How Many Bacteria Live on a Pair of Glasses?

A research team led by Prof. Dr. Markus Egert (Microbiology and Hygiene, Furtwangen University, Schwenningen campus) has published two separate studies on the bacterial colonisation of glasses.

The first study (2018, published in PLOS One) examined 31 pairs of glasses using classic culturing on nutrient media – 21 from university staff and students, 10 from residents of a care home. Result: every pair of glasses examined was bacterially colonised, with roughly 60% of species being potentially pathogenic.

The second, molecular-biology study (2020, published in Scientific Reports) went deeper: 30 pairs of glasses worn in a university setting were examined at the lenses, temple arms and nose pads using DNA analysis – a method that also captures species that can't be cultured. Result: 5,232 different bacterial species from 665 genera were identified in total.

Interestingly, the 2020 study found the highest species diversity on the lenses themselves – not, as often assumed, on the nose pads (where diversity was lowest, though with high bacterial density at points of direct skin contact). On the lenses, alongside skin and mucosal bacteria (Cutibacterium, Corynebacterium, Staphylococcus), typical environmental microbes such as Pseudomonas were also found, likely arriving through the air.

According to the 2020 study, more than 80% of the 13 most frequently identified species are potentially pathogenic – meaning they can cause disease under certain circumstances, particularly in people with weakened immune systems. For healthy wearers, the everyday risk is low; it becomes more relevant, according to the study authors, in clinical settings, such as for ophthalmologists or opticians who regularly handle other people's glasses.

Whether there's a direct link between this 'eyewear flora' and recurring eye infections is, according to Furtwangen University, still subject to further research.

An earlier study by the same research group showed that damp cleaning reduces the bacterial load on glasses by around 95%. Prof. Egert explains: damp cleaning with alcohol-based or surfactant-containing lens wipes – or simply water and washing-up liquid by hand – is, based on current knowledge, a sensible strategy, including for removing viruses such as SARS-CoV-2.

An important clarification: this statement refers to manually wiping the glasses with a dampened cloth – not to using washing-up liquid inside an ultrasonic cleaner. That's a technically different use case (see Why you should never put washing-up liquid in an ultrasonic cleaner), where washing-up liquid is unsuitable for other, non-hygiene-related reasons (foaming).

Absolute bacteria counts aren't directly comparable between studies using different methodologies (culturing vs. molecular analysis). But the underlying finding – that regular face and skin contact leads to substantial bacterial colonisation – matches findings for other everyday objects like smartphones or keyboards (see The dirtiest objects in your household).