Staphylococci bacteria are everywhere. They can be found on humans from our heads to our toes, on our cats and dogs, all the way to the sausages in your fridge.

Their name comes from their grape like appearance under the microscope , with staphle meaning a bunch of grapes.

To date there are 86 other Staphylococci species that have been lumped into a group called the coagulase negative staphylococci, or ‘CoNS’. However, it is another species of staphylococci, Staphylococcus aureus which gets most of the attention and bad reputation, often as a result of being identified as the “Superbug” MRSA.

The most well known Staphlococcus; MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) has been studied more because it is the most virulent of the Staphylococci. It is a great threat to patients in hospitals and it is more and more common worldwide. MRSA is also easy to identify and link to disease. It is characterised by having picked up a gene giving it resistance to methicillin (called mecA).

If you have had an operations you have likely been swabbed for MRSA and if positive, given an antimicrobial wash to try and decolonise your body and prevent this superbug getting into the blood or tissues during surgery, where they can cause painful and difficult to treat infections.

So many other Staphlococcus species

Many other species of Staphlococcus in the CoNS group live happy lives on our skin and even have benefits. It’s thought S. epidermidis is massively important at the very beginnings of life, colonising babies skin at birth and protecting them from the dreaded superbug MRSA. However, if these species get into the wrong place – say a prosthetic joint during surgery – these protective friends of ours can cause serious damage. When Staphlococci form communities known as biofilms on prosthetic heart values or hip joints they can have disastrous outcomes.

The remaining lesser-known CoNS that call our bodies home often get mixed up with MRSA samples taken in hospital. Eeven when we are careful, we can accidently pick up these commensal bacteria when taking samples like bloods and urine samples to lookfor infections. As Staphlococci can cause disease but are usually harmless it can be hard to differentiate between the good bacteria and the bad bacteria. For these reasons, we are unsure on the real burden staphylococcal infections caused by CoNS have on our health system.

What makes some Staphylococcus species switch from good bacteria to bad bacteria?

My current favourite Staphylococcus is Staphlococcus capitis. There are two sides to S. capitis.

There is the good side where S. capitis lives a happy life in oily rich areas including on your head. In a.

On the other side, Staphlococcus capitis sometimes can infect babies’ blood and cause late onset sepsis which is very dangerous. When you get into the nitty gritty, there is tiny difference in the DNA of those strains which cause disease and those which do not. When I say tiny, this may be less than 10 changes in the 2.5 million letters that make up their DNA.

So, if it is not large genetic changes that causes the switch from good to bad, then what else is it?

I see this problem a lot. S. epidermidis can cause a prosthetic joint infection, but a near identical isolate is living happily. on skin. Another strain of Staphylococcus, S. haemolyticus can cause sepsis in an adult or but also live content in a neonatal gut.

Why are some bad and some good when they are near on identical? Is it the conditions they find themselves in? Do the patients have weakened immune systems or lack of previous exposure to such bacteria?

What I think this behaviour tells us is that even species which are usually harmless can be a problem in the wrong circumstances and that there are many different strains within these species.

We need to understand what makes a good bacteria occasionally turn bad. We cannot eradicate bacteria and other microorganisms from babies or skin– these microbes are part of our natural world and removing them would potentially expose us to many more harmful organisms.

The future will hopefully include a better understanding of when and how good bacteria turn bad and find ways for us to identify quickly when this is happening and apply quick and effective treatments when needed. We cannot live without our companion bacteria so we need to learn how to live together in as harmonious a way as we can.

• More on Heather’s research into tracking Staphylococcus capitis behind life-threatening sepsis in premature babies
• Watch Nature’s Skin Microbiome video