Microbial Growth Testing

Microbiological contamination refers to the unintended introduction of microorganisms or microbes, such as bacteria, yeast, mold, fungi, and/or their by-products, into a given environment, solution, or product.

History of Microbial Growth Testing

The history of microbial growth testing traces back centuries, beginning with early observations of microorganisms and further developing into modern microbiological techniques. Today, microbial growth testing is required in the fields of medicine, food production, environmental science, and biotechnology. It plays a role in ensuring product safety, identifying pathogens, and advancing biotechnological innovations. Microbial growth is observed to be an exponential growth curve, as common microbes have rapid reproductive rates when exposed the preferable conditions. Due to the nature of microbes, their presence may not be detected right away.

The Monod equation derived by Jacques Monod, is a practical mathematical model for the exponential growth of microorganisms in laboratory environments [1]. This equation quantifies microbial growth rate in testing.

μ = μmax[S]


µ is the growth rate of a microorganism
µmax is the maximum rate of growth for the organism in question
[S] is the concentration of the given growth substrate
Ks is the velocity that yields half of the growth substrate concentration


Image 1: Reagent microbial growth quality control testing performed at Boston BioProducts. In this image, an incubator-shaker is used for this process, maintaining temperature viable for microbial growth over this period. Any growth in product samples will yield a positive test that will be rejected by our quality team and taken for further analysis.

Importance of Monitoring Microbial Contamination

The monitoring of microbial growth/contamination is crucial for ensuring the quality of all laboratory products. Final products must be free of microbial contamination, as confirmed by the absence volatile growth in a 96-hour time span. The presence of microbes could render these products unsuitable for experimentation involving bacterial transformations, DNA or RNA, or protein purification. Following exposure, the results of these experiments may become completely void. Clean surroundings, vessels for reagent preparation, and high-quality water are essential to prevent contamination.

Microbial Growth Screening in Buffers, Reagents, and Media

Contamination with microorganisms, including bacteria and fungi, can compromise the reliability of experiments and the reproducibility of results in many experiments. This testing not only confirms the absence of harmful microorganisms but also safeguards against unintended microbial interference in molecular biology and biotechnology applications. In manufacturing laboratories supporting research or further manufacturing through the production of buffers, reagents, and media, accuracy of derived products directly influence future applications. In this sense, the composition of your experiment is only as pure as the integrity of the chemicals put into the experiment. Confiding in your reagents prior to use by performing this quality test will save time and energy when it comes to your workflows.


Image 2: Positive (right) and Negative (left) Controls for Microbial Growth Testing.

Considerations and Limitations of Microbial Growth Testing

Maintain a clean work environment Any tools, reactants, and work environments that pose a risk to contaminating samples within the microbial growth test should be cleaned and monitored prior to use.
Quality of reactants High quality reactants should always be used. High-quality water free of microbial contamination, endotoxins, proteases, and nucleases is essential in receiving accurate results.

Preparation of testing facilities and tools

Agar and Media Any agar or media plates should be stored at 2-8°C in a clean, dark place prior to use. This will minimize risk of contamination and growth.
Biological Safety Cabinets Before a Biological Safety Cabinet is used, it must be wiped down with bleach/EtOH or Isopropanol, and UV light should be left on for at least an hour with clean air circulation.
Incubators Any incubators used for microbial growth should be clean and can be sanitized by spraying/wiping with ethanol or isopropanol.

In addition to Microbial Growth Testing, Boston BioProducts provides a comprehensive set of QC tests for custom reagents. Learn more about custom reagent development services.