Lab Media and Microbial Growth

Selective and Differential Media

Microbiologists use various types of media to isolate, identify, and differentiate microorganisms. Selective media favor the growth of specific microbes, while differential media distinguish between organisms based on biochemical reactions.

• Selective Media: Contain agents that inhibit certain microbes while allowing others to grow.

• Differential Media: Include indicators that reveal differences in metabolic activity.

Common Media Types

• Mannitol Salt Agar (MSA): Selective for Staphylococcus species due to high salt; differential for mannitol fermentation (yellow color indicates fermentation).

• MacConkey Agar: Selective for Gram-negative bacteria; differential for lactose fermentation (pink colonies indicate fermentation).

• Eosin Methylene Blue Agar (EMB): Selective for Gram-negative bacteria; differential for lactose fermentation (metallic green sheen for strong fermenters).

• Phenylethyl Alcohol Agar: Selective for Gram-positive bacteria by inhibiting Gram-negative growth.

• Blood Agar: Differential for hemolytic activity; supports growth of many bacteria.

Hemolysis Types on Blood Agar

• Beta Hemolysis: Complete lysis of red blood cells; clear zone around colonies.

• Alpha Hemolysis: Partial lysis; greenish discoloration around colonies.

• Gamma Hemolysis: No hemolysis; no change in the medium.

Microbial Oxygen Requirements

Sodium Thioglycollate Broth and Anaerobic Jar

These tools help determine the oxygen requirements of microbes. Sodium thioglycollate broth creates a gradient of oxygen, while an anaerobic jar removes oxygen for culturing anaerobes.

• Obligate Aerobe: Requires oxygen for growth.

• Obligate Anaerobe: Cannot tolerate oxygen; grows only in its absence.

• Facultative Anaerobe: Can grow with or without oxygen, but prefers oxygen.

Microbial Species and DNA Damage

Representative Bacteria and DNA Repair

Microbiology labs often use model organisms to study growth and DNA damage. Thymine dimers are DNA lesions caused by UV light, which can be repaired by photoreactivation or excision repair.

• Bacillus cereus: Gram-positive, spore-forming rod.

• E. coli: Gram-negative rod, common in gut.

• Staphylococcus aureus: Gram-positive cocci, often found on skin.

• Thymine Dimer: Covalent bond between adjacent thymine bases; disrupts DNA replication.

Example: Exposure of E. coli to UV light can induce thymine dimers, leading to mutations unless repaired.

Fungi: Structure and Reproduction

Fungal Morphology and Life Cycle

Fungi are eukaryotic organisms with diverse structures and reproductive strategies. They can be saprophytes (decomposers) or parasites (living on hosts).

• Hypha: Filamentous cell of fungi.

• Mycelium: Network of hyphae.

• Saprophyte: Obtains nutrients from dead organic matter.

• Parasite: Obtains nutrients from living hosts.

Fungal Spores

• Sporangiospores: Produced in a sac (sporangium).

• Conidiospores: Produced at tips or sides of hyphae, not enclosed.

• Basidiospores: Produced on club-shaped structures (basidia).

• Zygospores: Thick-walled spores formed by fusion of two cells.

Representative Fungi

• Saccharomyces cerevisiae: Yeast used in baking and brewing.

• Rhizopus nigricans: Bread mold, produces sporangiospores and zygospores.

• Aspergillus niger: Produces conidiospores; used in citric acid production.

• Coprinus sp.: Mushroom, produces basidiospores.

Protozoa: Classification and Life Cycle

Major Groups and Forms

Protozoa are unicellular eukaryotes classified by their movement and life cycle forms. They often alternate between cyst (dormant) and trophozoite (active) stages.

• Euglenozoa: Flagellated protozoa, includes Trypanosoma.

• Amebae: Move by pseudopodia, e.g., Entamoeba.

• Ciliata: Move by cilia, e.g., Balantidium coli.

• Apicomplexa: Non-motile, complex life cycles, e.g., Plasmodium vivax.

Life Cycle Forms

• Cyst: Dormant, resistant form.

• Trophozoite: Active, feeding form.

Representative Protozoa

• Giardia lamblia: Causes giardiasis, flagellated.

• Trypanosoma gambiense: Causes African sleeping sickness.

• Plasmodium vivax: Causes malaria.

• Balantidium coli: Only ciliate pathogenic to humans.

Example: Plasmodium vivax alternates between mosquito and human hosts, causing malaria.