Chapter 21, Problem 1R

Discuss the usual mode of entry of bacteria into the skin. Compare bacterial skin infections with infections caused by fungi and viruses with respect to mode of entry.

Summary Introduction

To review:

The mode of entry of bacteria into the skin; compare bacterial, viral, and fungal infections with respect to mode of entry

Introduction:

Bacterial infections often occur through cuts and openings on the skin surface. Fungal infections infect both the skin surface and subcutaneous tissue through skin openings. Viral infections occur both on the skin surface as well through the respiratory tract.

Explanation of Solution

Skin, being a protective layer, generally does not allow microbial colonization and infections due to the presence of natural antimicrobial substances and high salt concentrations. But microbes can enter through hair shafts, pores of sweat glands, or hair follicles to enter the deep mucosal surface. The skin microbiota contains high amounts of staphylococcus and micrococci species which makes it difficult for the pathogens to invade the skin layer. However, species belonging to staphylococci and streptococci cause the most common skin infections. Both genera produce invasive enzymes and skin damaging toxins to establish infections in the skin. Whenever the skin barrier is damaged, either through aberrations or invasive medical devices, it paves an entry for the normal flora to cause infections in the host.

The virus entry in to the cells requires breaching of the mucosal layer to establish viral replications and virus spread. The virus replication does not occur at the site of infection due to limited blood supply and lymphatic vessels. Viruses require vascularized dermis through bites of vectors like mosquitoes, ticks, sand flies, mites, and also through body piercing activities, and sexual contact with body fluids for deeper inoculation below the dermis layer. Warts, small pox, chicken pox, measles, and shingles are some of the common skin infections caused by viruses.

The fungal pathogens cause superficial, subcutaneous, and systemic infections. Dermatophytes are commonly associated with superficial skin infections. Skin trauma is the main cause for fungal entry to establish infections at the keratinized sites. They produce high amounts of keratinases which helps in the solubilisation of skin keratin for easy invasion. Common fungal pathogens in skin infections include Phaeoannellomyces werneckii, M. furfur, Trichosporon beigelii, and Piedraia hortae.

Conclusion

The skin serves as the largest protective organ of the human body. Pathogens, including bacteria, fungi, and viruses, cause superficial to deep sub-mucosal infections; these are based on host factors as well as the factors related to the pathogen’s virulence. Each pathogen has a specific mode of entry and is responsible for pathogenesis.