“I have a patient with positive blood cultures who we sent home 4 days ago”, they said in a slightly panicked voice.
“What did the blood culture grow?” asked the Microbiologist.
“They had a temperature of 38.5 oC and so had blood cultures taken, but clinically it looked like they had a viral upper respiratory tract infection, so we discharged them”, came the reply.
“But what did the blood culture grow?” asked the Microbiologist again slightly louder this time.
“The patient was fine, the temperature settled with Paracetamol. But I think I need to get them back in, repeat the blood cultures and start IV Piptazobactam for sepsis” continued the doctor starting to border on the hysterical.
“STOP TALKING, JUST TELL ME WHAT THE BLOOD CULTURE GREW!” shouted the Microbiologist in a last ditch effort to break through the A&E doctors ranting.
“Oh! It grew a Cutibacterium spp. and I’ve never heard of it before so thought it must be significant”.
“Okay. Take a deep breath and calm down” said the Microbiologist “you might not have heard of Cutibacterium but I am sure you have heard of Propionibacterium acnes?”
“Yea, that’s a skin contaminant but it definitely says Cutibacterium” muttered the A&E doctor now frustrated at the Microbiologist!
“Good, now let me explain the joy that is changes in names…” replied the Microbiologist, "while you're on the phone do you know what these used to be called too?!? I'm doing a quiz for Friday lunchtime..."
Up until 2016 Cutibacterium spp. were called Propionibacterium spp. The most commonly isolated organism in this family was Propionibacterium acnes which is now called Cutibacterium acnes. The organism was originally called “Propionibacterium” because it was able to produce propionic acid, a naturally occurring carboxylic acid with an unpleasant smell a bit like body odour. Propionic acid has antifungal and antibacterial properties so it is sometimes used in animal feeds and baked goods as a preservative (though why anyone would want to eat something that smells of body odour I really don’t know!)
The name for these bacteria was changed after genetic studies showed there were different types of “Propionibacteria”; the main group that affect humans and live on human skin were renamed the Cutibacterium spp. The other two species of Propionibacteria are Acidipropionibacterium spp. and Pseudopropionibacterium spp. so lets just be thankful that these environmental bacteria aren’t thought to cause human disease and therefore we don’t have to report them from clinical specimens!
How do you identify Cutibacterium spp.?
In my experience there are five simple factors that allow you to identify Cutibacterium spp.
- Gram film appearance
- Anaerobic culture
- Metronidazole resistance
- Slow growth
- Catalase reaction
The Gram film appearance is very striking; Cutibacterium spp. look like squashed spiders! Cutibacterium spp. are long, slender, filamentous Gram-positive bacilli which tend to coil up together to form a ball with sticking out bits. They basically look like someone has squashed a long-legged spider and the legs have splayed out in crooked directions. This identification is often enough for an experienced Biomedical Scientist to say the bacterium they are seeing is a Cutibacterium spp.
The next two identifying characteristics tend to be done at the same time. They are an anaerobic bacteria (not growing in the presence of oxygen) and in contrast to most anaerobes they are Metronidazole resistant. A Metronidazole disc is placed on a blood agar plate and incubated anaerobically. Cutibacterium spp. usually takes 48 hours to produce faint colonies on blood agar but in blood cultures it can take 4-5 days to grow enough to trigger automated incubators.
The final easy test for Cutibacterium spp. is the catalase reaction. Hydrogen peroxide is put on a glass microscope slide, a small amount of the bacterium is put on a cover slip, and then the cover slip is the turned upside down so that the organism comes into contact with the hydrogen peroxide between the slide and the cover slip. If the bacterium has the enzyme catalase this will react with the hydrogen peroxide to produce oxygen and water; bubbles of oxygen will form under the cover slip (catalase positive reaction). Cutibacterium spp. are catalase positive which is again unusual for an anaerobe. In theory this should allow Cutibacterium spp. to protect itself from oxygen free radicals and allow it to grow aerobically… and yet it doesn’t … before you ask I haven’t been able to find out why. If you know why it is still an anaerobe despite having the enzyme catalase then please let me know as I have wondered this for years!
How do infections with Cutibacterium spp. present?
Cutibacterium spp. are normal flora of human skin. They have very low virulence; that is they are not very good at causing infection. Most of the time Cutibacterium spp. are isolated they are contaminants from the skin. Often when a blood culture signals positive after day 4 it is only the anaerobic bottle that is positive and squashed spiders are seen in the Gram film. These are almost always skin contaminants of no clinical significance. I have seen thousands of contaminated blood cultures like this over the years and I have never seen a significant positive blood culture with this bacterium although it is theoretically possible.
