Microbiology Lab Procedures

By Donald Reinhardt

Types of Labs

There are many common procedures and methods used by clinical-medical, water, food and environmental microbiology diagnostic laboratories. A microbiology lab can perform many different tests and procedures to define and determine almost any known pathogen. These tests and procedures form the basic daily activities of the microbiology laboratory workers employed by these labs. Microbiology labs contain all the media, stains, microscope slides, microscopes, incubators, disinfectants and accessory equipment needed for isolation and identification of microbes.

Sample Acquisitions and Assignment

The lab receives appropriately-labeled specimens from various sender sources. An accession (identifying) number is assigned, and the type of specimen is described. For example, a clinical hospital lab which logs in a urine sample with the code "U101/R.S. 6009;1/10-28-09" identifies the sample as urine, the patient's ID as "R.S. 60091" and the date of 10/28/09. Each lab will have its own particular coding system.

Plating Media for Isolation, Screening and Identification of Microbes

Microbial media are important for the proper isolation and selection of actual or potential pathogens.The media used by a particular laboratory will depend on the "expected" types of pathogens from typical submissions to the lab. For example, in the above hospital urine sample, the following media could be used: blood agar, MacConkey or EMB agars.

Streak Isolation of Bacteria Put Onto Agar Dishes

This technique permits the microbiology laboratory worker to streak the surface of the agar with a sample of the specimen via microbiological wire or loop that can be sterilized by heat and used to pick up and transfer microbes from colonies. The goal here is to isolate pure, single bacterial colonies for further specific analysis. Pure, uncontaminated cultures are important for accurate test results.

Incubation of Streak Isolation Agar Dishes

This procedure is the growth phase period that permits the separate, single bacterial cells to grow, divide and multiply in 18 to 24 hours. In this way, each bacterium can become a visible colony containing millions of the same kind of bacterial cells.

Post-Incubation Visual Inspection and Recording of Isolation Plates

After suitable incubation, the dishes are checked for the number and type of colonies, and the microbiologist determines if staining, biochemical or other tests are needed for identification.

Selection of Required Further Tests and Inoculation of media

In some cases additional tests are selected and done, such as biochemical tests, staining, temperature tolerance and others.

Final Reading of Media Results

All the test results are compiled and checked. The microbiologist makes an identification to the genus level, or even to the species or serotype levels.

Issue Report

The lab certifies, by the final identification report, that it has identified the relevant microbe, or microbes, in the original specimen. A formal report is recorded and copies are retained by the lab. The specimen sender is sent a fax or mailed a report, and this completes the cycle and closes the specimen analysis.

About the Author

Medical conditions, procedures, epidemiology, infectious diseases