PERSPECTIVES ON AXILLARY ODOR 197 RELATIONSHIP OF RESIDENT BACTERIA TO ODOR Apocrine secretion is sterile when it issues as a droplet at the follicular mouth. It quickly becomes colonized by the resident bacteria and develops the characteristic pungent odor. This odor is also generated when organisms are incubated with apocrine secretion. There has been some controversy about the nature of these organisms. At first it was thought that a variety of organisms living in the warm, wet, secretion rich axilla could generate the typical odor. The responsible organisms were then narrowed down to gram positives, while gram negatives gave a quite different, fetid-type odor (21). We undertook a quantitative bacteriologic analysis of the axillary microflora and correlated the findings with the type and intensity of odor. The axillary flora of 12 females and 12 males was assessed on each of five consecutive days. The results showed that the microflora was quantitatively very stable from day to day and there were no differences between the right and left axillae. In a study of 200 young adults, males and females supported about the same numbers of organisms per sq. cm. about 500,000 to 800,000/cm 2. The composition of the microflora is shown in Table IV (22a). Table IV Prevalence and Density of Axillary Resident Bacterial Flora Males (N = 128) Females (N = 77) % Density* % Density* Total Aerobes 100 5.84 100 5.95 Micrococcaceae 100 5.51 100 5.56 Lipophilic diphtheroids 85 5.40 66 5.36 Large colony diphtheroids 26 4.43 25 4.57 Gram negative rods 20 3.36 19 3.32 Total Propionibacteria 70 3.71 47 4.23 P. aches 47 3.86 30 4.26 P. avidurn 34 3.62 21 4.18 P. granulosum 8 3.61 5 3.65 *log mean per square centimeter Aerobic cocci (Micrococcaceae) were found in all persons at comparatively high densities. These were mainly Staphylococcus epidermidis. Accordingly, these are not likely to be playing a decisive role in determining the quality of axillary odor. Lipophilic diphtheroids were found in 85% of males but only 66% of females, a significant difference statistically. The other category of diphtheroids, those which form large colonies, was much less abundant, occurring in about 25% with no sex difference. Various gram negatives (Escherichia, Klebsiella, Proteus, Enterobacter) were found in about 20% but in low numbers, averaging about 1000/cm 2. They do not make any significant contribution to apocrine odor production. Anaerobes (Propionibacterium species) were found in 70% of males and only 47% of females, again a significant sex difference. However, anaerobes are also not involved in odor generation. They live within the depths of the follicular pores where oxygen tension is low. Since the apocrine duct empties into the follicular canal, the secretion would bathe a dense growth of anaerobes. If the latter could use apocrine secretion as a substrate, the secretion would already be odorous when it reached the surface.

198 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Very low numbers of organisms can be recovered from the shafts of axillary hairs. These are probably displaced from the surface. Although odor is more intense in an unshaved axilla, the hairs serve mainly to disperse the preformed odor (9). When individuals were classified as to whether they had intense, typical apocrine (acrid) odor or the distinctively different "acid" odor, interesting differences were found with respect to the composition of the microflora. First of all those individuals with strong odor had 2 to 3 times as many organisms. There were differences in the representation of diphtheroids between these two odor groups. Every subject with an acrid odor had lipophilic diphtheroids compared to about a 50% prevalence in the acid group. Moreover, the mean density in the latter group was only 50,000/cm 2 in contrast to an average of 800,000/cm 2 in the group with acrid odor. In regard to large colony diphtheroids, the difference between the two groups was also striking but numerically of lesser magnitude. About 50% of acrid odor producers were colonized by these particular diphtheroids (mean--250,000/cm 2) compared to 9% of the acid group (mean--1500/cm•). These findings point unmistakeably to diphtheroids as the source of the acrid apocrine odor, especially the lipophilic ones, invariably present in high numbers in these subjects. These relationships are summarised in Fig. 1. The controlling role of diphtheroids in producing this odor was confirmed in an in vivo experiment (22a). Droplets of apocrine sweat were placed on forearms after deferming with 70% ethanol. To this was added 0.1 ml of suspensions containing 2 x 10 • cells of 7.0 6.0 I APOCRINE ODOR NON APOCRINE ODOR 5.0 4.0 o 5.0 2.0 1.0 m MICROCOCCI [•m LIPOPHILIC DIPTHEROIDS i•:[] LARGE COLONY DIPTHEROIDS [] GRAM NEGATIVE RODS [] PROPIONIBACTERIA o Figure 1. Diphtheroids as the source of the acrid apocrine odor.