Bacterial growth in diluted shampoo/conditioners

cutout paper composition of sanitizer between viruses and microbes

Is it a myth that there is bacterial growth in diluted products? 

I think most groomers nowadays know that it’s not a good idea to save diluted products for a prolonged period.
We have been educated by posts on social media and grooming magazines by shampoo manufacturers and grooming educators.

I still how ever sometimes see posts/comments from old timers like myself that says- yeah, you say that-but where is the proof? I have been doing this for 30 years and never had a problem.

To show that it’s not just an opinion from people who want to sell more shampoo or want to blame you instead of their products if anything goes wrong,I have put together a post with some links to scientific papers.

As always, I try to make it brief- but not sure if I can succeed in that 🤣

I learnt about this in the 90ths when I was working as a veterinary nurse. I worked at a large animal hospital, and we washed every dog before they went home.
We suddenly had an outbreak of unexplained skin issues in dogs that had recently left the hospital and a lot of effort was put into finding out what caused it.

After a lot of sampling of both skin samples and environmental testing, it was concluded that the cause of the issue was bacterial growth in the shampoo containers in the bathing area.
Once new routines were put in place, the issue was gone.

And this is not an isolated example, comments in grooming groups shows that other larger veterinary facilities have had similar experiences and the same results after all the testing was done.  

A lot of people say -I have always been using diluted products that’s been stored for many days and I have never had a problem in my many years of grooming.
And that might be completely true, they might have been lucky.

 But when you dive into it, it turns out that they occasionally do have dogs with “allergic” reactions and blame the owner for not telling them that the dog is allergic. Or they blame the shampoo manufacturer for selling bad products.
And those reactions might as well be a mild bacterial infection and not an allergy/ badly manufactured product.

What is it that happens ?

Our environment is full of bacteria…. A bit scary when you think about it 😱 They are most of the time no issues for us. But once in a while there will be too many of them and that’s when they turn into an issue.

One of the most common one that is an issue for us as groomers are Pseudomonas aeruginosa.
( read more about it on Wikipedia )

This is a bacteria that is present everywhere -in water, on the skin, in the soil and it thrives in dark moist areas that lack oxygen.
It can cause anything from mild skin infections to severe internal infections that kill you.
Our immune system can usually control these bacteria, but if you have a weakened immune system due to ongoing health issues or being on immunosuppressant drugs, then it can cause severe problems.
Or if you suddenly get exposed to a large amount of them in one go -like being washed with a product that contains a large amount. Then it’s hard for the immune system to fight them all in one go and you can get an infection.

When we have an issue with these bacteria in the grooming salon it is usually a combination of several factors that makes a recipe for the “perfect storm”

Here are some examples:

  You have a matted dog that you are brushing out, the dematting irritates the skin and that opens for bacteria to get further down into the skin and multiply. If you on top of that add products that are contaminated with bacteria, the bacterial load with be too high for the immune system to fight and you get a skin reaction.

  Or you have a shorthaired dog that is in for de shedding. If you scrub it too hard, the short stiff hairs will move around in the hair follicles and cause irritation around the follicle opening. That irritation will lead to inflammation and the inflammation makes the skin more vulnerable and opens it for bacteria to get down into the skin. If you then add a product that contains bacteria, that will again be too much for the immune system to handle and you get a skin infection.

  Another scenario is if you have a dog that has been on steroids for a prolonged time. The steroid treatment causes the skin to be thinner and more fragile. It also shuts down the immune system and the dog is now left without defence against bacterial infections.
You pluck the ears as they are very hairy. The plucking causes itching, and the dog is scratching around its head a little bit. That scratching does damage to the already fragile skin and if you wash the dog with a product that contains an excess amount of bacteria, the bacteria will make its way down into the skin and multiply as the immune system is not killing it as it is shut down.
Infection with Pseudomonas aeruginosa in dogs with a compromised immune system can lead to sepsis and death, so we must be careful when dealing with those dogs.


The warm water in itself helps the bacteria to get into the skin as it opens up the scales on the surface and make it possible for them to get in further down.

