Is green mucus a sign of a bacterial infection?

Can you tell the stage and severity of your cold by the colour of your snot? A very important question for this time of year!
04 February 2014



Is it true that if you have a chest or head cold, and the mucus turns yellow or green, this is a sign of a bacterial infection and requires antibiotics? Is this a way to tell viral from bacterial infection? Surely in this age of resistant bugs, it is better to let mild infections run their natural course even if they are bacterial? Also, can alcohol hand rubs prevent transmission of viral illness? They say "antibacterial" on the bottle.



Hannah - With post Christmas sniffles sticking around, what do you think about this one?

Sarah - Hi, Naked Scientists. My name is Sarah and I live in Tasmania, Australia. I'd like to know, is it true that if you have a chest or a head cold and then mucus turns yellow or green, this is a sign of a bacterial infection and requires antibiotics?

Hannah - So, can you detect the stage and severity of your cold by the colour of your snot? Over to Dr. Estee Torok, consultant in infectious diseases at Cambridge University.

Estee - Mucus is something that everybody has. It's produced by the goblet cells in the epithelial tissues which line the mouth, the nose, the sinuses, the throat, the lungs, and the intestinal tract. It acts as a protective blanket over these surfaces and prevents the tissues underneath from drying out. Mucus acts as part of the body's defence system by trapping unwanted substances like bacteria and dust before they get into the body. It also contains antibodies that recognise invaders like bacteria and viruses, and enzymes that kill the invaders that it traps in the protein called mucin. Even when you're healthy, your body turns out 1 to 1.5 litres of mucus per day and most of this trickles down your throat so you don't even notice it. However, there may be times when you do notice your mucus and this isn't usually because you're producing more of it, but because a consistency has changed. The things that can trigger these are respiratory infections or allergies, or contact with something that's irritating.

Hannah - In which case, can the colour of your snot signify whether you have a bacterial infection?

Estee - If you look at your mucus, normally, it's sort of quite clear. But at times, it can be yellow or green or even red or brown. When you get an infection and that can be bacterial or viral, the bugs damage the epithelial cells lining in your nose and throat and can cause inflammation. Your body responds by sending white blood cells called neutrophils to fight the infection and these neutrophils produce enzymes called myeloperoxidase enzymes that release oxygen and free radicals to kill the bugs or the viruses. These enzymes contain iron and the iron is what gives the mucus a green colour. So the neutrophils can die during the process of killing the infection and the green mucus therefore doesn't mean that it's necessarily a bacterial infection. It may be a viral infection.

Hannah - So, green snot can signify active immune cells giving off iron to help fight either a viral or bacterial infection. It doesn't necessarily mean you need antibiotics though. Thanks to Estee and also to Sarah for getting in touch with the question.

Chris - And our listener (Dan Manachuck) got in touch agreeing, saying the colour of phlegm has nothing to do with being a cold or flu having a bacterial infection on top. The green colour he says comes from dead cells and Steven on Facebook says, "If it's a pucker cold, it's usually is clear when you have the sneezing fits and that increasingly turns green as the cold is on the way out or going chesty." I'm usually pleased when it goes green when I have a cold because the end is usually neigh. With that cleared up, let's hear back from Hannah what we got in store for next week's question...

Hannah - Do you pick your nose with your left hand or are you a right-handed picker? Our next question is in from (Ray)...

Rey - Why are people either right-handed or left-handed? What possible benefit does that have over being ambidextrous? I find I'm right-handed, right-footed, and even right-eyed. When I wore a single muff headset on my job though, I preferred it on my left ear and not my right. So, that's a bit of a question. And do animals also display handedness?

Hannah - So, left or right-handed? Are other animals like this? Does it scramble your brain if you mix your left or right feet, arms and ears up?


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