We’re sometimes asked if finding particular bacteria in a sample could indicate the presence of a specific disease — or even guarantee the absence of any medical condition.
The answer to both of these questions is no. In order to understand why, let’s take a quick look at the taxonomy of biological organisms.
The way that biologists classify life forms today is by grouping like with like, in a series of nested classes. Bacteria – and all organisms – are classified based on shared descent from their nearest common ancestor.
The levels within this system are categorized as follows:
To see this in action, let’s look at the classification of a familiar pet – the common house cat.
When people read about the bacteria in their microbiome, they might think we are referring to the disease-causing strains, when they are in fact being told about the genus level. Let’s take a closer look at one of the bacteria you may find in your results.
Streptococcus is one of the most common bacteria in your microbiome and is typically found in your nose and throat. However, Streptococcus is simply the genus name. If we look at the various species within this genus, we find they perform very different functions. For example:
- The helpful Streptoccus sanguis inhabits most healthy human mouths. It is most often found in dental plaque, where it helps eliminate the other strains of bacteria that cause cavities.
- Live cultures of Streptococcus thermophilus can be used to help people who are lactose intolerant digest dairy products. These bacteria break down lactose, which is the sugar in milk that people have difficulty digesting.
- Streoticiccys pyogenes is the species directly responsible for a range of infections, including strep throat.
Many of the Streptococcus species are working to help keep you healthy. If all the species within thisgenus resulted in strep throat, we would all have run out of lozenges long ago!
We identify bacteria at genus level. In other words, our method of sequencing would be able to tell you that a member of the Felix genus is in your backyard. What we cannot tell you is if you have a jungle cat on your hands, or if it’s your neighbour’s tabby William Shakespurr, hoping for a saucer of milk.