How do the bacteria get into the products?

Grooming products are full of preservatives to prevent the growth of bacteria. But they only last for a certain amount of time and then the effect start to wear off. If your shampoo has been on the shelf for a long time, the protection has weakened. This can also happen if it is stored at high temperatures.

Once you dilute the product, then you also dilute the preservatives, and the protection is now almost gone. And remember what I said in the beginning, this bacteria is present in the water. Which means that you are adding bacteria to the mix.
So, you now have a mix with a small number of bacteria and very few preservatives to fight it. That will be no issue if you use the diluted product within a couple of hours as it is only a small number of bacteria.
But if you then decide to store it for a day or two, that gives the bacteria time to multiply in that bottle. Remember that it thrives in moist, dark, and warm environments….

You can also have bacterial growth in undiluted products despite the full amount of preservatives. The preservatives can only deal with so much bacteria and will at one point not be able to deal with it anymore.
This happens when you have water present in the product container. The bacteria will grow in the water, not in the product itself, but every time you pour some product into your mixing bottle, some of the contaminated water will also be transferred.

There are several ways the water ends up in the product container- the two most common are:

        condensation in the container from the humid air in the bathing area.

        Water entering the container when you wash the dogs

If you store your products close to the bath or in the bath, then it’s a high risk that you have water entering the container every time you wash a dog. This is the most common reason for issues with undiluted products.

The best way to avoid this issue is to store them in an area that is either so far away from the bath that it’s no risk they get showered or keep them covered if you must keep them close to the bath. (In a cabinet with closed doors) Keeping the container inside the bathtub is not a good idea, as that will really open up for water to enter it.

So, where is the science now in all these claims?  I have never had any issues……

Here are some scientific papers that discuss the issues:

    “Bacterial contamination of shampoo or cream rinse seems to be pivotal in the development of Post grooming furunculosis.
In particular, self-serve dog washing facilities and grooming parlours using large communal containers of bathing products have been implicated.
Recommendations for avoiding post-grooming furunculosis include diluting shampoo or cream rinse on the day it is used, sterilizing all community containers (including pump nozzles) on a daily basis”

Recognizing Pyoderma, More Difficult than it May Seem
Kinga Gortel, DVM, MS

”    Pseudomonas aeruginosa contamination of dog grooming shampoos and conditioners was significantly associated with product dilution. Contaminated grooming products may predispose dogs to severe bacterial skin infections.”

.Pseudomonas aeruginosa isolation from dog grooming products used by private owners or by professional pet grooming salons: prevalence and risk factors
Elad Perry, corresponding author 1 Gila Abells Sutton, 1 Lotem Haggag, 1 Marcelo Fleker, 2 Shlomo Eduardo Blum, 2 and Ronnie Kaufmann 1

”     This case report describes a dog with post-grooming furunculosis in which Pseudomonas aeruginosa with an identical genetic fingerprint was isolated from the skin lesions as well as from the shampoo used before the disease onset.
Conclusion and clinical importance: Our clinical investigation suggests a link between Pseudomonas-contaminated shampoo and the development of post grooming furunculosis, and underscores the need for hygienic management of shampoos to help limit this disease.”

Molecular confirmation of shampoo as the putative source of Pseudomonas aeruginosa-induced post grooming furunculosis in a dog Heng L. Tham*, Megan E. Jacob† and Petra Bizikova*‡

”    The dog was diagnosed with post-grooming furunculosis and mitral valve endocardiosis ACVIM stage B2. The culture of the lesional skin and the shampoo used to wash the dog prior to the onset of the skin lesions revealed the presence of the same bacteria, evidencing a clear link between bathing and the development of the skin lesions.”

Post-grooming furunculosis in a dog
L. Rutten, P. Smets, S. Vandenabeele

”   furunculosis following water immersion or grooming has been described in dogs. Furunculosis has been proposed as a sequela to follicular trauma from vigorous manipulation of the skin and coat (ie, brushing or coat-stripping), followed by bathing with contaminated shampoos or conditioners, particularly bulk formulations that are diluted for use in commercial grooming facilities.”

