Microbial Nutrition and Growth: Culturing Microorganisms

Microbial Nutrition and Growth

Introduction

This section covers the fundamental concepts of microbial nutrition and growth, focusing on the methods used to culture microorganisms, the characteristics of bacterial colonies, and the techniques for obtaining pure cultures. Understanding these principles is essential for laboratory work in microbiology and for interpreting clinical and environmental microbiological data.

Culturing Microorganisms

Inoculum and Culture

• Inoculum: The sample of microorganisms introduced into a growth medium. Sources include:

  • Environmental specimens (e.g., soil, water)

  • Clinical specimens (e.g., blood, urine, tissue)

  • Stored specimens (previously preserved cultures)

• Culture: The act of cultivating microorganisms or the collection of microorganisms that are cultivated in the laboratory.

Characteristics of Bacterial Colonies

Bacterial colonies can be distinguished by their physical characteristics, which are important for identification and classification.

• Shape: Circular, rhizoid, irregular, filamentous, spindle

• Margin (edge): Entire, undulate, lobate, curled, filiform

• Elevation: Flat, raised, convex, pulvinate, umbonate, crateriform

• Size: Punctiform (very small), small, moderate, large

• Surface: Smooth or rough

• Appearance: Glistening (shiny) or dull

• Pigmentation: Nonpigmented (e.g., cream, tan, white) or pigmented (e.g., purple, red, yellow)

• Optical properties: Opaque, translucent, transparent

Example: A colony that is circular, raised, smooth, and red-pigmented may be characteristic of Serratia marcescens.

Clinical Specimens and Collection Methods

Proper collection of clinical specimens is crucial for accurate diagnosis and prevention of contamination.

Type or Location of Specimen	Collection Method
Skin, accessible membrane (e.g., outer ear, nose, throat, vagina, cervix, urethra), or open wounds	Sterile swab brushed across the surface; care should be taken not to contact neighboring tissues
Blood	Needle aspiration from vein; anticoagulants are included in the specimen transfer tube
Cerebrospinal fluid	Needle aspiration from subarachnoid space of spinal column
Stomach	Intubation, which involves inserting a tube into the stomach, often via a nostril
Urine	In aseptic collection, a catheter is inserted into the bladder through the urethra; in the "clean catch" method, initial urination washes the urethra, and the specimen is midstream urine
Lungs	Collection of sputum either dislodged by coughing or acquired via a catheter
Diseased tissue	Surgical removal (biopsy)

Obtaining Pure Cultures

Definition and Importance

• Pure culture: A culture composed of cells arising from a single progenitor cell.

• Colony-forming unit (CFU): The single progenitor cell from which a pure culture arises.

• Aseptic technique: Procedures that prevent contamination of sterile substances or objects, essential for obtaining pure cultures.

Isolation Techniques

• Streak plate method: A technique where a loop is used to spread an inoculum across the surface of an agar plate in a pattern that thins out the sample and isolates individual colonies.

• Pour plate method: A technique where a diluted sample is mixed with molten agar and poured into a petri dish, allowing colonies to grow both on the surface and within the medium.

Example: The streak plate method is commonly used to isolate bacteria from a mixed culture, such as separating Staphylococcus aureus from Escherichia coli in a clinical sample.