Bacteria In Granite


This topic is a surprise to many readers, that granite can harbour living organisms.  Yet in fact, all that is needed for life to thrive is a protected spot to live, moisture and something to eat. Granite provides all three of these needs.

Since a sealer cannot completely block out water, thus moisture will be available. Even deep underground, with no sunlight or nutrients, bacteria thrive. One type of bacteria turns minerals into food, while another type uses the waste products left over and so on.

However, in a counter top, there is food available, grease, crumbs, all manner of proteins, sugars, and carbohydrates smeared around and ground down into a paste by use and cleaning, then forced into crevices and pores.

Do a Google search on shopping carts and bacteria, see how long they last with only moisture from hands and nutrients from skin oils.  Countertops are a haven for bacteria, which is why deep cleaning and sealing is necessary on a regular basis.

This is a sore point with stone shops – the bacterial aspects of granite.  Studies have proven the susceptibility of granite to bacterial colonisation and other studies have shown the rapid increase in food borne illness, yet no one has done a study to find any correlation between the two.  That will be done in the near future, first a paper study to bring together what has been done already, then food borne illness records will be checked to see what percentage have granite worktops in the home.  Denying scientific proof that granite is unsanitary is just trying to justify a purchase or trying to sell more stone tops.

One study on granite and bacteria was very thorough, yet it has been attacked by the granite industry.  If you do the maths on the results of the study you see why they are worried about the impact on their product.  Here is a short version of what NASA found and why a homeowner would want to know.

Imagine, NASA’s findings of over 100,000 bacteria per gram of granite, plus many, many bacteria spore.  Here is the maths, stone averages about 16 pounds per square foot (3 cm) x 453.6 grams per pound = 7257.6 grams per square foot.

7257.6 x 100,000 bacteria per gram (NASA’s figure)= 725,760,000 bacteria per square foot of countertop.  Average top is around 75 square foot, so 725,760,000 bacteria per square foot x 75 average square foot top = 54,432,000,000 bacteria per countertop.

Over 54 Billion bacteria, just in the cracks and crevices alone, not counting what is on the surface.  If a 5 log reduction could be done reliably, there would still be over a half million bacteria left in the cracks and crevices after disinfection. Twenty minutes after disinfection, the bacteria would have doubled to one million, forty minutes, two million, an hour later four million, and so on.  In about four hours, the bacteria have multiplied to 2,229,534,720.  In seven hours, the bacteria will have multiplied to 150 billion bacteria.

Keep in mind that these NASA figures were for rock taken from a desert environment, with little food and water available for the bacteria.  Imagine the average stone top in a London home, soaking in moisture from cleaning the top surface, and humidity from the bottom and edges.

On the studies page, there is a Brazilian/Portuguese study that showed up to 590% more bacteria retained on a granite surface than a plastic surface such as polypropylene or polyethylene.  The study concluded that granite was more prone to colonization by bacteria and that sanitizing granite countertops was very important.

This thread was from a stone fabrication site: Smelly Granite Bacteria or mould? And while they didn’t mention bacteria or mould, the advice to use a cleaning product containing bleach left no doubt as to what was causing the problems.  

Another study showed Salmonella bacteria thriving in granite outcroppings. Even surviving desiccation for extended periods.  Prior to this study, it was thought that Salmonella were present in human and mammal intestinal tracts and were spread by poor practices in slaughter houses.  Now it is know that regardless of the source, from the stone when it was mined or from meat juice contamination, the granite provides a suitable home for the bacteria to thrive.

Another study shows the dangers of not having a regular deep clean causing bacteria found in the soil because of the natural antibiotics secreted by plants and other bacteria.  Some will say that the natural bacteria that normally colonizes granite will keep the quantity of dangerous bacteria down by competition, yet this study shows that sometimes it makes the dangerous bacteria more resistant to household cleaners.

Granite is a proven home for bacteria, despite other claims.  Cleaning granite countertops must be done on a regular basis to keep the population of bacteria down to a level that a healthy human immune system can handle. This is easily done, yet the harsh cleaning solutions quickly strip any sealer present on the surface of the granite as well as degrading any resins used to fill pits and fissures in the slabs during polishing. So you have a choice,  unsanitary countertops, easily stained countertops or contact a professional stone mason who is able to provide an effective method of cleaning deep down in the granite, followed by sealing the natural stone and restoring its shine.