Mold and Granite Counter Tops | Importance of Media and Incubation Temperature Selection in Bacterial Culture Assays
By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board
We think of granite as impervious to everything, and indeed, it is both hard and strong. Granite is a major part of the continental crust and is composed primarily of quartz, feldspar, mica, and horneblend. Chemically, it is approximately 72% SiO2 and 14% Al2O3 with other compounds comprising less than 5% each. Depending on the impurities present during formation, granite can be of many different colors.
Granite is very hard with a hardness rating between 6 and 7, with diamond as a reference of 10. It is also resistant to many chemicals and is even used in underwater applications. This makes it very useful as a counter top material. Normal cutlery used in homes and most food liquids will not harm granite provided they are cleaned soon after use.
Fungi are present on and even in granite in the natural environment. The fungal spores germinate on the surface of granite, and if water is present, the hyphae are able to force their way into the spaces between crystals using osmotic pressure. The fungi also release organic acids that can assist in penetration of the rock. This process occurs over long periods of time, and results from this are generally not relevant to granite counter tops in the home. Although water may be present, it is usually transient and there is no time for fungi to gain a foothold.
We have done a literature search looking for documented invasive fungal growth on granite and have found no studies. There are studies of bacteria sticking to granite materials, but no more so than to other kinds of counter top material. The two studies most often quoted are by Patricia Fajardo-Cavazos and Wayne Nicholson (Appl. Environ. Microbiol. 2006 April; 72(4): 2856-2863), and Berenice Thomason, James Biddle, and William Cherry (Appl. Microbiol., Nov. 1975, p. 764-767 Vol. 30, No. 5). The first of these studies was done by NASA and focused on extreme environments and the genus Bacillus, which makes extremely resistant endospores. They did find these spores in the natural environment, which is not surprising, but it does not extrapolate to any health risk in the home environment. The second study was done by the Centers for Disease Control, and examined the prevalence of Salmonella strains in natural environments. Again, they were found, but the data are not relevant to the home environment other than emphasizing that there are microorganisms everywhere except in places where they are specifically excluded.
As with any other surface used for handling food, scrupulous hygiene is important. The most important cause of food contamination is handling or preparing food for cooking on a surface such as chicken, then preparing uncooked food on the same surface without washing. Hydrogen peroxide is a good disinfectant as is very dilute bleach.
As with any other material, keeping granite dry is the best defense against the growth of mold and other microorganisms. Sealing the surface of polished granite can make it hydrophobic so that it is less likely to soak up any water. Even unsealed granite absorbs very little water. If the material remains wet for days at a time, surface mold growth will occur. Once the surface is dry, the mold can be removed using soap and water. You can also use proprietary granite cleaning solutions.
Granite contains 10-20 ppm uranium, which decays to produce radon gas. Although health physicists do agree that granite countertops may emit radon, the levels are insignificant compared with background levels.
By Alex Spears, EMLab P&K Senior Bacteriologist
At EMLab P&K, samples submitted for bacterial culture analysis are plated on TSA (tryptic soy agar) and incubated at room temperature (typically 23-26°C). This media and incubation temperature selection allows for the growth of a wide range of organisms, and provides the environmental professional with good overview of the bacterial population in their submitted sample. However, if the environmental professional is looking for the presence of a particular organism or group of organisms (e.g. MRSA - methicillin resistant Staphylococcus aureus or fecal coliforms), this approach may not be sufficient. Knowledge of what organisms are of importance to the environmental professional can allow the bacteriologist to choose appropriate growth media and incubation conditions to increase the odds of recovering the organism or organisms of interest.
Media used in bacteriology can be generally grouped into four categories, the categories being general growth, selective, differential, and enriched. A general growth media, like the name implies, is suitable for the growth of a wide range of organisms, but generally lacks any selective or differential properties. An example of a general growth media is TSA, which is a "jack of many trades and master of none". Isolating a specific organism or group of organisms from TSA can be difficult, due to lack of differential properties and possibility of overgrowth from competing organisms. Selective media, unlike general growth media, select for a specific organism or group of organisms, usually by the addition of inhibitory compounds such as antibiotics. Selective media includes media such as MacConkey (for the selection of gram negative organisms) and colistin-nalidixic acid (CNA) (for the selection of gram positive organisms). Differential media allow for organism selection due to differing colony morphologies on the media. Xylose lysine deoxycholate (XLD) agar is a selective agar that allows for the differentiation of members of the coliform group of organisms. Many media that are selective can also be differential, such as MacConkey (enables differentiation by lactose fermentation) and CNA (enables differentiation due to hemolysis). Enriched media contain special growth factors needed for the recovery of certain organisms, examples being buffered charcoal yeast extract (BCYE) agar (for isolation of Legionella) and chocolate agar (for isolation of Haemophilus). TSA with 5% sheep blood is also an enriched agar, and some environmental organisms (especially injured or stressed organisms) may fail to grow on it.
In addition to media selection, incubation temperature also plays a large role in the recovery of organisms. For a general overview of a sample bacterial population, room temperature (23-26°C) incubation is satisfactory. This incubation temperature allows for the growth of most organisms. However, if one is looking for a specific organism or family of organisms (such as potential human pathogens), this incubation temperature is not optimal. For the recovery of potential pathogens (such as MRSA), incubation at 35-37°C is necessary. This is the temperature range of the human body, and allows for the rapid growth of the target organism (if present), and also reduces or eliminates the growth of some environmental organisms that might overgrow the culture. If one is looking for other groups of organisms, even higher incubation temperatures may be necessary to enhance organism recovery. Fecal (thermotolerant) coliforms should be incubated at 44.5°C, Pseudomonas aeruginosa cultures incubate at 41.5°C, and thermophilic actinomycetes need incubation temperatures of at least 50°C.
In conclusion, with hundreds of different types of media available and a wide range of incubation temperatures, it can be a daunting task to determine the correct conditions necessary to optimize target organism recovery. At EMLab P&K, our standard bacterial culture assay has been optimized to provide conditions agreeable to the growth of the broadest range of organisms possible. However, if you as a client are looking for a particular organism or group of organisms, please note what you are looking for on the chain of custody or discuss the appropriate media and incubation conditions with your project manager and/or our bacteriologists. This will enhance target organism recovery, and provide you with the best possible results.
This article was originally published on November 2008.