Microbiology

Nutrient Agar

June 14, 2025 by Olivia R

A. Definition & Function

• Nutrient Agar (NA) is a solid growth medium used to grow various types of non-fastidious bacteria (do not require complex nutrients) under laboratory conditions
• Included in the general-purpose media category
• Does not contain selective or differential components, so it supports the growth of many bacteria

B. Main Function

• Primary Culture of Clinical Specimens

Used as the first medium to grow bacteria from:

1. Urine specimens (detection of urinary tract infections)
2. Blood (after being modified as a supporting medium)
3. Wounds/exudates (identification of bacteria that cause infection)
4. Sputum (initial screening of respiratory tract infections)
5. Allows visualization of bacterial colony growth before being transferred to specific media


• Isolate Purity Test

Ensures that bacterial cultures are not contaminated before:

1. Biochemical identification (catalase, coagulase tests, etc.)
2. Gram Staining
3. Antibiotic sensitivity test


• Supporting Media for Antibiotic Sensitivity Tests

Although not the main medium for Kirby-Bauer, NA can be used as:

1. Basic media for certain bacterial resistance tests
2. Control media in the validation of the AST (Antimicrobial Susceptibility Testing) method


• Detection of Contamination in Equipment/Specimens

Laboratory Quality Control (QC):

1. Checking the sterility of medical equipment (eg: catheters, needles) by wiping the surface and inoculating into NA
2. Monitoring air contamination in the laboratory room (settled plate method)


• Temporary Storage of Bacterial Cultures

Storing bacterial isolates in the short term (2–8°C) before:

1. Transferring to enriched media (eg: Blood Agar)
2. Sending to a reference laboratory


• Application in Rapid Diagnostics

Advanced Microscopic Examination:

1. Bacterial colonies from NA can be taken for:
• Gram staining (Gram-positive/negative classification)
• Rapid tests such as catalase/coagulase
• Composition of nutrient agar