Quantitative Studies on Competitive Activities of Skin Bacteria Growing on Solid Media

Abstract

&ATE m v IINTERACTIONS among skin micro-organisms have begun to receive attention in recent years, but detailed quantitative work has apparently been restricted to growth studies in liquid media (Marsh and Selwyn, 1977). However, such culture systems bear little or no relationship to the skin surface. As a first step towards a more realistic approach, we have devised a quantitative method to study pure and mixed cultures growing on the surface of agar media, and experiments were carried out on antagonistic activities between skin bacteria by means of this procedure. The method, and representative results obtained with it, are described in this paper. MATERIALS AND METHODSBacterial strains. The three commensal organisms used were an antibiotic-producing strain of Staphylococcus epidermidis biotype 4 (Baird-Parker, 1974) referred to as S6+, a penicillin-resistant non-antibiotic-producing strain of Staph. epidermidis of biotype 4 referred to as S6-, a strain of Micrococcus sp. classified as M7 according to Baird-Parker (1965), and a group-D diphtheroid strain (Evans, 1968). These were all isolated from normal skin.The three pathogenic organisms used were a penicillin-resistant strain of Staphylococcus aureus (bacteriophage group 111) and a strain of Streptococcus pyogenes, both from infected skin lesions, and a strain of Corynebacterium diphtheriae var. mitis from a stock culture. The Staph. aureus strain showed relatively small inhibition zones in direct antagonism tests with S6' when compared with all but one of the other bacteria studied. The exception was the non-antibiotic-producing strain, S6-, which was totally resistant to the action of S6 + (Marsh and Selwyn, 1977).Media. Nutrient agar and blood agar were used as growth media, and stock cultures were kept in nutrient broth at 4°C. The media were prepared as described by Cruickshank (1965) with Proteose Peptone and Lab-Lemco (Oxoid), New Zealand Agar and Defibrinated Horse Blood (Tissue Culture Services), and, in addition, 0 3 % (w/v) Liver Digest and 0 3 % (w/v) Yeast Extract (Oxoid). The selective media devised by Marsh and Selwyn (1977) made it possible to obtain differential counts of the bacteria in mixtures (table I).SampZing method. The quantitative study of bacteria growing on a solid surface was carried out with special Petri dishes (Sterilin, code 504 S) which were originally designed for direct contact culture of contaminated surfaces. Each dish had a surface area of 24 cm* and a grid divided into centimetre squares. The inoculum was prepared by diluting a 24-h culture to give approximately 10' colonyforming units (c.f.u.) per ml. For pure culture studies, a 0.2-ml volume was placed on the surface of the medium after it had dried for 20 min. Each plate was gently rotated to distribute the inoculum evenly over the surface (8 x lo3 c.f.u. per cm2). In mixed-culture studies, 1-ml volumes containing 1 x lo6 c.f.u. of each strain were mixed and 0.2 ml of the mixture was used as the inoculum. Readings of viable counts taken at intervals permitted the c...

Skin Microflora.

“…Selwyn and Ellis detected bacteria that produce inhibitory compounds on 20% of normal adults [57]. Later work has revealed a strain of 5 epidermidis that produces a factor that kills strains of Micrococctts, C diphtheriac, and Streptococcti': pyoi^eite- [ 58].…”

Antagonism within populations of micro-organisms from normal human skin

“…The results show that inhibitory strains do not necessarily dominate healthy skin numerically but that they may control the population densities of sensitive resident organisms. Milyani and Selwyn (1978) showed that the inhibitory effect of their S6+ strain was more pronounced on solid than in liquid media and that this can be attributed to dilution of the antibiotic in the latter. However, the inhibition of a less sensitive strain of S. aureus on solid media depended on the absolute and relative size of the S6+ inoculum; if this was too low, the S. aureus grew normally.…”

“…However, the inhibition of a less sensitive strain of S. aureus on solid media depended on the absolute and relative size of the S6+ inoculum; if this was too low, the S. aureus grew normally. Milyani and Selwyn (1978) stated that the physical distribution of the antibiotic-producing strain was very important and that antibiotic production would be insufficient to suppress the growth of sensitive strains if the producer strain was too widely dispersed. In a recent study with the scanning electron microscope, Malcolm and Hughes (1 980) showed bacteria widely scattered in small colonies on the skin surface, a spatial relationship that may affect antibiotic sensitivity on the skin.…”

The skin microbiome: Associations between altered microbial communities and disease

“…66,67 The transplantation of 'protective' microbes can provide valuable antimicrobial therapy, if the correct conditions, such as pH, moisture and nutrients that support transplantation can also be co-administered. The use of prebiotics and probiotics on the skin is an area of research with great promise, as several interspecies competition mechanisms have already been explored.…”

“…For example, P. acnes has been utilised to outcompete methicillin-resistant S. aureus using fermentation, and S. epidermidis has been shown to limit growth of Micrococcus, Corynebacterium, and Streptococcus species in vitro. 66,67 The transplantation of 'protective' microbes can provide valuable antimicrobial therapy, if the correct conditions, such as pH, moisture and nutrients that support transplantation can also be co-administered. 13 In addition, therapies to augment the increased growth of commensal microorganisms or maintain the limited growth of potential pathogens would also be beneficial.…”