.Clinical and histopathologic features of dorsally located furunculosis in dogs following water immersion or exposure to grooming products: 22 cases (2005-2013)
Christine L. Cain, DVM, and Elizabeth A. Mauldin, DVM

Its not just grooming related

The issue is not isolated to dog grooming . It happens everywhere where you have liquid products that we handle.
This  Japanese study done on products used in hairdressing/barber shops  shows a large volume of bacteria in the products that are used

“To determine the extent of contamination by bacteria of hair-washing shampoo and rinse used professionally at barber shops and hair-dressing saloons, quantitative isolation of bacteria was performed by using a total of 39 samples of shampoo and rinse fluid obtained from 17 facilities.

It was found that a maximal number of 1•~107/ml colony forming units/ml of bacteria were isolated from 60.7% (17 out of 28 samples) of the shampoo and 45.5% (5 out of 11) of the rinse. Gram-negative bacilli were the predominant strains (87.9%) involved in bacterial contamination and the major isolates were Serratia marcescens (43.3%, most frequently isolated), Pseudomonas cepacia, P. fluorescens, P. aeruginosa and Klebsiella pneumoniae, all which are widely recognized as nosocomial-infection causing pathogens.

 These results indicate that hair-washing liquids for professional use are contaminated with a great number of gram-negative bacteria, being possible causes of nosocomial infections, and much attention should be paid to the sanitation and cleanliness of the shampoo and rinse for hair-washing.”

Bacterial Contamination of Hair Washing Liquids
Kazuhiko Amemiya, Fumiaki Taguchi

This paper proves that you can have contamination in diluted products and the  danger this contaminated shampoo can be to immunocompromised individuals

“The diluted shampoo used was heavily contaminated with Pseudomonas aeruginosa that infected the small nicks from shaving. These infections fulminated in these immunosuppressed patients and proved fatal to one.”

Hair Clipping: Another Hazard for Granulocytopenic Patients?
Victor Fainstein, Nita Andres, Jan Umphrey, Roy Hopfer

How can I reduce the risks?

     The main takeaway is that you should not store diluted products.

     Mix what you need for the day and dispose of any leftovers at the end of the workday.

     Rinse out your mixing bottles with hot water and it’s a good idea to also give them a quick rinse with a disinfectant and then store them upside down. That allows for any leftover water to drain out and the inside of the bottle to dry. The bacteria like moisture, so a dry container won’t be a beneficial environment for it.

     Never put your fingers inside the product container, that is a sure way of adding bacteria. A recent study showed bacteria growth inside a shampoo bottle cap in a grooming salon,so make sure you dont touch the inside if you are going to reuse the cap.

      Wash your hands before handling a pump that you are going to put into a container 

       If you are using pumps for your products, make sure they are cleaned inside out and disinfected before you put them in a new container. Pull it apart and clean out any remaining products from it.

       Put a cap on the pump opening to reduce the amount of humid air that can enter it.

       Don’t store products in a hot environment. And don’t bulk buy products that end up being stored for long a time. Preservatives efficiency is reduced over time and that allows for bacterial growth.  

       Don’t keep your products in the bath/just beside it so that water can get into the container.

       Put the opening date on your containers to make it easier for you to keep track on how long they have been opened.

       If you have a container that has been open for a couple of months, make it a habit to check the content daily before you use it. Smell it and look at it, you don’t want a foul smell or change in colour.


If you want to learn more about storage of your products and general product safety -listen to this episode of the GroomPod where Dave Campanella that is a manufacturer talks about the subject.
Barbara Bird that is one of the hosts also talk about pseudomonas aeruginosa.

Click here to listen


I hope that my brief take on the subject have given you a bit more information about the subject and also showed you the importance of proper handling of the products that we use. 

I know I have talked a lot about the risks for the dogs, but the risk applies to you as a groomer as well.
If you have damaged skin on your hands and a compromised immune system, you are at high risk to develop a severe skin infection from contaminated products – please dont take that risk, remember the cancer patient that died.

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