Acid Mantle Eco System

“Acid mantle microflora” symbiosis, the first line of “skin barrier defence”

Skin Flora

The surface of the skin itself comprises several distinct environments. Areas such as the axilla (armpit), the perineum (groin) and the toe webs provide typically moister regions for bacterial growth. These “tropical forest” environments often harbor the largest diversity amongst the skin flora. Typical organisms include Staphylococcus aureus, Corynebacterium and some Gram-negative bacteria.

The bulk of the human skin surface, however, is much drier and is predominantly inhabited by Staphylococcus epidermidis and Propionobacterium. Proprionibacterium granulosum and Malassezia furfur will also be found.

The microflora of the acid mantle is an important part of human skin contributing to its function and activities.

The various resident species are of an advantage in most cases but under some circumstances and with some groups of people the skin microflora is involved in minor to major pathological processes, e.g. acne and nosocomial infections in compromised hosts.

Human skin surface lipid is composed of secretions from both the epidermis and the sebaceous glands. Sebum is a holocrine secretion formed from the disintegration of the sebaceous gland cells, and at the point of secretion is composed primarily of triglycerides (56%), wax esters (26%) and squalene (15%). In combination with a complex mixture of other lipid species from the epidermis, primarily ceramides 40% triglycerides 25% Neutral lipids 78% and free fatty acids 22%.

The skin surface and the pilosebaceous follicles are generally colonised by a stable mixed population of Propionibacteria, coagulase-negative staphylococci, micrococci, aerobic coryneforms and the yeast Malassezia furfur. The degree of colonisation varies widely between different individuals and different sites on the same individual.


Normal skin flora are microbes, mostly bacteria, that live symbiotically in and on the body with, usually, no harmful effects to us. Normal microflora may be harmless, beneficial or disease causing.

The spectrum of microorganisms changes with the growth and development of the individual. The womb provides a sterile environment for the developing foetus. Upon birth, the newborn is colonised with numerous different bacteria and fungi. During growth and maturation, different microorganisms predominate at various sites on the body.

Most human skin is populated with Staphylococcus epidermidis, Propionibacterium acnes and other microorganisms. These bacteria are normal microflora and are beneficial to skin. They help maintain the low pH of skin; which inhibits the growth of more harmful bacteria. They also consume the limited amount of nutrients available on skin; making it hard for other bacteria to establish themselves.

We have about 1013 cells in our bodies and 1014 bacteria, most of which live in the large intestine. There are 103104 microbes per cm2 on the skin (Staphylococcus aureus, Staph. epidermidis, diphtheroids, streptococci, Candida, etc.). Various bacteria live in the nose and mouth. Lactobacilli live in the stomach and small intestine.

The upper intestine has about 104 bacteria per gram; the large bowel has 1011 per gram, of which 9599% are anaerobes (An anaerobe is a microorganism that can live without oxygen, while an aerobe requires oxygen.) or bacteroides. Various bacteria and diphtheroids lightly colonize the urogenitary tract. After puberty, the vagina is colonised by Lactobacillus aerophilus that ferment glycogen to maintain an acid pH.

Skin Fungi

Normal flora fill almost all of the available ecological niches in the body and produce bacteriocidins, defensins, cationic proteins, and lactoferrin all of which work to destroy other bacteria that compete for their niche in the body. The normal flora has already colonised all available sites and has evolved to survive in its particular niche. It is very difficult for another organism to come in and take hold.

However, if the environment changes (antibiotic treatment, eating lots of processed sugar, defects in immunity) bacteria that are normally minor components of the flora can overgrow the site.

The resident bacteria can become problematic when they invade spaces in which they were not meant to be. As examples:

Staphylococcus living on the skin can gain entry to the body through small cuts/nicks.

Some antibiotics, in particular clindamycin, kill some of the bacteria in our intestinal tract. This causes an overgrowth of Clostridium difficile, which results in pseudomembranous colitis, a rather painful condition wherein the inner lining of the intestine cracks and bleeds.

