The Word on Microbial Growth

January 12, 2006
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Q: When the S500 refers to 72 hours for microbial growth, is it referring to mold or bacteria?

A: Actually, the "IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration" does not refer to 72 hours in any instance. There is one reference to 48 hours in the Standard section 5.2 that says, "Time and temperature aggravate category 2 water contamination levels significantly. Gray water in flooded structures that remains untreated for longer than 48 hours may change to category 3 - black water..." I would like to point out that it refers to gray water and not clean water. In addition, the Standard says it "may change."

It is true that some molds can colonize in 72 hours. Bacteria can start to multiply in just a few hours. However, there is more to the story. Most of the references that deal with the time frame for molds to germinate or to colonize are based upon ideal growing conditions and not necessarily the conditions that you find in the indoor environment.

The intent of the S500 and the "IICRC S520 Standard and Reference Guide for Mold Remediation" is to provide the restorer or remediator with a general time frame that allows for an evaluation for the indoor environment. If the time frame is greater than 48 hours, then there may be some reason for concern. Then again, there may not.

Mold spores take up water at varying rates. When mold spores germinate in an environment that is conducive to growth, their vegetative hyphae will grow into the substrate at which time it can be said that the mold has colonized the substrate. At this point, all that has happened is that there has been a microscopic colony established that will not change the appearance of the substrate surface. Could it be said that the mold has colonized? Yes! In some cases the term "colonized" means that the mold spore has not only germinated and become established, but it has also developed additional spores that have germinated, become established and the amount of growth is such that the colony is visible without the aid of magnification. This process can take many more days. You may not see visual evidence of mold growth for six to 14 days. In some cases, the growth environment may not be favorable and you will not see any evidence of mold growth for weeks, if ever.

Microbial organisms, e.g. molds and bacteria, go through four phases of growth:

Lag Phase
Once the growing conditions become favorable, mold spores can germinate. This growth starts slowly and accelerates gradually.

Exponential or the Log Phase
Exponential growth occurs for a brief period as the hyphae starts to develop. The new hypha extends into the substrate. The mass of hyphae is referred to as mycelium. As long as the growing conditions are favorable and there are nutrients available, the growth remains constant.

Stationary Phase
When the nutrients are depleted or the growing environment is not longer favorable growth slows down or is completely stopped. During the stationary phase, hyphal growth stops. In some cases nutrients are transferred to developing spores as a propagation mechanism. The spores are dispersed by air movement.

The Death Phase
During the death phase, the mycelium eventually dies off.

The lag and exponential phases vary in length of time depending upon what molds are present. Generally those molds that are xerophilic (capable of growth in relatively dry conditions) germinate and grow first. They are also referred to as primary colonizers. Xerophilics can germinate in conditions where the water activity (Aw) is below .80 and perhaps as low as 0.61. Penicillium, Aspergillus and Eurotium can germinate and colonize in these conditions. In ideal growing conditions, micro-colonies can develop in 48 to 72 hours. Secondary colonizers (mesophilic) can germinate and grow where the Aw is between 0.8 and 0.9. Secondary colonizers include species of Cladosporium, Ulocladium, Alternaria and some species of Aspergillus. Tertiary colonizers (hydrophilic) can germinate in conditions where the Aw is above 0.9. The presence of tertiary colonizers in a building is an indication that there has been a very wet condition for extended periods of time. Stachybotrys, Chaetomium, Trichoderma, Aureobasidium, actinomycetes and other bacteria require very wet conditions. Tertiary colonizers (molds) can take seven to 14 days to develop micro-colonies. In all cases, it will take more time for the colonies to grow sufficiently to become visible.

Keep in mind that in any environment, bacteria is also present. When you have a very wet surface, bacteria can multiply very rapidly. In some cases, the odors that you might detect in wet buildings are actually bacteria. Bacteria go through the same phase of development and death as molds. As mentioned above, generally bacteria require a very high Aw in order to multiply. The lag phase for bacteria may only be 1 to 3 hours before it starts multiplying exponentially. The cells divide at a constant rate, doubling the population. In ideal growing conditions (culture), E. coli bacteria will double in population every 17 to 20 minutes. In the intestinal tract, E. coli's generation time is estimated to be 12-24 hours. At this growth rate, one E. coli bacterium could become a billion (1,000,000,000) bacteria in just 10 hours. In the case of Clostridium tetani which can result in Tetanus, the generation rate is even faster, perhaps doubling as quickly as every 6 minutes.

The S500 deals with bacteria primarily and molds as a secondary consideration. When reading the S500, keep in mind that the time frames mentioned are for microbial organisms that include more than just mold. Also keep in mind that these time frames are for ideal growing conditions. It was not intended for the time frames to be absolutes, but starting points at which we might want to be concerned.

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