Microbiology and Immunology

Acridine orange WORLD OF MICROBIOLOGY AND IMMUNOLOGY

2

•

In this altered hormonal environment, bacteriaplay a

role in the development of acne. The principal bacterial

species associated with acne is Proprionibacterium acnes.

This microorganism is a normal resident on the skin and inside

hair follicles. Normally, the outward flow of oil will wash the

bacteria to the surface and be removed when the face is

washed. However, in the androgen-altered hair follicles, the

cells lining the cavity shed more frequently, stick together,

mix with the excess oil that is being produced, and pile up in

clumps inside the cavity. The accumulated material is a ready

nutrient source for the Proprionibacterium acnesin the cavity.

The bacteria grow and multiply rapidly.

Two other bacterial species that live and grow on the

surface of the skin can be associated with acne. These are

Proprionibacterium granulosumand Staphylocccus epider-

midis. Their significance is less than Proprionibacterium

acnes, however.

As the numbers of bacteria increase, the by-products of

their metabolic activities cause even more inflammation. Also,

the bacteria contain enzymesthat can degrade the oil from the

oil glands into what are known as free fatty acids. These free

fatty acids are very irritating to the skin. Various other bacte-

rial enzymes contribute to inflammation, including proteases

and phosphatases.

The immune systemdoes react to the abnormal growth

of the bacteria by trying to clear the bacteria. Death of bacte-

ria combined with the immune response generates the material

known as pus. A hallmark of acne is often the pus that is

exuded from the crusty sores on the skin.

The altered environment within the hair follicle that

facilitates the explosive growth of Proprionibacterium acnes

can be stimulated by factors other than the altered hormone

production of puberty. The external environment, particularly

a warm and moist one, is one factor.

The damage caused by bacteria in acne ranges from

mild to severe. In a mild case of acne, only a so-called black-

heads or whiteheads are evident on the skin. More severe

cases are associated with more blackheads, whiteheads and

pimples, and also with inflammation. The most severe form,

called cystic acne, may produce marked inflammation over the

entire upper body, and requires a physician’s attention to

reduce the bacterial populations.

Reduction in the bacterial number involves slowing

down the secretion of the oil from the oil glands and making

the follicle pore more open, so that the normal outward flow

can occur. Oil production can be slowed in the presence of 12-

cis-retinoic acid (Accutane). Use of this medication is

reserved for severe cases of acne, as the retinoic acid can have

significant adverse side effects. Antibacterial agents can also

be useful. For example, many antibacterial creams and face

washes contain the compound called benzoyl peroxide, which

is very active against Proprionibacterium acnes.

Because the bacteria active in acne are normal residents

of the skin, there is no “cure” for acne. Rather, the condition is

lessened until biochemical or lifestyle changes in the individ-

ual lessen or eliminate the conditions that promote bacterial

overgrowth.

See alsoMicrobial flora of the skin; Skin infections

AAcridine orangeCRIDINE ORANGE

Acridine orange is a fluorescent dye. The compound binds to

genetic material and can differentiate between deoxyribonu-

cleic acid(DNA) and ribonucleic acid(RNA).

A fluorescent dye such as acridine orange absorbs the

energy of incoming light. The energy of the light passes into

the dye molecules. This energy cannot be accommodated by

the dye forever, and so is released. The released energy is at a

different wavelength than was the incoming light, and so is

detected as a different color.

Acridine orange absorbs the incoming radiation because

of its ring structure. The excess energy effectively passes

around the ring, being distributed between the various bonds

that exist within the ring. However, the energy must be dissi-

pated to preserve the stability of the dye structure.

The ring structure also confers a hydrophobic(water-

hating) nature to the compound. When applied to a sample in

solution, the acridine orange will tend to diffuse sponta-

neously into the membrane surrounding the microorganisms.

Once in the interior of the cell, acridine orange can form a

complex with DNA and with RNA. The chemistries of these

complexes affect the wavelength of the emitted radiation. In

the case of the acridine orange–DNA complex, the emitted

radiation is green. In the case of the complex formed with

RNA, the emitted light is orange. The different colors allow

DNA to be distinguished from RNA.

Binding of acridine orange to the nucleic acid occurs in

living and dead bacteriaand other microorganisms. Thus, the

dye is not a means of distinguishing living from dead

microbes. Nor does acridine orange discriminate between one

species of microbe versus a different species. However, acri-

dine orange has proved very useful as a means of enumerating

the total number of microbes in a sample. Knowledge of the

total number of bacteria versus the number of living bacteria

Facial acne caused by Propionibacterium acne.

womi_A 5/6/03 1:05 PM Page 2