By Siouxsie Wiles 29/04/2018

Recently, a lab-based study was published about menstrual cups, tampons, and toxic shock syndrome. In this post, microbiologist Siouxsie Wiles reflects on what the study can and can’t tell us about menstrual cups and toxic shock.

If you need reminding, toxic shock syndrome is a very rare complication of a bacterial infection that can quickly result in organ failure and death. In a previous post (published here and here) I wrote about one of my take-home messages from the recent study (1): that bacteria may stick to menstrual cups as they are used over the course of a period, and if you just follow the advice of wiping or rinsing after each use, you could be putting a cup inside your vagina that’s becoming more and more covered in potentially harmful bacteria. My advice was to get two and sterilise after each use.

As I described in my posts, in the study, the researchers tested a bunch of tampons and menstrual cups by putting each one in a sterile bag and adding 15 mL of brain heart infusion broth. That’s a nutritious bacterial growth medium made from calf brains, cow hearts and a bunch of other less yucky sounding ingredients. Then they added some S. aureus that they know produces the toxic shock toxin.

Microbiologists tend to quantify bacteria as colony forming units (CFU) per mL, to make it easier to make comparisons. In the paper, the authors write that they added one hundred thousand CFU per mL so that’s 1.5 million bacteria in total. After 8 hours they measured how much the bacteria had grown, how much of the toxic shock toxin the bacteria had made if any, and whether the bacteria had stuck to the tampons or cups. They also took measurements from bags that contained no tampons or cups. This is what’s known as a positive control and shows that the bacteria are behaving as they should in lab-based conditions.

I’ve summarised the study findings in the graph below. Basically, the higher the bar, the more bacteria, and the bigger the scary-looking toxic symbol, the more toxin they produced. Please note, the axis showing how the bacteria have grown is on a log scale, so each jump up is 10x more than the previous. The data presented is the number of bacteria per mL, so to get the total number of bacteria, multiply by 15. As it’s all relative, you don’t have to do any maths if you don’t want to.

No, you aren’t more at risk using no sanitary products at all…

I’ve had some feedback that one of the columns in that graph is a bit confusing. It’s the one that shows that the bacteria grow really well (and in many cases, better) without tampons or menstrual cups. As I said earlier, that’s the positive control that shows the bacteria are growing ok. Some people have interpreted that data as meaning you are MORE at risk if you don’t use any sanitary products at all. That’s not the case. It is interesting that the bacteria grow less well when most of the tampons are added, but there could be lots of reasons for that: absorbance of the growth media by the tampon, inhibitory molecules in the tampons. The fact the bacteria do grow, and by several orders of magnitude in some cases, is what is important.

All models are wrong, but some are useful…

As I said in my original post, this is a lab-based study so while I think it tells us some interesting and important things, the reality is that it is just a ‘model’ system and obviously very different to an actual tampon or cup-wearing person. For example, humans clearly aren’t menstruating 15 mLs of brain heart infusion broth. They are menstruating blood and mucosal tissue from the lining of their uterus. And vaginas obviously aren’t sterile plastic bags, they are dynamic ecosystems with their very own mix of microbial residents. And they are an organ present in an actual person with an immune system whose job is to try to protect us from infection. What impact do these things have on how S. aureus grows? And how relevant is the amount of bacteria the researchers used to start their experiments? Are there 1.5 million S. aureus cells present in someone’s vagina? We don’t know. Would the results have been different with more or fewer bacteria? Possibly, possibly not. As statistician George Box said, all models are wrong, but some are useful.

A dearth of studies…

Here’s the interesting thing. When looking into what studies have been done on menstrual cups, I found the answer to be bugger all. It looks we’re making judgements about the safety of menstrual cups based on their use in less than 800 women and young girls. I only found one study that specifically looked at vaginal colonisation of cup wearers by S. aureus. It was a study of 604 menstruating 14 to 16-year-olds in western Kenya (2). The authors reported no instances of toxic shock syndrome in the 11 months they followed their cohort. But that’s hardly surprising. If the incidence was the same as seen in tampon-users then they would have needed more than 25,000 people in the trial to have a chance of seeing one case of toxic shock syndrome. Yet that is what we’re using as evidence that there is no risk of toxic shock syndrome.

If you are as appalled as I am at how little this has actually been studied and want to help me change that, then read my post here. I want to see how feasible it might be for us to use crowdsourcing and crowdfunding to answer all the questions people have about menstrual cups. Just how much do bacteria grow on them, and what kinds of bacteria? How should they be cleaned? Let’s figure this out!


  1. Nonfoux, et al. Impact of currently marketed tampons and menstrual cups on Staphylococcus aureus growth and TSST-1 production in vitro. Applied and Environmental Microbiology. doi: 10.1128/AEM.00351-18
  2.  Juma, et al (2017). Examining the safety of menstrual cups among rural primary school girls in western Kenya: observational studies nested in a randomised controlled feasibility study. BMJ Open. 7(4):e015429. doi: 10.1136/bmjopen-2016-015429.