hemolysis Types

• Beta Hemolysis: Complete lysis of RBCs; clear zone.

• Alpha Hemolysis: Partial lysis; greenish discoloration.

• Gamma Hemolysis: No lysis; no change in medium.

Coagulase Test

• Principle: Detects enzyme that clots plasma.

• Procedure: Inoculate plasma, incubate 24 hr; positive = clot formation.

Example: Staphylococcus aureus is catalase positive, ferments mannitol, is beta-hemolytic, and produces coagulase.

Streptococcus: Characteristics and Identification

General Features

• Gram Reaction and Morphology: Gram-positive cocci in chains.

• Family: Streptococcaceae

• Pyogenic: Some species form pus.

• Diseases: Pneumonia, meningitis, pharyngitis, endocarditis, glomerulonephritis.

• Habitat: Pharyngeal cavity, teeth, saliva, colon.

• Catalase Negative: Distinguishes from Staphylococcus.

• Small Colonies: Typically smaller than Staphylococcus.

• Medically Significant Species: S. pyogenes, S. agalactiae, S. pneumoniae, Enterococcus.

Lancefield Groupings

• Developed by Rebecca Lancefield, classifying Streptococcus species based on cell wall antigens (Groups A, B, C, D, etc.).

Identification Methods

• Hemolysis on Blood Agar:

  • Beta: Complete (clear zone)

  • Alpha: Partial (greenish)

  • Gamma: None

• Bacitracin Susceptibility: Beta-hemolytic; any zone = Group A (S. pyogenes).

• SXT Susceptibility: Beta-hemolytic; any zone = Group C.

• CAMP Test: Beta-hemolytic; arrowhead hemolysis = Group B (S. agalactiae positive).

• Bile Esculin Hydrolysis: Alpha-hemolytic; blackening = Group D.

• Salt Tolerance: Alpha-hemolytic; turbidity = Group D enterococci.

• Optochin Susceptibility: Alpha-hemolytic; measured zone = S. pneumoniae positive.

Comparison Table: Staphylococcus vs. Streptococcus

Feature	Staphylococcus	Streptococcus
Gram Reaction	Positive	Positive
Shape	Cocci in clusters	Cocci in chains
Catalase	Positive	Negative
Hemolysis	Beta, alpha, gamma	Beta, alpha, gamma
Diseases	Abscesses, TSS, MRSA	Pneumonia, pharyngitis

Additional info: The CAMP test is used to identify Group B Streptococcus (S. agalactiae), which enhances hemolysis by S. aureus toxins.

White Blood Cell (WBC) Study

Types and Identification of WBCs

White blood cells are crucial components of the immune system, classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

• Granulocytes:

  • Neutrophils (50-70%)

  • Eosinophils (1-5%)

  • Basophils (<1%)

• Agranulocytes:

  • Lymphocytes (20-30%)

  • Monocytes (2-6%)

Identification Criteria

• Relative size compared to RBCs

• Nuclear morphology

• Presence and color of cytoplasmic granules

Normal Percentages and Clinical Significance

WBC Type	Normal %	Notes
Neutrophils	50-70%	Increased in bacterial infections
Eosinophils	1-5%	Increased in parasitic infections, allergies
Basophils	<1%	Increased in allergic reactions
Lymphocytes	20-30%	Increased in viral infections
Monocytes	2-6%	Increased in chronic infections

Example: A high neutrophil count suggests a bacterial infection, while elevated lymphocytes may indicate a viral infection.

Epidemiology and Synthetic Epidemic Exercise

Key Definitions

• Infectious Disease: Caused by microorganisms that invade and multiply in host tissues, often communicable.

• Epidemiology: Study of the distribution and determinants of diseases in populations.

• Epidemic: Sudden increase in cases in a specific area and time.

• Endemic: Disease constantly present in a population or region.

• Pandemic: Epidemic that spreads across continents.

Types of Epidemics

• Common Source: All cases arise from a single source (e.g., contaminated water).

• Propagated: Spread from person to person.

Case Study: Mary Mallon ("Typhoid Mary")

• Who: Mary Mallon, a cook in New York City.

• What: Asymptomatic carrier of Salmonella typhi, causing Typhoid Fever.

• When/Where: Late 1800s to early 1900s, New York City.

• How: Infected others through poor hygiene; bacteria shed from gall bladder via feces.

Additional info: The Mary Mallon case is a classic example of an asymptomatic carrier contributing to the spread of infectious disease, highlighting the importance of public health measures and epidemiological investigation.