Microbes found on Rembrandt's The Night Watch in Amsterdam, including a Japanese sugar-refinery bacterium

Samples taken from the canvas

Microbiologists from Micropia, the microbe museum at Artis zoo in Amsterdam, worked with restorers at the Rijksmuseum to sample Rembrandt's The Night Watch (1642). Restorers pressed sterile cellulose discs against the canvas, and the samples were placed in Petri dishes until visible colonies formed. Reports differ on the number: German and Spanish outlets cite about 60 samples, while a Dutch report describes ten samples taken at six moments. The sampling covers varnished and unvarnished areas and is set to run over roughly six months. A negative control was used to confirm that the microbes came from the painting and not from the sampling itself. The results are now on show in the exhibition 'The Night Watch Lives', where the colonies sit in Petri dishes beside a full-size reproduction of the painting.

What the microbes are

Researchers identified at least 20 types of microbes, according to one report. Many are bacteria commonly found on human skin, which can help protect against pathogens, while others typically occur on plant roots or in soil. Lysinibacillus sphaericus, a soil bacterium, tolerates high levels of heavy metals, and Micrococcus luteus belongs to the normal skin and mucosal flora of humans. Museum officials were relieved that no threatening pests turned up on the painting. Van Bloois says the microbes do not harm the artwork.

A bacterium from a Japanese sugar refinery

Among the findings is a bacterium from the genus Saccharophyllum, first described in 2013 after being found at a sugar refinery in Japan. It turned up on a spot where the varnish had been removed during restoration. Other reporting says the species is usually found in a region of Japan that has a sugar refinery. How it reached a 17th-century painting in Amsterdam is a puzzle the researchers cannot yet answer. The first results were presented on Tuesday, 10 November, while the sampling is still under way.

How old the microbes are

Scientists cannot yet say how long the microbes have been on the canvas, because a single microbe cannot show whether it has been there for 400 years or just a day. Van Bloois said it is not likely that the microbes come from Rembrandt himself. Pijnenborg, the project lead and head of the Artis museums, described the study as partly an offbeat experiment and partly a marketing stunt. Microbes form the basis of life on Earth, yet most people do not realise how widely they are present, which is the gap the exhibition is meant to close. The exhibition is meant to spark public curiosity about the invisible life around people.

Operation Night Watch

The microbe study forms part of Operation Night Watch, described as the largest research and restoration project on the painting, which has been running for about seven years in view of visitors at the Rijksmuseum. Restorer Esther van Duijn and ten colleagues have spent the past two years removing yellowed varnish, though so far only two narrow strips at the side have been done. Earlier findings from the project include a hidden sketch, changes in the composition, a lead layer applied to protect the canvas from damp, arsenic pigments and a dog inspired by another artist. No completion date for the restoration has been set.