The most common “infection” caused by Cutibacterium acnes, as its name suggests, is acne vulgaris. Acne vulgaris, or acne for short, is the common inflammation of the hair follicle and its associated sebaceous gland seen primarily in adolescents. It is so common in adolescents (up to 90%) that it is almost considered a “normal part of teenage-life” although it can be extremely psychologically, emotionally and socially debilitating. It is said to continue into late adult life (>50 years) in 10-15% of adults. During puberty adolescent sebaceous glands start to produce increased amounts of sebum (an oily substance that protects skin) and Cutibacterium acnes is able to use this sebum as an energy source and so the bacterium proliferates in the gland causing inflammation leading to acne.
Other than in acne, Cutibacterium spp. are opportunistic pathogens exploiting a failure in the bodies host defence to prevent infection. The most common reason for this is the presence of metal or plastic which the bacterium uses as a site to grow. Cutibacterium spp. can therefore cause infections in prosthetic joints, intravenous access devices such as Central Venous Catheters (CVCs), prosthetic heart valves (which would give significant positive blood cultures), central nervous system shunts and breast implants. In all of these cases the infection is thought to occur because the prosthetic material is contaminated at the time it is put in rather than afterwards by haematogenous spread. Because they are slow growing bacteria, infections of prosthetic material with Cutibacterium spp. can take months and years to present. The longest time I have seen is eleven years after spinal surgery!
How are infections with Cutibacterium spp. treated?
The mainstay of treatment of Cutibacterium spp. is to remove the prosthetic material. In my experience it is impossible to get rid of the infection with the prosthetic material in place because the bacteria have been there for a very long time and have built up biofilms (slime cities) which antibiotics alone are unable to treat.
Having removed the prosthetic material antibiotics are usually given to mop up any residual bacteria. Antibiotics should be based on sensitivity testing and the site of infection, for example infected bone requires higher doses of antibiotics.
Most Cutibacterium spp. are sensitive to the beta-lactams and the glycopeptides and these are often used empirically until sensitivities are known. The duration of therapy is dependent on the site of infection, for example CVC infection 5-7 days after removal of CVC, endocarditis 4-6 weeks after valve surgery.
The treatment of mild acne is relatively straightforward with topical retinoids and topical antimicrobials such as Benzoyl peroxide. In more severe cases care is best done under an expert such as a Dermatologist; possible treatments in terms of antibiotics commonly include the use of the macrolides such as Erythromycin or Clarithromycin and the tetracyclines such as Doxycycline or Minocycline.
How do you prevent infection with Cutibacterium spp.?
It is impossible to prevent Cutibacterium spp. getting on the skin and becoming part of the normal skin flora. It is also impossible to prevent these bacteria getting into hair follicles and sebaceous glands to cause acne.
To prevent deeper infections of prosthetic material requires excellent aseptic surgical technique. Factors such as operating theatre cleanliness, hand hygiene, sterile gloves, surgical gowns and patient skin preparation are all important. Proper maintenance of the operating theatre, with correct air flow and pressure differences also help prevent any skin bacteria that might settle in the operative site from doing so.
So once the A&E doctor assured the Microbiologist that their patient did not have any metal or plastic prosthetic material including joint replacements, IV devices, prosthetic heart valves, shunts or breast implants, both were reassured that no further action was required. The patient stayed comfortably at home unaware of their imminent recall for repeat blood cultures and a long course of antibiotics. The A&E Doctor returned to the never ending patients in the waiting area, muttering “what a waste of time! Microbiologists don’t help themselves by changing bacterial names... and a lunchtime quiz? ...who has time for lunch?!”. The Microbiologist returned to authorising the laboratory results muttering under his breath about “non-urgent” calls for advice before 2pm and the wholly “unnecessary” bacterial name changes confusing the heck out of everyone. Who thought that was a good idea…academia grr!!
Here’s a list of name changes on a postcard bookmark for anyone with an old edition of Nuts & Bolts, it’s in the appendix of the 3rd edition :-)
Lunchtime Quiz Answers:
1) Marathon, 2) Opal Fruits, 3) Jif, 4) BT Cellnet, 5) Abbey National, Alliance & Leicester & Bradford & Bingley, 6) The Beach Boys, 7) Brad's Drink, 8) Blue Ribbon Sports, 9) Pete's Super Submarines, 10) Jerry and David’s Guide to the World Wide Web