At puberty, the number of bacteria on the skin surface increases. These include:

Proprionibacterium acnes (P. acnes)

Proprionibacterium granulosum Staphylococcus epidermidis The yeast, Malassezia furfur also increases.

The number and activity of P. acnes bacteria varies according to oxygen supply, nutrient supply and pH. Propionibacterium acnes is the most common gram-positive, non-spore forming, anaerobic rod encountered. P. acnes typically grows as an obligate anaerobe, however, some strains are aero-tolerant, but still show better growth as an anaerobe.

P. acnes can produce active enzymes and inflammatory mediators which contribute to the activity of acne.


Close-up of P-acnes bacteria

These include:

  • Lipases
  • Proteases
  • Hyaluronate lyase
  • Phosphatase
  • Smooth-muscle contracting substances

The lipases can convert triglyceride in sebum to free fatty acids. The free fatty acids increase clumping of the bacteria and thus colonisation of duct by more of them. The inflammatory mediators penetrate surrounding skin and are a cause of inflammation.

Skin microbiology As has been mentioned, the skin has its own ecology, with an abundance of microorganisms living on our surfaces. Typically these are harmless provided the skin is unbroken,

The organisms found on the skin are quite characteristic species and do not represent just a random selection of the organisms we encounter in our daily lives. These microbes respond in some way to factors produced by the skin that allow them to colonise this habitat. Primary interest is in P. acnes because of its potential importance in acne.

Acid Mantle and Acne

Few people go through their teenage years without experiencing some acne to a greater or lesser extent. It usually comes at a time when adolescents are undergoing considerable psychological change too, and is a serious problem in up to 15 per cent of people at this time in their lives. If the acne is inadequately treated, permanent scaring may result.

During puberty, we do know that it is often associated with excessive production of sebum. This occurs because of hormonal changes in adolescence and can result in obstruction of the tiny ducts through which the sebum passes on to the surface of the skin. This promotes colonisation at that site with a normally harmless bacterium called Propionibacterium acnes leading to the production of material that diffuses into the surrounding skin and causes the inflammation usually associated with acne.

A number of treatments are aimed at reducing the sebum excretion, and for severe cases some of the drugs used, such as isotretinoin, are given orally. Other treatments try to reduce the P. acnes colonisation and associated inflammation, and here antibiotics are used. Regrettably, we are now seeing a high incidence of antibiotic resistance to commonly used antibiotics. Androgen inhibitors are rarely used for adolescent acne but have proven to be very effective with adult acne.

Researchers in Microbiology are investigating the factors that influence the emergence and spread of antibiotic resistance in acne patients and in untreated individuals.

At the immunological level research has shown that the immuno-pathology of inflamed acne lesions is consistent with a classical type IV cutaneous hypersensitivity reaction. (Rosacea grade 5)

This may be caused by the leakage of a substance called interleukin-1alpha from the inflamed region around the ‘spot’ into the dermis. However, it has to be said that evidence for a direct role for microorganisms in acne is still lacking, the bacteria may simply exacerbate the inflammatory response.

Certainly, P. acnes are resistant to killing by phagocytes, the white cells whose role is to destroy invading organisms, and can induce a chronic inflammatory response.

Impaired Acid Mantle

Removal of the acid mantle on a daily basis through harsh alkaline washes and toners will lead to an imbalance of the micro flora. As has been mentioned, the skin has its own ecology, with an abundance of microorganisms living on the surface. Typically these are harmless provided the skin is unbroken, and the acid mantle remains intact.

Many skin care lines that are designed for active, oily or acne skin are over used by the sufferer, leading to an underlying sensitivity and an impaired acid mantle. The associated dryness that comes from this action exacerbates the blocking of the pilosebaceous duct, thus accelerating the micro-comedone problem that leads to the papule, pustule inflammatory action.

To maintain the delicate “micro flora”, the use of cleansers with a more acidic and balanced pH would maintain the first line of “skin barrier defence” the acid mantle.

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