Human skin microbiota has been described as a “microbial fingerprint” due to observed differences between individuals. Current understanding of the cutaneous microbiota is based on sampling the outermost layers of the epidermis, while the microbiota in the remaining skin layers has not yet been fully characterized. Environmental conditions can vary drastically between the cutaneous compartments and give rise to unique communities. We demonstrate that the dermal microbiota is surprisingly similar among individuals and contains a specific subset of the epidermal microbiota. Variability in bacterial community composition decreased significantly from the epidermal to the dermal compartment but was similar among anatomic locations (hip and knee). The composition of the epidermal microbiota was more strongly affected by environmental factors than that of the dermal community. These results indicate a well-conserved dermal community that is functionally distinct from the epidermal community, challenging the current dogma. Future studies in cutaneous disorders and chronic infections may benefit by focusing on the dermal microbiota as a persistent microbial community. IMPORTANCE Human skin microbiota is thought to be unique according to the individual's lifestyle and genetic predisposition. This is true for the epidermal microbiota, while our findings demonstrate that the dermal microbiota is universal between healthy individuals. The preserved dermal microbial community is compositionally unique and functionally distinct to the specific environment in the depth of human skin. It is expected to have direct contact with the immune response of the human host, and research in the communication between host and microbiota should be targeted to this cutaneous compartment. This novel insight into specific microbial adaptation can be used advantageously in the research of chronic disorders and infections of the skin. It can enlighten the alteration between health and disease to the benefit of patients suffering from long-lasting socioeconomic illnesses.

The presence of biofilms in DFO may explain why non-surgical treatment of DFO, relying on systemic antibiotic therapy, may not resolve some chronic infections caused by biofilm-producing strains.

A relationship has been suggested between lumbar disc herniation (LDH) and chronic bacterial infection frequently involving Propionibacterium acnes, which is known to cause chronic infection through the formation of biofilm aggregates. The objective of the study was to assess whether a disc infection involving biofilm formation is present in patients with LDH. A total of 51 LDH patients and 14 controls were included. Bacterial DNA was detected by real-time polymerase chain reaction (PCR) in 16/51 samples in the LDH group and 7/14 controls (p = 0.215). Sequencing identified bacteria in 9/16 and 6/7 PCR positive samples in the LDH and control groups, respectively. All samples were stained using fluorescence in situ hybridization (FISH) and examined by confocal laser scanning microscopy. Microscopy demonstrated tissue-embedded bacterial aggregates with host inflammatory cells in 7/51 LDH patients and no controls. The presence of both bacterial aggregates and inflammatory cells suggests a chronic infection in a subset of LDH patients. The finding of bacterial 16S rDNA in both LDH and control disc tissue highlights the importance of microscopic observation to discriminate infection vs contamination. Our findings may have therapeutic implications, as the treatment of biofilm infections is different and more challenging than traditional infections.

The ability of bacteria to aggregate and form biofilms impairs phagocytosis by polymorphonuclear leukocytes (PMNs). The aim of this study was to examine if the size of aggregates is critical for successful phagocytosis and how bacterial biofilms evade phagocytosis. We investigated the live interaction between PMNs and Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli and Staphylococcus epidermidis using confocal scanning laser microscopy. Aggregate size significantly affected phagocytosis outcome and larger aggregates were less likely to be phagocytized. Aggregates of S. epidermidis were also less likely to be phagocytized than equally-sized aggregates of the other three species. We found that only aggregates of approx. 5 μm diameter or smaller were consistently phagocytosed. We demonstrate that planktonic and aggregated cells of all four species significantly reduced the viability of PMNs after 4 h of incubation. Our results indicate that larger bacterial aggregates are less likely to be phagocytosed by PMNs and we propose that, if the aggregates become too large, circulating PMNs may not be able to phagocytose them quickly enough, which may lead to chronic infection.

Multiple approaches were employed to detect pathogens from bone margins associated with Diabetic Foot Osteomyelitis (DFO). Intra‐operative bone specimens of 14 consecutive subjects with suspected DFO were collected over a six‐month study period from Liverpool Hospital. Infected bone and a proximal bone margins presumed to be ‘clean/non‐infected’ were collected. Bone material was subjected to conventional culture, DNA sequencing and microscopy. In total, eight of 14 (57%) proximal bone margins had no growth by conventional culture but were identified in all proximal bone specimens by DNA sequencing. Proximal margins had lower median total microbial counts than infected specimens, but these differences were not statistically significant. Pathogens identified by sequencing in infected specimens were identified in proximal margins and the microbiomes were similar (ANOSIM = 0.02, p = 0.59). Using a combination of SEM and/or PNA‐FISH, we visualized the presence of microorganisms in infected bone specimens and their corresponding proximal margins of seven patients (50%) with DFO. We identify that bacteria can still reside in what seems to be proximal ‘clean’ margins. The significance and implications of clinical outcomes requires further analysis from a larger sample size that incorporates differences in surgical and post‐operative approaches, correlating any outcomes back to culture‐sequence findings.